.. meta::
   :description: A guide to torch.cuda, a PyTorch module to run CUDA operations
   :keywords: memory management, PYTORCH_ALLOC_CONF, optimize PyTorch, CUDA

.. _cuda-semantics:

CUDA semantics
==============


:mod:`torch.cuda` is used to set up and run CUDA operations. It keeps track of
the currently selected GPU, and all CUDA tensors you allocate will by default be
created on that device. The selected device can be changed with a
:any:`torch.cuda.device` context manager.

However, once a tensor is allocated, you can do operations on it irrespective
of the selected device, and the results will be always placed on the same
device as the tensor.

Cross-GPU operations are not allowed by default, with the exception of
:meth:`~torch.Tensor.copy_` and other methods with copy-like functionality
such as :meth:`~torch.Tensor.to` and :meth:`~torch.Tensor.cuda`.
Unless you enable peer-to-peer memory access, any attempts to launch ops on
tensors spread across different devices will raise an error.

Below you can find a small example showcasing this::

    cuda = torch.device('cuda')     # Default CUDA device
    cuda0 = torch.device('cuda:0')
    cuda2 = torch.device('cuda:2')  # GPU 2 (these are 0-indexed)

    x = torch.tensor([1., 2.], device=cuda0)
    # x.device is device(type='cuda', index=0)
    y = torch.tensor([1., 2.]).cuda()
    # y.device is device(type='cuda', index=0)

    with torch.cuda.device(1):
        # allocates a tensor on GPU 1
        a = torch.tensor([1., 2.], device=cuda)

        # transfers a tensor from CPU to GPU 1
        b = torch.tensor([1., 2.]).cuda()
        # a.device and b.device are device(type='cuda', index=1)

        # You can also use ``Tensor.to`` to transfer a tensor:
        b2 = torch.tensor([1., 2.]).to(device=cuda)
        # b.device and b2.device are device(type='cuda', index=1)

        c = a + b
        # c.device is device(type='cuda', index=1)

        z = x + y
        # z.device is device(type='cuda', index=0)

        # even within a context, you can specify the device
        # (or give a GPU index to the .cuda call)
        d = torch.randn(2, device=cuda2)
        e = torch.randn(2).to(cuda2)
        f = torch.randn(2).cuda(cuda2)
        # d.device, e.device, and f.device are all device(type='cuda', index=2)

.. _tf32_on_ampere:

TensorFloat-32 (TF32) on Ampere (and later) devices
---------------------------------------------------

After Pytorch 2.9, we provide a new sets of APIs to control the TF32 behavior in a more fine-grained way, and
suggest to use the new APIs for better control.
We can set float32 precision per backend and per operators. We can also override the global setting for a specific operator.

.. code:: python

  torch.backends.fp32_precision = "ieee"
  torch.backends.cuda.matmul.fp32_precision = "ieee"
  torch.backends.cudnn.fp32_precision = "ieee"
  torch.backends.cudnn.conv.fp32_precision = "tf32"
  torch.backends.cudnn.rnn.fp32_precision = "tf32"

The fp32_precision can be set to `ieee` or `tf32` for `cuda/cudnn`.
`ieee` fp32_precision indicate that we will use `FP32` as internal computation precision.
`tf32` fp32_precision indicate that we will allow to use `TF32` as internal computation precision.

We can override a generic setting for a specific operator if the fp32_precision is set to `ieee`.

.. code:: python

  torch.backends.cudnn.fp32_precision = "tf32"
  torch.backends.cudnn.conv.fp32_precision = "ieee"
  torch.backends.cudnn.rnn.fp32_precision = "ieee"

We can also override a generic setting for a specific backend if the fp32_precision is set to `ieee`.

.. code:: python

  torch.backends.fp32_precision = "tf32"
  torch.backends.cudnn.fp32_precision = "ieee"
  torch.backends.cudnn.conv.fp32_precision = "ieee"
  torch.backends.cudnn.rnn.fp32_precision = "ieee"

For above 2 cases, both `torch.backends.cudnn.conv.fp32_precision` and `torch.backends.cudnn.rnn.fp32_precision`
is overridden to `ieee`.

We suggest to use the new settings for better control. And we do not support to use mix of old and new settings.

.. warning::

  Old settings with `allow_tf32` as follows is going to be deprecated. We suggest to use the above new settings for
  better control. And we do not support to use mix of old and new settings.

Starting in PyTorch 1.7, there is a new flag called `allow_tf32`. This flag
defaults to True in PyTorch 1.7 to PyTorch 1.11, and False in PyTorch 1.12 and later.
This flag controls whether PyTorch is allowed to use the TensorFloat32 (TF32) tensor cores,
available on NVIDIA GPUs since Ampere, internally to compute matmul (matrix multiplies
and batched matrix multiplies) and convolutions.

TF32 tensor cores are designed to achieve better performance on matmul and convolutions on
`torch.float32` tensors by rounding input data to have 10 bits of mantissa, and accumulating
results with FP32 precision, maintaining FP32 dynamic range.

matmuls and convolutions are controlled separately, and their corresponding flags can be accessed at:

.. code:: python

  # The flag below controls whether to allow TF32 on matmul. This flag defaults to False
  # in PyTorch 1.12 and later.
  torch.backends.cuda.matmul.allow_tf32 = True

  # The flag below controls whether to allow TF32 on cuDNN. This flag defaults to True.
  torch.backends.cudnn.allow_tf32 = True

The precision of matmuls can also be set more broadly (limited not just to CUDA) via :meth:`~torch.set_float32_matmul_precision`.
Note that besides matmuls and convolutions themselves, functions and nn modules that internally uses
matmuls or convolutions are also affected. These include `nn.Linear`, `nn.Conv*`, cdist, tensordot,
affine grid and grid sample, adaptive log softmax, GRU and LSTM.

To get an idea of the precision and speed, see the example code and benchmark data (on A100) below:

.. code:: python

  a_full = torch.randn(10240, 10240, dtype=torch.double, device='cuda')
  b_full = torch.randn(10240, 10240, dtype=torch.double, device='cuda')
  ab_full = a_full @ b_full
  mean = ab_full.abs().mean()  # 80.7277

  a = a_full.float()
  b = b_full.float()

  # Do matmul at TF32 mode.
  torch.backends.cuda.matmul.allow_tf32 = True
  ab_tf32 = a @ b  # takes 0.016s on GA100
  error = (ab_tf32 - ab_full).abs().max()  # 0.1747
  relative_error = error / mean  # 0.0022

  # Do matmul with TF32 disabled.
  torch.backends.cuda.matmul.allow_tf32 = False
  ab_fp32 = a @ b  # takes 0.11s on GA100
  error = (ab_fp32 - ab_full).abs().max()  # 0.0031
  relative_error = error / mean  # 0.000039

From the above example, we can see that with TF32 enabled, the speed is ~7x faster on A100, and that
relative error compared to double precision is approximately 2 orders of magnitude larger. Note that
the exact ratio of TF32 to single precision speed depends on the hardware generation, as properties
such as the ratio of memory bandwidth to compute as well as the ratio of TF32 to FP32 matmul throughput
may vary from generation to generation or model to model.
If full FP32 precision is needed, users can disable TF32 by:

.. code:: python

  torch.backends.cuda.matmul.allow_tf32 = False
  torch.backends.cudnn.allow_tf32 = False

To toggle the TF32 flags off in C++, you can do

.. code:: C++

  at::globalContext().setAllowTF32CuBLAS(false);
  at::globalContext().setAllowTF32CuDNN(false);

For more information about TF32, see:

- `TensorFloat-32`_
- `CUDA 11`_
- `Ampere architecture`_

.. _TensorFloat-32: https://blogs.nvidia.com/blog/2020/05/14/tensorfloat-32-precision-format/
.. _CUDA 11: https://devblogs.nvidia.com/cuda-11-features-revealed/
.. _Ampere architecture: https://devblogs.nvidia.com/nvidia-ampere-architecture-in-depth/

.. _fp16reducedprecision:

Reduced Precision Reduction in FP16 GEMMs
-----------------------------------------

(Distinct from full FP16 accumulation that is intended for hardware that has higher throughput
with FP16 accumulation than FP32 accumulation, see :ref:`Full FP16 accumulation<fp16accumulation>`)

fp16 GEMMs are potentially done with some intermediate reduced precision reductions (e.g., in fp16 rather than fp32). These selective reductions in precision can allow for higher performance on certain workloads (particularly those with a large `k` dimension) and GPU architectures at the cost of numerical precision and potential for overflow.

Some example benchmark data on V100:

.. code::

  [--------------------------- bench_gemm_transformer --------------------------]
        [  m ,  k  ,  n  ]    |  allow_fp16_reduc=True  |  allow_fp16_reduc=False
  1 threads: --------------------------------------------------------------------
        [4096, 4048, 4096]    |           1634.6        |           1639.8
        [4096, 4056, 4096]    |           1670.8        |           1661.9
        [4096, 4080, 4096]    |           1664.2        |           1658.3
        [4096, 4096, 4096]    |           1639.4        |           1651.0
        [4096, 4104, 4096]    |           1677.4        |           1674.9
        [4096, 4128, 4096]    |           1655.7        |           1646.0
        [4096, 4144, 4096]    |           1796.8        |           2519.6
        [4096, 5096, 4096]    |           2094.6        |           3190.0
        [4096, 5104, 4096]    |           2144.0        |           2663.5
        [4096, 5112, 4096]    |           2149.1        |           2766.9
        [4096, 5120, 4096]    |           2142.8        |           2631.0
        [4096, 9728, 4096]    |           3875.1        |           5779.8
        [4096, 16384, 4096]   |           6182.9        |           9656.5
  (times in microseconds).

