An implementation of BLAS using the SYCL open standard for acceleration on OpenCL devices
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README.md

SYCL BLAS Implementation

SYCL BLAS implements BLAS - Basic Linear Algebra Subroutines - using SYCL 1.2, the Khronos abastraction layer for OpenCL.

SYCL BLAS is a current work in progress research project from an ongoing collaboration with the High Performance Computing & Architectures (HPCA) group from the Universitat Jaume I UJI.

SYCL BLAS is written using modern C++. The current implementation uses C++11 features but we aim to move to C++14 in the short term. See Roadmap for details on the current status and plans for the project.

Table of Contents

Motivation

The same numerical operations are computed to solve many scientific problems and engineering applications, such as image and signal processing, telecommunication, computational finance, materials science simulations, structural biology, data mining, bio-informatics, fluid dynamics, and many other areas. Thus, it was identified that around the 90% percent of the computational cost is consumed on the 10% of the code, and therefore any improvement in this 10% of code would have a great impact in the performances of the applications. Numerical Linear Algebra is the science area in charge of identifying the most common operations and seeking their best implementation. To do this, the researchers should consider the numerical stability of the selected algorithm, and the platform on the operation will be solved. The first analysis studies the accuracy of the solution while the second one compares the performances of the different implementations to select the best one.

Nowadays, all the numerical computations are based on a set of standard libraries on which the most common operations are implemented. These libraries are different for dense matrices (BLAS, LAPACK, ScaLAPACK, ...) and for sparse matrices (SparseBLAS, ...). Moreover, there are vendor implementations which are adjusted to the platform features:

  • For multicores: ACML (AMD), ATLAS, Intel-MKL, OpenBLAS, ...
  • For GPUs: cuBLAS(nVidia), clBLAS, MAGMA, ...

But, in any case, BLAS is always the lowest level in the hierarchy of numerical libraries, such that a good BLAS implementation improves the performances of all the other libraries. The development of numerical libraries on SYCL is one of the most important objectives, because it will improve the performance of other SYCL applications. Obviously, it makes sense SYCL-BLAS was the first step in this task.

On GPUs, the data communication to/from the device and the grain of the kernels play an important rule on the performances of the developments. On one hand, to reduce the communication cost, the most of the data should be mapped on the device, even the scalars. On the other hand, growing the size of the kernels allows the CPU to complete other tasks while the GPU is computing or to enter an energy-efficient C-state, reducing the energy consumption.

To enlarge the grain of the kernels is a complex task, in which many aspects should be considered as the dependency between kernels, the grid topology, the grid sizes, etc. This complexity justifies that, usually, the fused kernels are manually written. An alternative to simplify this task could be to build a expression tree on which all the single operation which are required to solve a problem appears. This structure could be analysed by the compiler to decide how to merge the different kernel and the best grid topology to execute the fused kernel. The use of expression trees is one of most important features of SYCL-BLAS.

Basic Concepts

SYCL BLAS uses C++ Expression Tree templates to generate SYCL Kernels via kernel composition. Expression Tree templates are a widely used technique to implement expressions on C++, that facilitate development and composition of operations. In particular, Kernel composition in SYCL has been used in various projects to create efficient domain specific embedded languages that enable users to easily fuse GPU kernels.

SYCL-BLAS is a header-only library. All the relevant files can be found in the include directory. There are four components in SYCL-BLAS, the View, the Operations, the Executors and the Interface itself.

Views

The input data to all the operations in SYCL-BLAS is passed to the library using Views. A View represents data on top of a container, passed by reference. Views do not store data, they only map a visualization of the data on top of a container. This enables the library to implement the different indexing modes of the BLAS API, such as strides. Note than a view can be of a different size than a container.

All views derive from the base view class or the base matrix view class, which represents a view of a container as a vector or as a matrix. The container does not need to be multi-dimensional to store a matrix. The current restriction is that container must obey the RandomAccessIterator properties of the C++11 standard.

Operations

Operations among elements of vectors (or matrices) are expressed in the set of Operation Classes. Operations are templated classes that take templated types as input. Operations form the nodes of the SYCL-BLAS expression tree. Refer to the documentation of each node type for details.

Composing these is how the compile-time Expression tree is created: Given an operation node, the leaves of the node are other Operations. The leaf nodes of an Expression Tree are Views or Scalar types (data). The intermediate nodes of the Expression Tree are operations (e.g, binary operations, unary operations, etc).

Executors

An executor traverses the Expression Tree to evaluate the operations that it defines. Executors use different techniques to evaluate the expression tree. The basic C++ executor performs a for loop on the size of the data and calls the evaluation function on each item.

The SYCL evaluator transform the tree into a device tree (i.e, converting buffer to accessors) and then evaluates the Expression Tree on the device.

Interface

The different headers on the interface directory implements the traditional BLAS interface. Files are organised per BLAS level (1,2,3).

When the SYCL-BLAS BLAS interface is called, the Expression Tree for each operation is constructed, and then executed. Some API calls may execute several kernels (e.g, when a reduction is required). The expression trees in the API allow to compile-time fuse operations.

Note that, although this library features a BLAS interface, users are allowed to directly compose their own expression trees to compose multiple operations. The CG example shows an implementation of the Conjugate Gradient that uses various expression tree to demonstrate how to achieve compile-time kernel fusion of multiple BLAS operations.

Requirements

SYCL-BLAS is designed to work with any SYCL 1.2.1 implementation. We do not use any OpenCL interoperability, hence, the code is pure C++. The project is developed using (ComputeCpp CE Edition 1.0.2)[http://www.computecpp.com] using Ubuntu 16.04 on Intel OpenCL CPU and AMD GPU. In order to build the sources, GCC 5.4 or higher is required. The build system is CMake version 3.2.2 or higher. We rely on the FindComputeCpp.cmake imported from the Computecpp SDK to build the project.

Setup

  1. Clone the SYCL-BLAS repository, making sure to pass the --recursive option, in order to clone submodule(s), such as the computecpp-sdk.
  2. Create a build directory
  3. Run CMake from the build directory:
$ cd build; cmake ../ -DComputeCpp_DIR=/path/to/computecpp

Doxygen documentation can be generated by running

$ doxygen doc/Doxyfile

Tests

The sample code of the project is designed also as a test. The project uses Ctest to run the different test. The default platform reported by ComputeCpp is used as the execution platform.

Contributing to the project

SYCL-BLAS is an Open Source project maintained by the HPCA group and Codeplay Software Ltd. Feel free to create an issue on the github tracker to request features or report bugs.