Tutorial: OpenCL SGEMM tuning for Kepler

Note: the complete source-code is available at GitHub.
Note: a tuned OpenCL BLAS library based on this tutorial is now available at GitHub.

Background: Matrix-multiplication

Before we look at OpenCL code, let us define the computation we are about to perform. According to the definition of BLAS libraries, the single-precision general matrix-multiplication (SGEMM) computes the following:
C := alpha * A * B + beta * C

In this equation, A is a K by M input matrix, B is an N by K input matrix, C is the M by N output matrix, and alpha and beta are scalar constants. For simplicity, we assume the common case where alpha is equal to 1 and beta is equal to zero, yielding:
C := A * B

This computation is illustrated in the following image: to compute a single element of C (in purple), we need a row of A (in green) and a column of B (in blue).

Matrix-multiplication illustrated

This version of SGEMM can be implemented in plain C using 3 nested loops (see below). We assume data to be stored in column-major format (Fortran-style), following cuBLAS's default. If we wanted, we could easily change this to row-major by swapping the A and B matrices and the N and M constants, so this is not a real limitation of our code.
for (int m=0; m<M; m++) {
    for (int n=0; n<N; n++) {
        float acc = 0.0f;
        for (int k=0; k<K; k++) {
            acc += A[k*M + m] * B[n*K + k];
        }
        C[n*M + m] = acc;
    }
}
To keep things simple for now, we assume that the matrix sizes are nice multiples of 32, such that we can for example create OpenCL workgroups of size 32 by 32 without having to worry about boundary conditions. There will be assumptions along these lines in the next couple of pages, but that's mainly because we want to keep the code simple and the main story-line easy to follow.



Tutorial written by Cedric Nugteren, (c) 2014 SURFsara
Last updated in 2014 - information might be outdated
tutorial_picture
Navigation:
- Introduction
- Matrix-multiplication
- Kernel 1
- Kernel 2
- Kernel 3
- Kernel 4
- Kernel 5
- Kernel 6
- Kernel 7
- Kernel 8
- Kernel 9
- Kernel 10
- What's next?

Extra pages:
- Inside clBlas
- clBlas on AMD GPUs

Contact:
www.cedricnugteren.nl