.. _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 in 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)

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.

.. _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.

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.

.. _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`) 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``.

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')

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. You overuse of pinned memory can cause serious
    problems if you'll be 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`.