If full precision reductions are needed, users can disable reduced precision reductions in fp16 GEMMs with:

.. code:: python

  torch.backends.cuda.matmul.allow_fp16_reduced_precision_reduction = False

To toggle the reduced precision reduction flags in C++, one can do

.. code:: C++

  at::globalContext().setAllowFP16ReductionCuBLAS(false);

.. _bf16reducedprecision:

Reduced Precision Reduction in BF16 GEMMs
-----------------------------------------

A similar flag (as above) exists for BFloat16 GEMMs.
Note that this switch is set to `True` by default for BF16, if you observe
numerical instability in your workload, you may wish to set it to `False`.

If reduced precision reductions are not desired, users can disable reduced
precision reductions in bf16 GEMMs with:

.. code:: python

  torch.backends.cuda.matmul.allow_bf16_reduced_precision_reduction = False

To toggle the reduced precision reduction flags in C++, one can do

.. code:: C++

  at::globalContext().setAllowBF16ReductionCuBLAS(true);

.. _fp16accumulation:

Full FP16 Accumulation in FP16 GEMMs
-------------------------------------

Certain GPUs have increased performance when doing _all_ FP16 GEMM accumulation
in FP16, at the cost of numerical precision and greater likelihood of overflow.
Note that this setting only has an effect on GPUs of compute capability 7.0 (Volta)
or newer.

This behavior can be enabled via:

.. code:: python

  torch.backends.cuda.matmul.allow_fp16_accumulation = True

To toggle the reduced precision reduction flags in C++, one can do

.. code:: C++

  at::globalContext().setAllowFP16AccumulationCuBLAS(true);

Asynchronous execution
----------------------

By default, GPU operations are asynchronous.  When you call a function that
uses the GPU, the operations are *enqueued* to the particular device, but not
necessarily executed until later.  This allows us to execute more computations
in parallel, including operations on CPU or other GPUs.

In general, the effect of asynchronous computation is invisible to the caller,
because (1) each device executes operations in the order they are queued, and
(2) PyTorch automatically performs necessary synchronization when copying data
between CPU and GPU or between two GPUs.  Hence, computation will proceed as if
every operation was executed synchronously.

You can force synchronous computation by setting environment variable
``CUDA_LAUNCH_BLOCKING=1``.  This can be handy when an error occurs on the GPU.
(With asynchronous execution, such an error isn't reported until after the
operation is actually executed, so the stack trace does not show where it was
requested.)

A consequence of the asynchronous computation is that time measurements without
synchronizations are not accurate. To get precise measurements, one should either
call :func:`torch.cuda.synchronize()` before measuring, or use :class:`torch.cuda.Event`
to record times as following::

    start_event = torch.cuda.Event(enable_timing=True)
    end_event = torch.cuda.Event(enable_timing=True)
    start_event.record()

    # Run some things here

    end_event.record()
    torch.cuda.synchronize()  # Wait for the events to be recorded!
    elapsed_time_ms = start_event.elapsed_time(end_event)

As an exception, several functions such as :meth:`~torch.Tensor.to` and
:meth:`~torch.Tensor.copy_` admit an explicit :attr:`non_blocking` argument,
which lets the caller bypass synchronization when it is unnecessary.
Another exception is CUDA streams, explained below.

CUDA streams
^^^^^^^^^^^^

A `CUDA stream`_ is a linear sequence of execution that belongs to a specific
device.  You normally do not need to create one explicitly: by default, each
device uses its own "default" stream.

Operations inside each stream are serialized in the order they are created,
but operations from different streams can execute concurrently in any
relative order, unless explicit synchronization functions (such as
:meth:`~torch.cuda.synchronize` or :meth:`~torch.cuda.Stream.wait_stream`) are
used.  For example, the following code is incorrect::

    cuda = torch.device('cuda')
    s = torch.cuda.Stream()  # Create a new stream.
    A = torch.empty((100, 100), device=cuda).normal_(0.0, 1.0)
    with torch.cuda.stream(s):
        # sum() may start execution before normal_() finishes!
        B = torch.sum(A)

When the "current stream" is the default stream, PyTorch automatically performs
necessary synchronization when data is moved around, as explained above.
However, when using non-default streams, it is the user's responsibility to
ensure proper synchronization.  The fixed version of this example is::

    cuda = torch.device('cuda')
    s = torch.cuda.Stream()  # Create a new stream.
    A = torch.empty((100, 100), device=cuda).normal_(0.0, 1.0)
    s.wait_stream(torch.cuda.default_stream(cuda))  # NEW!
    with torch.cuda.stream(s):
        B = torch.sum(A)
    A.record_stream(s)  # NEW!

There are two new additions.  The :meth:`torch.cuda.Stream.wait_stream` call
ensures that the ``normal_()`` execution has finished before we start running
``sum(A)`` on a side stream.  The :meth:`torch.Tensor.record_stream` (see for
more details) ensures that we do not deallocate A before ``sum(A)`` has
completed.  You can also manually wait on the stream at some later point in
time with ``torch.cuda.default_stream(cuda).wait_stream(s)`` (note that it
is pointless to wait immediately, since that will prevent the stream execution
from running in parallel with other work on the default stream.)  See the
documentation for :meth:`torch.Tensor.record_stream` on more details on when
to use one or another.

Note that this synchronization is necessary even when there is no
read dependency, e.g., as seen in this example::

    cuda = torch.device('cuda')
    s = torch.cuda.Stream()  # Create a new stream.
    A = torch.empty((100, 100), device=cuda)
    s.wait_stream(torch.cuda.default_stream(cuda))  # STILL REQUIRED!
    with torch.cuda.stream(s):
        A.normal_(0.0, 1.0)
        A.record_stream(s)

Despite the computation on ``s`` not reading the contents of ``A`` and no
other uses of ``A``, it is still necessary to synchronize, because ``A``
may correspond to memory reallocated by the CUDA caching allocator, with
pending operations from the old (deallocated) memory.

.. _bwd-cuda-stream-semantics:

Stream semantics of backward passes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Each backward CUDA op runs on the same stream that was used for its corresponding forward op.
If your forward pass runs independent ops in parallel on different streams,
this helps the backward pass exploit that same parallelism.

The stream semantics of a backward call with respect to surrounding ops are the same
as for any other call. The backward pass inserts internal syncs to ensure this even when
backward ops run on multiple streams as described in the previous paragraph.
More concretely, when calling
:func:`autograd.backward<torch.autograd.backward>`,
:func:`autograd.grad<torch.autograd.grad>`, or
:meth:`tensor.backward<torch.Tensor.backward>`,
and optionally supplying CUDA tensor(s) as the  initial gradient(s) (e.g.,
:func:`autograd.backward(..., grad_tensors=initial_grads)<torch.autograd.backward>`,
:func:`autograd.grad(..., grad_outputs=initial_grads)<torch.autograd.grad>`, or
:meth:`tensor.backward(..., gradient=initial_grad)<torch.Tensor.backward>`),
the acts of

1. optionally populating initial gradient(s),
2. invoking the backward pass, and
3. using the gradients

have the same stream-semantics relationship as any group of ops::

    s = torch.cuda.Stream()

    # Safe, grads are used in the same stream context as backward()
    with torch.cuda.stream(s):
        loss.backward()
        use grads

    # Unsafe
    with torch.cuda.stream(s):
        loss.backward()
    use grads

    # Safe, with synchronization
    with torch.cuda.stream(s):
        loss.backward()
    torch.cuda.current_stream().wait_stream(s)
    use grads

    # Safe, populating initial grad and invoking backward are in the same stream context
    with torch.cuda.stream(s):
        loss.backward(gradient=torch.ones_like(loss))

    # Unsafe, populating initial_grad and invoking backward are in different stream contexts,
    # without synchronization
    initial_grad = torch.ones_like(loss)
    with torch.cuda.stream(s):
        loss.backward(gradient=initial_grad)

    # Safe, with synchronization
    initial_grad = torch.ones_like(loss)
    s.wait_stream(torch.cuda.current_stream())
    with torch.cuda.stream(s):
        initial_grad.record_stream(s)
        loss.backward(gradient=initial_grad)

BC note: Using grads on the default stream
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In prior versions of PyTorch (1.9 and earlier), the autograd engine always synced
the default stream with all backward ops, so the following pattern::

    with torch.cuda.stream(s):
        loss.backward()
    use grads

was safe as long as ``use grads`` happened on the default stream.
In present PyTorch, that pattern is no longer safe. If ``backward()``
and ``use grads`` are in different stream contexts, you must sync the streams::

    with torch.cuda.stream(s):
        loss.backward()
    torch.cuda.current_stream().wait_stream(s)
    use grads

even if ``use grads`` is on the default stream.

