Sparse BLAS Matrix Handle Contract between User and Library

Intel® oneAPI Math Kernel Library (oneMKL) - Data Parallel C++ Developer Reference

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ID 772045

Date 12/16/2022

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Sparse BLAS Matrix Handle Contract between User and Library

The sparse matrix handle (sparse::matrix_handle_t) takes in data from the User in a call that looks like sparse::set_csr_data(q, handle, /*user data*/). Unlike most other oneMKL domains, the sparse::matrix_handle_t along with user data persists outside of individual calls to the oneMKL library. When the User subsequently makes a call to a oneMKL Sparse BLAS API with that handle, the Library uses that data stored in the handle internally for the various operations. Because both users and library have access to the data at the same time while the matrix handle exists, there must be some agreements about what can and cannot be done by each party. We therefore introduce an implicit contract between the User and the Library that describes each party’s roles and responsibilities with respect to the Sparse BLAS SYCL APIs and the use of the sparse::matrix_handle_t.

Description of `sparse::matrix_handle_t` object

First, we note that the sparse::matrix_handle_t represents a generic sparse matrix object with some internal matrix format or representation. Providing device USM pointers for that matrix data in the sparse::matrix_handle_t object binds that handle to that particular sycl::context and sycl::device as well as any other devices in that sycl::context that are compatible (can read from the particular device peer to peer). It is the Users’ responsibility to make sure they are using the matrix handle with appropriate queues and devices within the given context. Using shared and host USM pointers or sycl::buffers for the matrix data binds that sparse::matrix_handle_t object to the sycl::context and could reasonably be used on any device in the context.

Second, the sparse::matrix_handle_t can best be described as a “view of User’s matrix data arrays with an opaque state attached to it”. That is, it is a lightweight view to begin with, but through the use of sparse::optimize_* APIs and in some cases, the execute APIs (like sparse::matmat()), the hidden state could grow with different internal optimizations/structures that can persist through the lifetime of the handle, with the goal of enabling superior performance for the desired operations.

User and Library agreements with respect to `sparse::matrix_handle_t` object

Finally the following agreements describe User and Library roles with respect to the handle use:

User agreements:

The User owns any data that is provided to the sparse::matrix_handle_t and is responsible to create and dispose of them correctly. That is, the data is created by the User then passed to the matrix handle using some Library API like sparse::set_csr_data(). The data can only be disposed of after all uses of the matrix handle and the release of the matrix handle have finished.
The User agrees not to modify the data arrays directly while they are in a sparse::matrix_handle_t. Any changes should happen while outside of a matrix handle or when available, indirectly through use of a Sparse BLAS library API that states it will modify the matrix handle data in specified ways.

Library agreements:

The Library may create and store it’s own data in the sparse::matrix_handle_t during a Sparse BLAS API library call. Such data is owned by the Library, that is, the Library is responsible for its disposal. The User cannot access this data, and the library-owned data, associated with a sparse::matrix_handle_t, is cleaned up at the respective handle release.
The Library agrees to not modify the user-provided data arrays in the sparse::matrix_handle_t unless through a library API that specifically states it may change the data (like sparse::sort_matrix() or sparse::omatcopy()).

With understanding of these agreements, the User can safely use the oneMKL SYCL Sparse BLAS APIs to accelerate their workloads and applications and the Library can rely on a consistent and known state of the user data through the lifetime of the handle.

Example of `sparse::matrix_handle_t` usage workflow

An example workflow to demonstrate the usage model is the following:

// (A) Allocate user memory for sparse matrix such as with SYCL USM device alloc or sycl::buffer
// (B) Do anything with matrix arrays, such as memcpy some data from host
// (C) Create a sparse matrix handle and pass in matrix arrays
// (D) Make some library calls of your choice (for instance with matrix handle)
// (E) Destroy the matrix handle via call to sparse::release_matrix_handle()
// (F) Do anything with user memory - modify, copy, etc
// (G) Create another sparse matrix handle using the same arrays
// (H) Make other library calls of your choice
// (I) Destroy the matrix handle via call to sparse::release_matrix_handle()
// (J) Do whatever you want with user memory

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