Data interchange mechanisms ¶
This section discusses the mechanism to convert one type of array into another. As discussed in the assumptions-dependencies section, functions provided by an array library are not expected to operate on array types implemented by another library. Instead, the array can be converted to a “native” array type.
The interchange mechanism must offer the following:
Data access via a protocol that describes the memory layout of the array in an implementation-independent manner. Rationale: any number of libraries must be able to exchange data, and no particular package must be needed to do so.
Support for all dtypes in this API standard (see Data Types ).
Device support. It must be possible to determine on what device the array that is to be converted lives. Rationale: there are CPU-only, GPU-only, and multi-device array types; it’s best to support these with a single protocol (with separate per-device protocols it’s hard to figure out unambiguous rules for which protocol gets used, and the situation will get more complex over time as TPU’s and other accelerators become more widely available).
Zero-copy semantics where possible, making a copy only if needed (e.g. when data is not contiguous in memory). Rationale: performance.
A Python-side and a C-side interface, the latter with a stable C ABI. Rationale: all prominent existing array libraries are implemented in C/C++, and are released independently from each other. Hence a stable C ABI is required for packages to work well together.
The best candidate for this protocol is DLPack. See the RFC to adopt DLPack for details.
The main alternatives to DLPack are device-specific methods:
The buffer protocol on CPU
__cuda_array_interface__for CUDA, specified in the Numba documentation here (Python-side only at the moment)
An issue with device-specific protocols are: if two libraries both support multiple device types, in which order should the protocols be tried? A growth in the number of protocols to support each time a new device gets supported by array libraries (e.g. TPUs, AMD GPUs, emerging hardware accelerators) also seems undesirable.
In addition to the above argument, it is also clear from adoption
patterns that DLPack has the widest support. The buffer protocol, despite
being a lot older and standardized as part of Python itself via PEP 3118,
hardly has any support from array libraries. CPU interoperability is
mostly dealt with via the NumPy-specific
(which, when called,
means the object it is attached to must return a
containing the data the object holds).
Syntax for data interchange with DLPack ¶
The array API will offer the following syntax for data interchange:
from_dlpack(x)function, which accepts (array) objects with a
__dlpack__method and uses that method to construct a new array containing the data from
__dlpack_device__methods on the array object, which will be called from within
from_dlpack, to query what device the array is on (may be needed to pass in the correct stream, e.g. in the case of multiple GPUs) and to access the data.
DLPack describe the memory layout of strided, n-dimensional arrays.
When a user calls
, the library implementing
“producer”) will provide access to the data from
to the library
(the “consumer”). If possible, this must be
will be a
). If not possible, that library
may make a copy of the data. In both cases:
the producer keeps owning the memory
ymay or may not be a view, therefore the user must keep the recommendation to avoid mutating
yin mind - see Copy-view behaviour and mutability .
ymay continue to be used just like arrays created in other ways.
If an array that is accessed via the interchange protocol lives on a
device that the requesting library does not support, it is recommended to
Stream handling through the
keyword applies to CUDA and ROCm (perhaps
to other devices that have a stream concept as well, however those haven’t been
considered in detail). The consumer must pass the stream it will use to the
producer; the producer must synchronize or wait on the stream when necessary.
In the common case of the default stream being used, synchronization will be
unnecessary so asynchronous execution is enabled.
Note that while this API standard largely tries to avoid discussing implementation details, some discussion and requirements are needed here because data interchange requires coordination between implementers on, e.g., memory management.
DLPack diagram. Dark blue are the structs it defines, light blue struct members, gray text enum values of supported devices and data types.
method will produce a
, which will be consumed immediately within
- therefore it is consumed exactly once, and it will not be
visible to users of the Python API.
The consumer must set the PyCapsule name to
, and call the
when it no longer needs the data.
field in the
, it indicates a
row-major compact array. If the array is of size zero, the data pointer in
should be set to either
DLPack version used must be
. For further
details on DLPack design and how to implement support for it,
DLPack contains a
, which will be the device ID (an integer,
) which the producer library uses. In practice this will likely be the same numbering as that of the consumer, however that is not guaranteed. Depending on the hardware type, it may be possible for the consumer library implementation to look up the actual device from the pointer to the data - this is possible for example for CUDA device pointers.
It is recommended that implementers of this array API consider and document
attribute of the array returned from
guaranteed to be in a certain order or not.