.. _CUDA stream: https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#streams

.. _cuda-memory-management:

Memory management
-----------------

PyTorch uses a caching memory allocator to speed up memory allocations. This
allows fast memory deallocation without device synchronizations. However, the
unused memory managed by the allocator will still show as if used in
``nvidia-smi``. You can use :meth:`~torch.cuda.memory_allocated` and
:meth:`~torch.cuda.max_memory_allocated` to monitor memory occupied by
tensors, and use :meth:`~torch.cuda.memory_reserved` and
:meth:`~torch.cuda.max_memory_reserved` to monitor the total amount of memory
managed by the caching allocator. Calling :meth:`~torch.cuda.empty_cache`
releases all **unused** cached memory from PyTorch so that those can be used
by other GPU applications. However, the occupied GPU memory by tensors will not
be freed so it can not increase the amount of GPU memory available for PyTorch.

To better understand how CUDA memory is being used over time,
:ref:`torch_cuda_memory` describes tools for capturing and visualizing traces of memory use.

For more advanced users, we offer more comprehensive memory benchmarking via
:meth:`~torch.cuda.memory_stats`. We also offer the capability to capture a
complete snapshot of the memory allocator state via
:meth:`~torch.cuda.memory_snapshot`, which can help you understand the
underlying allocation patterns produced by your code.

.. _cuda-memory-envvars:

Optimizing memory usage  with ``PYTORCH_ALLOC_CONF``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Use of a caching allocator can interfere with memory checking tools such as
``cuda-memcheck``.  To debug memory errors using ``cuda-memcheck``, set
``PYTORCH_NO_CUDA_MEMORY_CACHING=1`` in your environment to disable caching.

The behavior of the caching allocator can be controlled via the environment variable
``PYTORCH_ALLOC_CONF``. ``PYTORCH_CUDA_ALLOC_CONF`` is its alias and is provided only
for backward compatibility.
The format is ``PYTORCH_ALLOC_CONF=<option>:<value>,<option2>:<value2>...``
Available options:

* ``backend`` allows selecting the underlying allocator implementation.
  Currently, valid options are ``native``, which uses PyTorch's native
  implementation, and ``cudaMallocAsync``, which uses
  `CUDA's built-in asynchronous allocator`_.
  ``cudaMallocAsync`` requires CUDA 11.4 or newer. The default is ``native``.
  ``backend`` applies to all devices used by the process, and can't be
  specified on a per-device basis.
* ``max_split_size_mb`` prevents the native allocator
  from splitting blocks larger than this size (in MB). This can reduce
  fragmentation and may allow some borderline workloads to complete without
  running out of memory. Performance cost can range from 'zero' to 'substantial'
  depending on allocation patterns.  Default value is unlimited, i.e. all blocks
  can be split. The
  :meth:`~torch.cuda.memory_stats` and
  :meth:`~torch.cuda.memory_summary` methods are useful for tuning.  This
  option should be used as a last resort for a workload that is aborting
  due to 'out of memory' and showing a large amount of inactive split blocks.
  ``max_split_size_mb`` is only meaningful with ``backend:native``.
  With ``backend:cudaMallocAsync``, ``max_split_size_mb`` is ignored.
* ``roundup_power2_divisions`` helps with rounding the requested allocation
  size to nearest power-2 division and making better use of the blocks. In
  the native CUDACachingAllocator, the sizes are rounded up in multiple
  of blocks size of 512, so this works fine for smaller sizes. However, this
  can be inefficient for large near-by allocations as each will go to different
  size of blocks and reuse of those blocks are minimized. This might create
  lots of unused blocks and will waste GPU memory capacity. This option enables
  the rounding of allocation size to nearest power-2 division. For example, if
  we need to round-up size of 1200 and if number of divisions is 4,
  the size 1200 lies between 1024 and 2048 and if we do 4 divisions between
  them, the values are 1024, 1280, 1536, and 1792. So, allocation size of 1200
  will be rounded to 1280 as the nearest ceiling of power-2 division.
  Specify a single value to apply for all allocation sizes or specify an
  array of key value pairs to set power-2 division individually for each
  power of two interval. For example to set 1 division for all allocations
  under 256MB, 2 division for allocations between 256MB and 512MB, 4 divisions
  for allocations between 512MB and 1GB and 8 divisions for any larger allocations,
  set the knob value to: [256:1,512:2,1024:4,>:8].
  ``roundup_power2_divisions`` is only meaningful with ``backend:native``.
  With ``backend:cudaMallocAsync``, ``roundup_power2_divisions`` is ignored.
* ``max_non_split_rounding_mb`` will allow non-split blocks for better reuse, eg,
   a 1024MB cached block can be reused for a 512MB allocation request. In the default
   case, we only allow up to 20MB of rounding of non-split blocks, so a 512MB block
   can only be served with between 512-532 MB size block. If we set the value of this
   option to 1024, it will allow 512-1536 MB size blocks to be used for a 512MB block
   which increases reuse of larger blocks. This will also help in reducing the stalls
   in avoiding expensive cudaMalloc calls.
* ``garbage_collection_threshold`` helps actively reclaiming unused GPU memory to
  avoid triggering expensive sync-and-reclaim-all operation (release_cached_blocks),
  which can be unfavorable to latency-critical GPU applications (e.g., servers).
  Upon setting this threshold (e.g., 0.8), the allocator will start reclaiming
  GPU memory blocks if the GPU memory capacity usage exceeds the threshold (i.e.,
  80% of the total memory allocated to the GPU application). The algorithm prefers
  to free old & unused blocks first to avoid freeing blocks that are actively being
  reused. The threshold value should be between greater than 0.0 and less than 1.0.
  The default value is set at 1.0.

  ``garbage_collection_threshold`` is only meaningful with ``backend:native``.
  With ``backend:cudaMallocAsync``, ``garbage_collection_threshold`` is ignored.
* ``expandable_segments`` (experimental, default: `False`) If set to `True`, this setting instructs
  the allocator to create CUDA allocations that can later be expanded to better handle cases
  where a job changing allocation sizes frequently, such as having a changing batch size.
  Normally for large (>2MB) allocations, the allocator calls cudaMalloc to get allocations
  that are the same size as what the user requests. In the future, parts of these
  allocations can be reused for other requests if they are free. This works well
  when the program makes many requests of exactly the same size or of sizes that
  even multiples of that size. Many deep learning models follow this behavior.
  However, one common exception is when the batch size changes slightly from one
  iteration to the next, e.g. in batched inference. When the program runs
  initially with batch size `N`, it will make allocations appropriate for that size.
  If in the future, it runs at size `N - 1`, the existing allocations will still be
  big enough. However, if it runs at size `N + 1`, then it will have to make new
  allocations that are slightly larger. Not all the tensors are the same size.
  Some might be `(N + 1)*A` and others `(N + 1)*A*B` where `A` and `B` are some non-batch
  dimensions in the model. Because the allocator reuses existing allocations when
  they are big enough, some number of `(N + 1)*A` allocations will actually fit in
  the already existing `N*B*A` segments, though not perfectly. As the model runs it
  will partially fill up all of these segments leaving unusable free slices of
  memory at the end of these segments. The allocator at some point will need to
  `cudaMalloc` a new `(N + 1)*A*B` segment. If there is not enough memory, there is
  now no way to recover the slices of memory that are free at the end of existing
  segments. With models 50+ layers deep, this pattern might repeat 50+ times
  creating many slivers.

  `expandable_segments` allows the allocator to create a segment initially and then
  expand its size later when more memory is needed. Instead of making one segment
  per allocation, it tries to make one segment (per stream) that grows as
  necessary. Now when the `N + 1` case runs, the allocations will tile nicely into
  the one large segment until it fills up. Then more memory is requested and
  appended to the end of the segment. This process does not create as many slivers
  of unusable memory, so it is more likely to succeed at finding this memory.

* `pinned_use_cuda_host_register` option is a boolean flag that determines whether to
  use the CUDA API's cudaHostRegister function for allocating pinned memory instead
  of the default cudaHostAlloc. When set to True, the memory is allocated using regular
  malloc and then pages are mapped to the memory before calling cudaHostRegister.
  This pre-mapping of pages helps reduce the lock time during the execution
  of cudaHostRegister.

* `pinned_num_register_threads` option is only valid when pinned_use_cuda_host_register
  is set to True. By default, one thread is used to map the pages. This option allows
  using more threads to parallelize the page mapping operations to reduce the overall
  allocation time of pinned memory. A good value for this option is 8 based on
  benchmarking results.

* `pinned_use_background_threads` option is a boolean flag to enable background thread
  for processing events. This avoids any slow path associated with querying/processing of
  events in the fast allocation path. This feature is disabled by default.

* `pinned_reserve_segment_size_mb` option is a size in MB to reserve for pinned memory
  segment. This allocates a large segment of pinned memory upfront and then uses to allocate
  small size requests. This helps reduce the number of expensive device library calls.

* ``graph_capture_record_stream_reuse`` (experimental, default: `False`)
  If set to `True`, the CUDA caching allocator will attempt to reclaim device memory during
  CUDA Graph capture by using the graph topology (instead of CUDA events) to determine
  when a freed block is safe to reuse. This can reduce peak memory during long captures that free
  and reallocate buffers across multiple streams, especially when the capture DAG frequently
  reaches joined frontiers.

