add an interoperatibility page

Author: leofang

cuda_core/docs/source/index.rst

@@ -10,6 +10,7 @@ and other functionalities.
 
    release.md
    install.md
+   interoperability.rst
    api.rst
 
 

cuda_core/docs/source/interoperability.rst

@@ -0,0 +1,83 @@
+.. currentmodule:: cuda.core.experimental
+
+Interoperability
+================
+
+``cuda.core`` is designed to be interoperable with other Python GPU libraries. Below
+we cover a list of possible such scenarios.
+
+
+Current device/context
+----------------------
+
+The :meth:`Device.set_current` method ensures that the calling host thread has
+an active CUDA context set to current. This CUDA context can be seen and accessed
+by other GPU libraries without any code change. For libraries built on top of
+the CUDA runtime (``cudart``), this is as if ``cudaSetDevice`` is called.
+
+Since CUDA contexts are per-thread constructs, in a multi-threaded program each
+host thread should call this method.
+
+Conversely, if any GPU library already set a device (or context) to current, this
+method ensures that the same device/context is picked up by and shared with
+``cuda.core``.
+
+
+``__cuda_stream__`` protocol
+----------------------------
+
+The :class:`~_stream.Stream` class is a vocabulary type representing CUDA streams
+in Python. While we encourage new Python projects to start using streams (and other
+CUDA types) from ``cuda.core``, we understand that there are already several projects
+exposing their own stream types.
+
+To address this issue, we propose the ``__cuda_stream__`` protocol (currently version
+0) as follows: For any Python objects that are meant to be interpreted as a stream, they
+should add a ``__cuda_stream__`` attribute that returns a 2-tuple: The version number
+(``0``) and the address of ``cudaStream_t``:
+
+.. code-block:: python
+
+   class MyStream:
+
+       @property
+       def __cuda_stream__(self):
+           return (0, self.ptr)
+
+       ...
+
+Then such objects can be understood by ``cuda.core`` anywhere a stream-like object
+is needed.
+
+We suggest all existing Python projects that expose a stream class to also support this
+protocol wherever a function takes a stream.
+
+
+Memory view utilities for CPU/GPU buffers
+-----------------------------------------
+
+The Python community has defined protocols such as CUDA Array Interface (CAI) [1]_ and DLPack
+[2]_ (part of the Python array API standard [3]_) for facilitating zero-copy data exchange
+between two GPU projects. In particular, performance considerations prompted the protocol
+designs gearing toward *stream-ordered* operations so as to avoid unnecessary synchronizations.
+While the designs are robust, *implementing* such protocols can be tricky and often requires
+a few iterations to ensure correctness.
+
+``cuda.core`` offers a :func:`~utils.args_viewable_as_strided_memory` decorator for
+extracting the metadata (such as pointer address, shape, strides, and dtype) from any
+Python objects supporting either CAI or DLPack and returning a :class:`~utils.StridedMemoryView` object, see the
+`strided_memory_view.py <https://github.com/NVIDIA/cuda-python/blob/main/cuda_core/examples/strided_memory_view.py>`_
+example. Alternatively, a :class:`~utils.StridedMemoryView` object can be explicitly
+constructed without using the decorator. This provides a *concrete implementation* to both
+protocols that is **array-library-agnostic**, so that all Python projects can just rely on this
+without either re-implementing (the consumer-side of) the protocols or tying to any particular
+array libraries.
+
+The :attr:`~utils.StridedMemoryView.is_device_accessible` attribute can be used to check
+whether or not the underlying buffer can be accessed on GPU.
+
+.. rubric:: Footnotes
+
+.. [1] https://numba.readthedocs.io/en/stable/cuda/cuda_array_interface.html
+.. [2] https://dmlc.github.io/dlpack/latest/python_spec.html
+.. [3] https://data-apis.org/array-api/latest/design_topics/data_interchange.html