* ``per_process_memory_fraction`` option limits the amount of memory that can be allocated
  on all the CUDA devices to a specified fraction of the available memory. This is a value
  between 0 and 1. Attempting to allocate more memory will raise an out of memory error.

.. note::

    Some stats reported by the
    :ref:`CUDA memory management API<cuda-memory-management-api>`
    are specific to ``backend:native``, and are not meaningful with
    ``backend:cudaMallocAsync``.
    See each function's docstring for details.

.. _CUDA's built-in asynchronous allocator:
    https://developer.nvidia.com/blog/using-cuda-stream-ordered-memory-allocator-part-1/

.. _cuda-memory-custom-allocator:

Using custom memory allocators for CUDA
---------------------------------------

It is possible to define allocators as simple functions in C/C++ and compile
them as a shared library, the code below shows a basic allocator that just
traces all the memory operations.

.. code:: C++

   #include <sys/types.h>
   #include <cuda_runtime_api.h>
   #include <iostream>
   // Compile with g++ alloc.cc -o alloc.so -I/usr/local/cuda/include -shared -fPIC
   extern "C" {
   void* my_malloc(ssize_t size, int device, cudaStream_t stream) {
      void *ptr;
      cudaMalloc(&ptr, size);
      std::cout<<"alloc "<<ptr<<size<<std::endl;
      return ptr;
   }

   void my_free(void* ptr, ssize_t size, int device, cudaStream_t stream) {
      std::cout<<"free "<<ptr<< " "<<stream<<std::endl;
      cudaFree(ptr);
   }
   }


This can be used in python through the :class:`torch.cuda.memory.CUDAPluggableAllocator`.
The user is responsible for supplying the path to the `.so` file and the name
of the alloc/free functions that match the signatures specified above.

.. code:: python

   import torch

   # Load the allocator
   new_alloc = torch.cuda.memory.CUDAPluggableAllocator(
       'alloc.so', 'my_malloc', 'my_free')
   # Swap the current allocator
   torch.cuda.memory.change_current_allocator(new_alloc)
   # This will allocate memory in the device using the new allocator
   b = torch.zeros(10, device='cuda')


.. code:: python

   import torch

   # Do an initial memory allocator
   b = torch.zeros(10, device='cuda')
   # Load the allocator
   new_alloc = torch.cuda.memory.CUDAPluggableAllocator(
       'alloc.so', 'my_malloc', 'my_free')
   # This will error since the current allocator was already instantiated
   torch.cuda.memory.change_current_allocator(new_alloc)

.. cublas-workspaces:

Mixing different CUDA system allocators in the same program
-----------------------------------------------------------
Depending on your use case, :meth:`~torch.cuda.change_current_allocator` may not be what you
want to use, since it swaps the CUDA allocator for the entire program (similar to
``PYTORCH_ALLOC_CONF=backend:cudaMallocAsync``). For instance, if the swapped allocator doesn't
have caching mechanism, you will lose all the benefits of PyTorch's CUDACachingAllocator. Instead,
you can selectively mark a region of PyTorch code to use a custom allocator using
:class:`torch.cuda.MemPool`. This will let you use multiple CUDA system allocators in the same
PyTorch program, along with most of the benefits of the CUDACachingAllocator (e.g. caching).
Using :class:`torch.cuda.MemPool`, you can utilize custom allocators that enable several features,
such as:

* Allocating output buffers for an all-reduce using ``ncclMemAlloc`` allocator can enable NVLink
  Switch Reductions (NVLS). This can reduce contention between overlapping compute and communication
  kernels on GPU resources (SMs, and Copy Engines), especially on tensor-parallel workloads.
* For Grace CPU based systems, allocating host outputs buffers for an all-gather using ``cuMemCreate``
  and specifying ``CU_MEM_LOCATION_TYPE_HOST_NUMA`` can enable Extended GPU Memory (EGM) based memory transfers
  from source GPUs to the destination CPU. This accelerates the all-gather since the transfer
  happens over NVLinks, which otherwise would have happened over bandwidth-limited, Network Interface
  Card (NIC) links. Such an accelerated all-gather can in turn speed up model checkpointing.
* If you are crafting a model and don't want to think about the optimal memory placements of a memory
  intensive module at first (e.g. an embedding table), or perhaps you have a module which is not
  performance sensitive and doesn't fit in the GPU, then you could just allocate that module with
  ``cudaMallocManaged`` with preferred CPU location and get your model working first.

.. note::

    While ``cudaMallocManaged`` offers convenient automatic memory management using CUDA Unified Virtual Memory (UVM),
    it is not recommended for DL workloads. For DL workloads that fit in GPU memory, explicit placement consistently
    outperforms UVM, since there are no page faults and access patterns remain predictable. When GPU memory gets
    saturated, UVM has to perform costly double transfers, evicting pages to CPU before bringing in new ones.

The code below shows ``ncclMemAlloc`` wrapped in a :class:`torch.cuda.memory.CUDAPluggableAllocator`.

.. code:: python

   import os

   import torch
   import torch.distributed as dist
   from torch.cuda.memory import CUDAPluggableAllocator
   from torch.distributed.distributed_c10d import _get_default_group
   from torch.utils import cpp_extension


   # create allocator
   nccl_allocator_source = """
   #include <nccl.h>
   #include <iostream>
   extern "C" {

   void* nccl_alloc_plug(size_t size, int device, void* stream) {
     std::cout << "Using ncclMemAlloc" << std::endl;
     void* ptr;
     ncclResult_t err = ncclMemAlloc(&ptr, size);
     return ptr;

   }

   void nccl_free_plug(void* ptr, size_t size, int device, void* stream) {
     std::cout << "Using ncclMemFree" << std::endl;
     ncclResult_t err = ncclMemFree(ptr);
   }

   }
   """
   nccl_allocator_libname = "nccl_allocator"
   nccl_allocator = torch.utils.cpp_extension.load_inline(
       name=nccl_allocator_libname,
       cpp_sources=nccl_allocator_source,
       with_cuda=True,
       extra_ldflags=["-lnccl"],
       verbose=True,
       is_python_module=False,
       build_directory="./",
   )

   allocator = CUDAPluggableAllocator(
       f"./{nccl_allocator_libname}.so", "nccl_alloc_plug", "nccl_free_plug"
   ).allocator()

   # setup distributed
   rank = int(os.getenv("RANK"))
   local_rank = int(os.getenv("LOCAL_RANK"))
   world_size = int(os.getenv("WORLD_SIZE"))
   torch.cuda.set_device(local_rank)
   dist.init_process_group(backend="nccl")
   device = torch.device(f"cuda:{local_rank}")
   default_pg = _get_default_group()
   backend = default_pg._get_backend(device)

   # Note: for convenience, ProcessGroupNCCL backend provides
   # the ncclMemAlloc allocator as backend.mem_allocator
   allocator = backend.mem_allocator


You can now define a new memory pool by passing this allocator to :class:`torch.cuda.MemPool`:

.. code:: python

   pool = torch.cuda.MemPool(allocator)


The pool can then be used with the :class:`torch.cuda.use_mem_pool` context manager to
allocate tensors into that pool:

.. code:: python

   with torch.cuda.use_mem_pool(pool):
       # tensor gets allocated with ncclMemAlloc passed in the pool
       tensor = torch.arange(1024 * 1024 * 2, device=device)
       print(f"tensor ptr on rank {rank} is {hex(tensor.data_ptr())}")

   # register user buffers using ncclCommRegister (called under the hood)
   backend.register_mem_pool(pool)

   # Collective uses Zero Copy NVLS
   dist.all_reduce(tensor[0:4])
   torch.cuda.synchronize()
   print(tensor[0:4])


Note the usage of ``register_mem_pool`` in the above example. This is an extra step for
NVLS reductions, where the user buffers need to be registered with NCCL. A user can
de-register the buffers with a similar ``deregister_mem_pool`` call.

To reclaim memory, users will first need to ensure nothing is using the pool. When none
of the tensors are holding a reference to the pool, :meth:`~torch.cuda.empty_cache` will
be called internally on deletion of the pool, hence returning all the memory to the system.

.. code:: python

   del tensor, del pool


Users can optionally specify a ``use_on_oom`` bool (which is False by default) during MemPool
creation. If true, then the CUDACachingAllocator will be able to use memory in this pool as
a last resort instead of OOMing.

.. code:: python

    pool = torch.cuda.MemPool(allocator, use_on_oom=True)
    with torch.cuda.use_mem_pool(pool):
        a = torch.randn(40 * 1024 * 1024, dtype=torch.uint8, device="cuda")
    del a

    # at the memory limit, this will succeed by using pool's memory in order to avoid the oom
    b = torch.randn(40 * 1024 * 1024, dtype=torch.uint8, device="cuda")


The following :meth:`torch.cuda.MemPool.use_count` and :meth:`torch.cuda.MemPool.snapshot`
APIs can be used for debugging purposes:

.. code:: python

   pool = torch.cuda.MemPool(allocator)

   # pool's use count should be 1 at this point as MemPool object
   # holds a reference
   assert pool.use_count() == 1

   nelem_1mb = 1024 * 1024 // 4

   with torch.cuda.use_mem_pool(pool):
       out_0 = torch.randn(nelem_1mb, device="cuda")

       # pool's use count should be 2 at this point as use_mem_pool
       # holds a reference
       assert pool.use_count() == 2

   # pool's use count should be back to 1 at this point as use_mem_pool
   # released its reference
   assert pool.use_count() == 1

   with torch.cuda.use_mem_pool(pool):
       # pool should have 1 segment since we made a small allocation (1 MB)
       # above and so the CUDACachingAllocator packed it into a 2 MB buffer
       assert len(pool.snapshot()) == 1

       out_1 = torch.randn(nelem_1mb, device="cuda")

       # pool should still have 1 segment since we made another small allocation
       # (1 MB) that got packed into the existing 2 MB buffer
       assert len(pool.snapshot()) == 1

       out_2 = torch.randn(nelem_1mb, device="cuda")

       # pool now should have 2 segments since the CUDACachingAllocator had
       # to make a new 2 MB buffer to accommodate out_2
       assert len(pool.snapshot()) == 2


.. note::

   * :class:`torch.cuda.MemPool` holds a reference to the pool. When you use the
     :class:`torch.cuda.use_mem_pool` context manager, it will also acquire another reference
     to the pool. On exit of the context manager, it will release its reference. After that,
     ideally it should only be tensors holding references to the pool. Once the tensors release
     their references, the use count of the pool will be 1, reflecting that only the
     :class:`torch.cuda.MemPool` object is holding a reference. Only at that point, can the memory
     held by the pool be returned to the system when the pool's destructor is called using
     ``del``.
   * :class:`torch.cuda.MemPool` doesn't currently support ``expandable_segments`` mode of
     CUDACachingAllocator.
   * `NCCL has specific requirements`_ for a buffer to be compatible with NVLS reductions.
     These requirements can be broken in a dynamic workload, for instance, the buffer being
     sent to NCCL by the CUDACachingAllocator might be split and hence, not correctly aligned.
     In those cases, NCCL can use a fallback algorithm instead of NVLS.
   * Allocators like ``ncclMemAlloc`` can use more memory than requested, due to alignment
     requirements (``CU_MULTICAST_GRANULARITY_RECOMMENDED``, ``CU_MULTICAST_GRANULARITY_MINIMUM``),
     and can cause your workload to run out of memory.

.. _NCCL has specific requirements:
    https://docs.nvidia.com/deeplearning/nccl/user-guide/docs/usage/bufferreg.html#memory-allocator


Tuning NVLink Performance with Custom Memory Allocator on H100/H200 GPUs
------------------------------------------------------------------------
In rare cases, performance of NVLink on H100/H200 GPUs can be influenced by the physical memory
layout of data, creating an opportunity for developers to tune their applications for optimal
throughput.

An example of how physical memory layout of data affects performance is when communication
kernels issue unbalanced NVLink read/write operations. In the following figure, we can see
that each warp accesses memory addresses with a consistent strided pattern in each single wave.
We can have a more balanced load by tuning the stride size in the workload or we can implement
a custom CUDA allocator.

.. code::

  _______________________________  _______________________________      _______________________________
  | Warp 0 Reading | No-reading |  | Warp 1 Reading | No-reading |  ...  Warp N Reading | No-reading |
  _______________________________  _______________________________      _______________________________
  <----------------------------->
          Stride size

Such an allocator can maintain contiguous virtual memory addresses for the kernel while strategically
arranging the mapping to physical memory addresses (e.g., through shuffling). This technique allows
developers to explore different physical access patterns to find the most efficient one, unlocking
higher performance without modifying the kernel's logic. A practical implementation of such an allocator
can be achieved using PyTorch’s custom allocator support as mentioned before, where the malloc and free
functions are:

.. code:: C++

  // assuming a system with 8 GPUs
  struct CustomAllocInfo {
    void** devPtr;  // This will be the usable virtual memory address
    CUdeviceptr dptr;
    size_t totalSize;  // Total size of the allocated memory
    size_t padded_size;
    int device_id;
    std::vector<CUmemGenericAllocationHandle> handles;  // Handles to physical memory allocations
  };

  // loop over pages
  cudaError_t customCudaMalloc(CustomAllocInfo* info) {
      if (!info) return cudaErrorInvalidValue;

      CUdeviceptr dptr;

      // Handles to redundant physical memory allocations which help truncate stride pattern in physical memory
      std::vector<CUmemGenericAllocationHandle> handles_redundant;

      size_t granularity = 0;
      CUmemAllocationProp prop = {};

      int currentDev = info->device_id;
      size_t totalSize = info->totalSize;

      prop.type = CU_MEM_ALLOCATION_TYPE_PINNED;
      prop.location.type = CU_MEM_LOCATION_TYPE_DEVICE;
      prop.location.id = currentDev;
      cuMemGetAllocationGranularity(&granularity, &prop, CU_MEM_ALLOC_GRANULARITY_MINIMUM);
      size_t padded_size = ROUND_UP(totalSize, granularity);

      info->padded_size = padded_size;

      // loop over pages
      size_t iter_granularity = granularity * 64; // 64 * granularity with shift_size = 2 works
      uint32_t iteration_count = (totalSize + iter_granularity - 1) / iter_granularity;

      cuMemAddressReserve(&dptr, padded_size, 0ULL, 0ULL, 0ULL);

      const int shift_size = 2;
      for (size_t i = 0; i < iteration_count; i+=shift_size) {

          CUmemGenericAllocationHandle allocHandle[shift_size];
          for (int shift = 0; (shift < shift_size)&&(i+shift < iteration_count); shift++){
              CHECK_CUDA(cuMemCreate(&allocHandle[shift], iter_granularity, &prop, 0));
              info->handles.push_back(allocHandle[shift]);
          }

          for (int shift = 0; (shift < shift_size)&&(i+shift < iteration_count); shift++){

              // mapping makes the shift (shift -> (shift+1)%shift_size  )
              CHECK_CUDA(cuMemMap(dptr + (i+shift) * iter_granularity, iter_granularity, 0, allocHandle[(shift+1)%shift_size], 0));

              setupMultiGPUAccess(dptr + (i+shift) * iter_granularity, iter_granularity, {0, 1, 2, 3, 4, 5, 6, 7}); // Enable access for all 8 GPUs
          }

          // std::cout << "Here we allocate one redundant page (2MB)..." << std::endl;
          // this is an extra optimization on top of the swizzling. It helps "break"
          // the physical access pattern even more. It can be left out if workload is already
          // performing at SOL with just swizzling.
          CUmemGenericAllocationHandle allocHandle_redundant;
          CHECK_CUDA(cuMemCreate(&allocHandle_redundant, granularity, &prop, 0));
          handles_redundant.push_back(allocHandle_redundant);
      }

      *info->devPtr = (void*)dptr;
      info->dptr = dptr;

      // Release each redundant allocation
      for (auto handle : handles_redundant) {
          // std::cout << "Here we release one redundant page (2MB)..." << std::endl;
          CHECK_CUDA(cuMemRelease(handle));
      }

      return cudaSuccess;
  }

  void customCudaFree(CustomAllocInfo* info) {
      if (!info) return;

      // CHECK_CUDA(cudaSetDevice(info->device_id));

      CHECK_CUDA(cuMemUnmap(info->dptr, info->padded_size));

      // Unmap and release each allocation
      for (auto handle : info->handles) {
          CHECK_CUDA(cuMemRelease(handle));
      }

      // Unreserve the virtual address space
      // CHECK_CUDA(cuMemAddressFree((CUdeviceptr)*info->devPtr, info->padded_size));
      CHECK_CUDA(cuMemAddressFree(info->dptr, info->padded_size));
  }


cuBLAS workspaces
-----------------

For each combination of cuBLAS handle and CUDA stream, a cuBLAS workspace will be allocated
if that handle and stream combination executes a cuBLAS kernel that requires a workspace.
In order to avoid repeatedly allocating workspaces, these workspaces are not deallocated unless
``torch._C._cuda_clearCublasWorkspaces()`` is called. The workspace size per allocation can be
specified via the environment variable ``CUBLAS_WORKSPACE_CONFIG`` with the format ``:[SIZE]:[COUNT]``.
As an example, the default workspace size per allocation is ``CUBLAS_WORKSPACE_CONFIG=:4096:2:16:8``
which specifies a total size of ``2 * 4096 + 8 * 16 KiB``. To force cuBLAS to avoid using workspaces,
set ``CUBLAS_WORKSPACE_CONFIG=:0:0``.

.. _cufft-plan-cache:

cuFFT plan cache
----------------

For each CUDA device, an LRU cache of cuFFT plans is used to speed up repeatedly
running FFT methods (e.g., :func:`torch.fft.fft`) on CUDA tensors of same geometry
with same configuration. Because some cuFFT plans may allocate GPU memory,
these caches have a maximum capacity.

You may control and query the properties of the cache of current device with
the following APIs:

* ``torch.backends.cuda.cufft_plan_cache.max_size`` gives the capacity of the
  cache (default is 4096 on CUDA 10 and newer, and 1023 on older CUDA versions).
  Setting this value directly modifies the capacity.

* ``torch.backends.cuda.cufft_plan_cache.size`` gives the number of plans
  currently residing in the cache.

* ``torch.backends.cuda.cufft_plan_cache.clear()`` clears the cache.

To control and query plan caches of a non-default device, you can index the
``torch.backends.cuda.cufft_plan_cache`` object with either a :class:`torch.device`
object or a device index, and access one of the above attributes. E.g., to set
the capacity of the cache for device ``1``, one can write
``torch.backends.cuda.cufft_plan_cache[1].max_size = 10``.

.. _cuda-just-in-time-compilation:

Just-in-Time Compilation
------------------------

PyTorch just-in-time compiles some operations, like torch.special.zeta, when
performed on CUDA tensors. This compilation can be time consuming
(up to a few seconds depending on your hardware and software)
and may occur multiple times for a single operator since many PyTorch operators actually
select from a variety of kernels, each of which must be compiled once, depending on their input.
This compilation occurs once per process, or just once if a kernel cache is used.

By default, PyTorch creates a kernel cache in $XDG_CACHE_HOME/torch/kernels if
XDG_CACHE_HOME is defined and $HOME/.cache/torch/kernels if it's not (except on Windows,
where the kernel cache is not yet supported). The caching behavior can be directly
controlled with two environment variables. If USE_PYTORCH_KERNEL_CACHE is set to 0 then no
cache will be used, and if PYTORCH_KERNEL_CACHE_PATH is set then that path will be used
as a kernel cache instead of the default location.

Best practices
--------------

Device-agnostic code
^^^^^^^^^^^^^^^^^^^^

Due to the structure of PyTorch, you may need to explicitly write
device-agnostic (CPU or GPU) code; an example may be creating a new tensor as
the initial hidden state of a recurrent neural network.

The first step is to determine whether the GPU should be used or not. A common
pattern is to use Python's ``argparse`` module to read in user arguments, and
have a flag that can be used to disable CUDA, in combination with
:meth:`~torch.cuda.is_available`. In the following, ``args.device`` results in a
:class:`torch.device` object that can be used to move tensors to CPU or CUDA.

::

    import argparse
    import torch

    parser = argparse.ArgumentParser(description='PyTorch Example')
    parser.add_argument('--disable-cuda', action='store_true',
                        help='Disable CUDA')
    args = parser.parse_args()
    args.device = None
    if not args.disable_cuda and torch.cuda.is_available():
        args.device = torch.device('cuda')
    else:
        args.device = torch.device('cpu')

.. note::

    When assessing the availability of CUDA in a given environment (:meth:`~torch.cuda.is_available`), PyTorch's default
    behavior is to call the CUDA Runtime API method `cudaGetDeviceCount`_. Because this call in turn initializes the
    CUDA Driver API (via `cuInit`_) if it is not already initialized, subsequent forks of a process that has run
    :meth:`~torch.cuda.is_available` will fail with a CUDA initialization error.

    One can set ``PYTORCH_NVML_BASED_CUDA_CHECK=1`` in your environment before importing PyTorch modules that execute
    :meth:`~torch.cuda.is_available` (or before executing it directly) in order to direct
    :meth:`~torch.cuda.is_available` to attempt an NVML-based assessment (`nvmlDeviceGetCount_v2`_). If the
    NVML-based assessment is successful (i.e. NVML discovery/initialization does not fail),
    :meth:`~torch.cuda.is_available` calls will not poison subsequent forks.

    If NVML discovery/initialization fails, :meth:`~torch.cuda.is_available` will fallback to the standard CUDA Runtime
    API assessment and the aforementioned fork constraint will apply.

    Note that the above NVML-based CUDA availability assessment provides a weaker guarantee than the default CUDA
    Runtime API approach (which requires CUDA initialization to succeed). In some circumstances, the NVML-based check
    may succeed while later CUDA initialization fails.

Now that we have ``args.device``, we can use it to create a Tensor on the
desired device.

::

    x = torch.empty((8, 42), device=args.device)
    net = Network().to(device=args.device)

This can be used in a number of cases to produce device agnostic code. Below
is an example when using a dataloader:

::

    cuda0 = torch.device('cuda:0')  # CUDA GPU 0
    for i, x in enumerate(train_loader):
        x = x.to(cuda0)

When working with multiple GPUs on a system, you can use the
``CUDA_VISIBLE_DEVICES`` environment flag to manage which GPUs are available to
PyTorch. As mentioned above, to manually control which GPU a tensor is created
on, the best practice is to use a :any:`torch.cuda.device` context manager.

::

    print("Outside device is 0")  # On device 0 (default in most scenarios)
    with torch.cuda.device(1):
        print("Inside device is 1")  # On device 1
    print("Outside device is still 0")  # On device 0

If you have a tensor and would like to create a new tensor of the same type on
the same device, then you can use a ``torch.Tensor.new_*`` method
(see :class:`torch.Tensor`).
Whilst the previously mentioned ``torch.*`` factory functions
(:ref:`tensor-creation-ops`) depend on the current GPU context and
the attributes arguments you pass in, ``torch.Tensor.new_*`` methods preserve
the device and other attributes of the tensor.

This is the recommended practice when creating modules in which new
tensors need to be created internally during the forward pass.

::

    cuda = torch.device('cuda')
    x_cpu = torch.empty(2)
    x_gpu = torch.empty(2, device=cuda)
    x_cpu_long = torch.empty(2, dtype=torch.int64)

    y_cpu = x_cpu.new_full([3, 2], fill_value=0.3)
    print(y_cpu)

        tensor([[ 0.3000,  0.3000],
                [ 0.3000,  0.3000],
                [ 0.3000,  0.3000]])

    y_gpu = x_gpu.new_full([3, 2], fill_value=-5)
    print(y_gpu)

        tensor([[-5.0000, -5.0000],
                [-5.0000, -5.0000],
                [-5.0000, -5.0000]], device='cuda:0')

    y_cpu_long = x_cpu_long.new_tensor([[1, 2, 3]])
    print(y_cpu_long)

        tensor([[ 1,  2,  3]])


If you want to create a tensor of the same type and size of another tensor, and
fill it with either ones or zeros, :meth:`~torch.ones_like` or
:meth:`~torch.zeros_like` are provided as convenient helper functions (which
also preserve :class:`torch.device` and :class:`torch.dtype` of a Tensor).

::

    x_cpu = torch.empty(2, 3)
    x_gpu = torch.empty(2, 3)

    y_cpu = torch.ones_like(x_cpu)
    y_gpu = torch.zeros_like(x_gpu)


.. _cuda-memory-pinning:

Use pinned memory buffers
^^^^^^^^^^^^^^^^^^^^^^^^^

.. warning::

    This is an advanced tip. If you overuse pinned memory, it can cause serious
    problems when running low on RAM, and you should be aware that pinning is
    often an expensive operation.

Host to GPU copies are much faster when they originate from pinned (page-locked)
memory. CPU tensors and storages expose a :meth:`~torch.Tensor.pin_memory`
method, that returns a copy of the object, with data put in a pinned region.

Also, once you pin a tensor or storage, you can use asynchronous GPU copies.
Just pass an additional ``non_blocking=True`` argument to a
:meth:`~torch.Tensor.to` or a :meth:`~torch.Tensor.cuda` call. This can be used
to overlap data transfers with computation.

You can make the :class:`~torch.utils.data.DataLoader` return batches placed in
pinned memory by passing ``pin_memory=True`` to its constructor.

.. _cuda-nn-ddp-instead:

Use nn.parallel.DistributedDataParallel instead of multiprocessing or nn.DataParallel
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Most use cases involving batched inputs and multiple GPUs should default to
using :class:`~torch.nn.parallel.DistributedDataParallel` to utilize more
than one GPU.

There are significant caveats to using CUDA models with
:mod:`~torch.multiprocessing`; unless care is taken to meet the data handling
requirements exactly, it is likely that your program will have incorrect or
undefined behavior.

It is recommended to use :class:`~torch.nn.parallel.DistributedDataParallel`,
instead of :class:`~torch.nn.DataParallel` to do multi-GPU training, even if
there is only a single node.

The difference between :class:`~torch.nn.parallel.DistributedDataParallel` and
:class:`~torch.nn.DataParallel` is: :class:`~torch.nn.parallel.DistributedDataParallel`
uses multiprocessing where a process is created for each GPU, while
:class:`~torch.nn.DataParallel` uses multithreading. By using multiprocessing,
each GPU has its dedicated process, this avoids the performance overhead caused
by GIL of Python interpreter.

If you use :class:`~torch.nn.parallel.DistributedDataParallel`, you could use
`torch.distributed.launch` utility to launch your program, see :ref:`distributed-launch`.

.. _cudaGetDeviceCount:
    https://docs.nvidia.com/cuda/cuda-runtime-api/group__CUDART__DEVICE.html#group__CUDART__DEVICE_1g18808e54893cfcaafefeab31a73cc55f

.. _cuInit:
    https://docs.nvidia.com/cuda/cuda-driver-api/group__CUDA__INITIALIZE.html#group__CUDA__INITIALIZE_1g0a2f1517e1bd8502c7194c3a8c134bc3

.. _nvmlDeviceGetCount_v2:
    https://docs.nvidia.com/deploy/nvml-api/group__nvmlDeviceQueries.html#group__nvmlDeviceQueries_1ga93623b195bff04bbe3490ca33c8a42d

.. _cuda-graph-semantics:

CUDA Graphs
-----------

A CUDA graph is a record of the work (mostly kernels and their arguments) that a
CUDA stream and its dependent streams perform.
For general principles and details on the underlying CUDA API, see
`Getting Started with CUDA Graphs`_ and the
`Graphs section`_ of the CUDA C Programming Guide.

PyTorch supports the construction of CUDA graphs using `stream capture`_, which puts a
CUDA stream in *capture mode*. CUDA work issued to a capturing stream doesn't actually
run on the GPU. Instead, the work is recorded in a graph.

After capture, the graph can be *launched* to run the GPU work as many times as needed.
Each replay runs the same kernels with the same arguments. For pointer arguments this
means the same memory addresses are used.
By filling input memory with new data (e.g., from a new batch) before each replay,
you can rerun the same work on new data.

Why CUDA Graphs?
^^^^^^^^^^^^^^^^

Replaying a graph sacrifices the dynamic flexibility of typical eager execution in exchange for
**greatly reduced CPU overhead**. A graph's arguments and kernels are fixed, so a graph replay
skips all layers of argument setup and kernel dispatch, including Python, C++, and CUDA driver
overheads. Under the hood, a replay submits the entire graph's work to the GPU with
a single call to `cudaGraphLaunch`_.  Kernels in a replay also execute slightly faster
on the GPU, but eliding CPU overhead is the main benefit.

You should try CUDA graphs if all or part of your network is graph-safe (usually this means
static shapes and static control flow, but see the other :ref:`constraints<capture-constraints>`)
and you suspect its runtime is at least somewhat CPU-limited.

.. _Getting Started with CUDA Graphs:
    https://developer.nvidia.com/blog/cuda-graphs/
.. _Graphs section:
    https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#cuda-graphs
.. _stream capture:
    https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#creating-a-graph-using-stream-capture
.. _cudaGraphLaunch:
    https://docs.nvidia.com/cuda/cuda-runtime-api/group__CUDART__GRAPH.html#group__CUDART__GRAPH_1g1accfe1da0c605a577c22d9751a09597

PyTorch API
^^^^^^^^^^^

.. warning::
    This API is in beta and may change in future releases.

PyTorch exposes graphs via a raw :class:`torch.cuda.CUDAGraph` class
and two convenience wrappers,
:class:`torch.cuda.graph` and
:class:`torch.cuda.make_graphed_callables`.

:class:`torch.cuda.graph` is a simple, versatile context manager that
captures CUDA work in its context.
Before capture, warm up the workload to be captured by running
a few eager iterations. Warmup must occur on a side stream.
Because the graph reads from and writes to the same memory addresses in every
replay, you must maintain long-lived references to tensors that hold
input and output data during capture.
To run the graph on new input data, copy new data to the capture's input tensor(s),
replay the graph, then read the new output from the capture's output tensor(s).
Example::

    g = torch.cuda.CUDAGraph()

    # Placeholder input used for capture
    static_input = torch.empty((5,), device="cuda")

    # Warmup before capture
    s = torch.cuda.Stream()
    s.wait_stream(torch.cuda.current_stream())
    with torch.cuda.stream(s):
        for _ in range(3):
            static_output = static_input * 2
    torch.cuda.current_stream().wait_stream(s)

    # Captures the graph
    # To allow capture, automatically sets a side stream as the current stream in the context
    with torch.cuda.graph(g):
        static_output = static_input * 2

    # Fills the graph's input memory with new data to compute on
    static_input.copy_(torch.full((5,), 3, device="cuda"))
    g.replay()
    # static_output holds the results
    print(static_output)  # full of 3 * 2 = 6

    # Fills the graph's input memory with more data to compute on
    static_input.copy_(torch.full((5,), 4, device="cuda"))
    g.replay()
    print(static_output)  # full of 4 * 2 = 8

See
:ref:`Whole-network capture<whole-network-capture>`,
:ref:`Usage with torch.cuda.amp<graphs-with-amp>`, and
:ref:`Usage with multiple streams<multistream-capture>`
for realistic and advanced patterns.

:class:`~torch.cuda.make_graphed_callables` is more sophisticated.
:class:`~torch.cuda.make_graphed_callables` accepts Python functions and
:class:`torch.nn.Module`\s. For each passed function or Module,
it creates separate graphs of the forward-pass and backward-pass work. See
:ref:`Partial-network capture<partial-network-capture>`.

.. _capture-constraints:

Constraints
~~~~~~~~~~~

A set of ops is *capturable* if it doesn't violate any of the following constraints.

Constraints apply to all work in a
:class:`torch.cuda.graph` context and all work in the forward and backward passes
of any callable you pass to :func:`torch.cuda.make_graphed_callables`.

Violating any of these will likely cause a runtime error:

* Capture must occur on a non-default stream. (This is only a concern if you use the raw
  :meth:`CUDAGraph.capture_begin<torch.cuda.CUDAGraph.capture_begin>` and
  :meth:`CUDAGraph.capture_end<torch.cuda.CUDAGraph.capture_end>` calls.
  :class:`~torch.cuda.graph` and
  :func:`~torch.cuda.make_graphed_callables` set a side stream for you.)
* Ops that synchronize the CPU with the GPU (e.g., ``.item()`` calls) are prohibited.
* CUDA RNG operations are permitted, and when using multiple :class:`torch.Generator` instances within a graph,
  they must be registered using :meth:`CUDAGraph.register_generator_state<torch.cuda.CUDAGraph.register_generator_state>` before graph capture.
  Avoid using :meth:`Generator.get_state<torch.get_state>` and :meth:`Generator.set_state<torch.set_state>` during capture;
  instead, utilize :meth:`Generator.graphsafe_set_state<torch.Generator.graphsafe_set_state>` and :meth:`Generator.graphsafe_get_state<torch.Generator.graphsafe_get_state>`
  for managing generator states safely within the graph context. This ensures proper RNG operation and generator management within CUDA graphs.


Violating any of these will likely cause silent numerical errors or undefined behavior:

* Within a process, only one capture may be underway at a time.
* No non-captured CUDA work may run in this process (on any thread) while capture is underway.
* CPU work is not captured. If the captured ops include CPU work, that work will be elided during replay.
* Every replay reads from and writes to the same (virtual) memory addresses.
* Dynamic control flow (based on CPU or GPU data) is prohibited.
* Dynamic shapes are prohibited. The graph assumes every tensor in the captured op sequence
  has the same size and layout in every replay.
* Using multiple streams in a capture is allowed, but there are :ref:`restrictions<multistream-capture>`.

Non-constraints
~~~~~~~~~~~~~~~

* Once captured, the graph may be replayed on any stream.

.. _whole-network-capture:

Whole-network capture
^^^^^^^^^^^^^^^^^^^^^^

If your entire network is capturable, you can capture and replay an entire iteration::

    N, D_in, H, D_out = 640, 4096, 2048, 1024
    model = torch.nn.Sequential(torch.nn.Linear(D_in, H),
                                torch.nn.Dropout(p=0.2),
                                torch.nn.Linear(H, D_out),
                                torch.nn.Dropout(p=0.1)).cuda()
    loss_fn = torch.nn.MSELoss()
    optimizer = torch.optim.SGD(model.parameters(), lr=0.1)

    # Placeholders used for capture
    static_input = torch.randn(N, D_in, device='cuda')
    static_target = torch.randn(N, D_out, device='cuda')

    # warmup
    # Uses static_input and static_target here for convenience,
    # but in a real setting, because the warmup includes optimizer.step()
    # you must use a few batches of real data.
    s = torch.cuda.Stream()
    s.wait_stream(torch.cuda.current_stream())
    with torch.cuda.stream(s):
        for i in range(3):
            optimizer.zero_grad(set_to_none=True)
            y_pred = model(static_input)
            loss = loss_fn(y_pred, static_target)
            loss.backward()
            optimizer.step()
    torch.cuda.current_stream().wait_stream(s)

    # capture
    g = torch.cuda.CUDAGraph()
    # Sets grads to None before capture, so backward() will create
    # .grad attributes with allocations from the graph's private pool
    optimizer.zero_grad(set_to_none=True)
    with torch.cuda.graph(g):
        static_y_pred = model(static_input)
        static_loss = loss_fn(static_y_pred, static_target)
        static_loss.backward()
        optimizer.step()

    real_inputs = [torch.rand_like(static_input) for _ in range(10)]
    real_targets = [torch.rand_like(static_target) for _ in range(10)]

    for data, target in zip(real_inputs, real_targets):
        # Fills the graph's input memory with new data to compute on
        static_input.copy_(data)
        static_target.copy_(target)
        # replay() includes forward, backward, and step.
        # You don't even need to call optimizer.zero_grad() between iterations
        # because the captured backward refills static .grad tensors in place.
        g.replay()
        # Params have been updated. static_y_pred, static_loss, and .grad
        # attributes hold values from computing on this iteration's data.

.. _partial-network-capture:

Partial-network capture
^^^^^^^^^^^^^^^^^^^^^^^^^

If some of your network is unsafe to capture (e.g., due to dynamic control flow,
dynamic shapes, CPU syncs, or essential CPU-side logic), you can run the unsafe
part(s) eagerly and use :func:`torch.cuda.make_graphed_callables` to graph only
the capture-safe part(s).

By default, callables returned by :func:`~torch.cuda.make_graphed_callables`
are autograd-aware, and can be used in the training loop as direct replacements
for the functions or :class:`nn.Module<torch.nn.Module>`\ s you passed.

:func:`~torch.cuda.make_graphed_callables` internally creates
:class:`~torch.cuda.CUDAGraph` objects, runs warmup iterations, and maintains
static inputs and outputs as needed.  Therefore (unlike with
:class:`torch.cuda.graph`) you don't need to handle those manually.

In the following example, data-dependent dynamic control flow means the
network isn't capturable end-to-end, but
:func:`~torch.cuda.make_graphed_callables`
lets us capture and run graph-safe sections as graphs regardless::

    N, D_in, H, D_out = 640, 4096, 2048, 1024

    module1 = torch.nn.Linear(D_in, H).cuda()
    module2 = torch.nn.Linear(H, D_out).cuda()
    module3 = torch.nn.Linear(H, D_out).cuda()

    loss_fn = torch.nn.MSELoss()
    optimizer = torch.optim.SGD(chain(module1.parameters(),
                                      module2.parameters(),
                                      module3.parameters()),
                                lr=0.1)

    # Sample inputs used for capture
    # requires_grad state of sample inputs must match
    # requires_grad state of real inputs each callable will see.
    x = torch.randn(N, D_in, device='cuda')
    h = torch.randn(N, H, device='cuda', requires_grad=True)

    module1 = torch.cuda.make_graphed_callables(module1, (x,))
    module2 = torch.cuda.make_graphed_callables(module2, (h,))
    module3 = torch.cuda.make_graphed_callables(module3, (h,))

    real_inputs = [torch.rand_like(x) for _ in range(10)]
    real_targets = [torch.randn(N, D_out, device="cuda") for _ in range(10)]

    for data, target in zip(real_inputs, real_targets):
        optimizer.zero_grad(set_to_none=True)

        tmp = module1(data)  # forward ops run as a graph

        if tmp.sum().item() > 0:
            tmp = module2(tmp)  # forward ops run as a graph
        else:
            tmp = module3(tmp)  # forward ops run as a graph

        loss = loss_fn(tmp, target)
        # module2's or module3's (whichever was chosen) backward ops,
        # as well as module1's backward ops, run as graphs
        loss.backward()
        optimizer.step()

.. _graphs-with-amp:

Usage with torch.cuda.amp
^^^^^^^^^^^^^^^^^^^^^^^^^

For typical optimizers, :meth:`GradScaler.step<torch.cuda.amp.GradScaler.step>` syncs
the CPU with the GPU, which is prohibited during capture. To avoid errors, either use
:ref:`partial-network capture<partial-network-capture>`, or (if forward, loss,
and backward are capture-safe) capture forward, loss, and backward but not the
optimizer step::

    # warmup
    # In a real setting, use a few batches of real data.
    s = torch.cuda.Stream()
    s.wait_stream(torch.cuda.current_stream())
    with torch.cuda.stream(s):
        for i in range(3):
            optimizer.zero_grad(set_to_none=True)
            with torch.cuda.amp.autocast():
                y_pred = model(static_input)
                loss = loss_fn(y_pred, static_target)
            scaler.scale(loss).backward()
            scaler.step(optimizer)
            scaler.update()
    torch.cuda.current_stream().wait_stream(s)

    # capture
    g = torch.cuda.CUDAGraph()
    optimizer.zero_grad(set_to_none=True)
    with torch.cuda.graph(g):
        with torch.cuda.amp.autocast():
            static_y_pred = model(static_input)
            static_loss = loss_fn(static_y_pred, static_target)
        scaler.scale(static_loss).backward()
        # don't capture scaler.step(optimizer) or scaler.update()

    real_inputs = [torch.rand_like(static_input) for _ in range(10)]
    real_targets = [torch.rand_like(static_target) for _ in range(10)]

    for data, target in zip(real_inputs, real_targets):
        static_input.copy_(data)
        static_target.copy_(target)
        g.replay()
        # Runs scaler.step and scaler.update eagerly
        scaler.step(optimizer)
        scaler.update()

.. _multistream-capture:

Usage with multiple streams
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Capture mode automatically propagates to any streams that sync with a capturing stream.
Within capture, you may expose parallelism by issuing calls to different streams,
but the overall stream dependency DAG must branch out from the
initial capturing stream after capture begins and rejoin the initial stream
before capture ends::

    with torch.cuda.graph(g):
        # at context manager entrance, torch.cuda.current_stream()
        # is the initial capturing stream

        # INCORRECT (does not branch out from or rejoin initial stream)
        with torch.cuda.stream(s):
            cuda_work()

        # CORRECT:
        # branches out from initial stream
        s.wait_stream(torch.cuda.current_stream())
        with torch.cuda.stream(s):
            cuda_work()
        # rejoins initial stream before capture ends
        torch.cuda.current_stream().wait_stream(s)

.. note::

    To avoid confusion for power users looking at replays in nsight systems or nvprof:
    Unlike eager execution, the graph interprets a nontrivial stream DAG in capture
    as a hint, not a command. During replay, the graph may reorganize independent ops
    onto different streams or enqueue them in a different order (while respecting your
    original DAG's overall dependencies).

Usage with DistributedDataParallel
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

NCCL < 2.9.6
~~~~~~~~~~~~

NCCL versions earlier than 2.9.6 don't allow collectives to be captured.
You must use :ref:`partial-network capture<partial-network-capture>`,
which defers allreduces to happen outside graphed sections of backward.

Call :func:`~torch.cuda.make_graphed_callables` on graphable network sections
*before* wrapping the network with DDP.

NCCL >= 2.9.6
~~~~~~~~~~~~~

NCCL versions 2.9.6 or later allow collectives in the graph.
Approaches that capture an :ref:`entire backward pass<whole-network-capture>`
are a viable option, but need three setup steps.

1. Disable DDP's internal async error handling::

    os.environ["NCCL_ASYNC_ERROR_HANDLING"] = "0"
    torch.distributed.init_process_group(...)

2. Before full-backward capture, DDP must be constructed in a side-stream context::

    with torch.cuda.stream(s):
        model = DistributedDataParallel(model)

3. Your warmup must run at least 11 DDP-enabled eager iterations before capture.

.. _graph-memory-management:

Graph memory management
^^^^^^^^^^^^^^^^^^^^^^^

A captured graph acts on the same virtual addresses every time it replays.
If PyTorch frees the memory, a later replay can hit an illegal memory access.
If PyTorch reassigns the memory to new tensors, the replay can corrupt the values
seen by those tensors.  Therefore, the virtual addresses used by the graph must be
reserved for the graph across replays. The PyTorch caching allocator achieves this
by detecting when capture is underway and satisfying the capture's allocations
from a graph-private memory pool. The private pool stays alive until its
:class:`~torch.cuda.CUDAGraph` object and all tensors created during capture
go out of scope.

Private pools are maintained automatically. By default, the allocator creates a
separate private pool for each capture. If you capture multiple graphs,
this conservative approach ensures graph replays never corrupt each other's values,
but sometimes needlessly wastes memory.

Sharing memory across captures
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To economize the memory stashed in private pools, :class:`torch.cuda.graph`
and :func:`torch.cuda.make_graphed_callables` optionally allow different
captures to share the same private pool.
It's safe for a set of graphs to share a private pool if you know they'll always
be replayed in the same order they were captured,
and never be replayed concurrently.

:class:`torch.cuda.graph`'s ``pool`` argument is a hint to use a particular private pool,
and can be used to share memory across graphs as shown::

    g1 = torch.cuda.CUDAGraph()
    g2 = torch.cuda.CUDAGraph()

    # (create static inputs for g1 and g2, run warmups of their workloads...)

    # Captures g1
    with torch.cuda.graph(g1):
        static_out_1 = g1_workload(static_in_1)

    # Captures g2, hinting that g2 may share a memory pool with g1
    with torch.cuda.graph(g2, pool=g1.pool()):
        static_out_2 = g2_workload(static_in_2)

    static_in_1.copy_(real_data_1)
    static_in_2.copy_(real_data_2)
    g1.replay()
    g2.replay()

It's also safe to share a memory pool across separate graphs that do not depend
on each other's outputs, provided they never run concurrently.
Be aware that replaying one graph can clobber another graph's outputs when
they share a pool, unless :meth:`~torch.Tensor.clone` is called on the outputs
beforehand.
This pattern is frequently used in inference servers that accept variable batch
sizes at runtime.
vLLM is a notable example; see `here <https://github.com/vllm-project/vllm/blob/938a81692ea318e59ead4750e7e7425bfd6a4896/vllm/platforms/interface.py#L508-L515>`__
and `here <https://github.com/vllm-project/vllm/blob/938a81692ea318e59ead4750e7e7425bfd6a4896/vllm/compilation/cuda_graph.py#L86-L89>`__.

With :func:`torch.cuda.make_graphed_callables`, if you want to graph several
callables and you know they'll always run in the same order (and never concurrently)
pass them as a tuple in the same order they'll run in the live workload, and
:func:`~torch.cuda.make_graphed_callables` will capture their graphs using a shared
private pool.

If, in the live workload, your callables will run in an order that occasionally changes,
or if they'll run concurrently, passing them as a tuple to a single invocation of
:func:`~torch.cuda.make_graphed_callables` is not allowed. Instead, you must call
:func:`~torch.cuda.make_graphed_callables` separately for each one.
