# plain matrix multiplication very slow (implemented with Matlab mex)

I implemented a very simple plain matrix multiplication with Matlab mex.
But it is about 30x slower than the matlab build-in function (A*B with A and B being both gpuArrays).

It takes about 14 seconds to multiply two 8000x8000 matrices. Matlab build-in takes about 0.5 second.

Why is there so large difference?

There could be two reasons:
(1) I haven’t made full use of the GPU using my code
(2) The fancy algorithm used by Matlab build-in function is 30x faster (with 8000x8000 matrices) than my naive matrix multiplication. But my 14 seconds seems really too long given the power of my GPU (Tesla K80).

Anybody has any idea?

Thank you very much!

https://www.dropbox.com/s/cq6tiqcyc5539po/mat_mult.tar?dl=0

Main code:

``````// modified from http://www.mathworks.com/help/distcomp/run-mex-functions-containing-cuda-code.html

#include "mex.h"
#include "gpu/mxGPUArray.h"

/*
* Device code
*/

void __global__ MatMult(float const * const A,
float const * const B,
float * const C,
int const N, int const M, int const K)
{
int const ROW = blockDim.x * blockIdx.x + threadIdx.x;
int const COL = blockDim.y * blockIdx.y + threadIdx.y;
if (ROW < N && COL < M) {
float tmpSum = 0.0f;
for (int i = 0; i < K; i++) {
tmpSum += A[i * N + ROW] * B[COL * K + i];
}
C[COL * N + ROW] = tmpSum;
}
}

/*
* Host code
*/
void mexFunction(int nlhs, mxArray *plhs[],
int nrhs, mxArray const *prhs[])
{
/* Declare all variables.*/
mxGPUArray const *A;
mxGPUArray const *B;
mxGPUArray *C;
float const *d_A;
float const *d_B;
float *d_C;
int N,M,K;
char const * const errId = "parallel:gpu:mexGPUExample:InvalidInput";
char const * const errMsg = "Invalid input to MEX file.";

/* Choose a reasonably sized number of threads for the block. */

/* Initialize the MathWorks GPU API. */
mxInitGPU();

/* Throw an error if the input is not a GPU array. */
if ( (nrhs!=2) || !(mxIsGPUArray(prhs[0])) || !(mxIsGPUArray(prhs[1])) ) {
mexErrMsgIdAndTxt(errId, errMsg);
}

A = mxGPUCreateFromMxArray(prhs[0]);
B = mxGPUCreateFromMxArray(prhs[1]);

/*
* Verify that A really is a float array before extracting the pointer.
*/
if (mxGPUGetClassID(A) != mxSINGLE_CLASS) {
mexErrMsgIdAndTxt(errId, errMsg);
}
if (mxGPUGetClassID(B) != mxSINGLE_CLASS) {
mexErrMsgIdAndTxt(errId, errMsg);
}

/*
* Now that we have verified the data type, extract a pointer to the input
* data on the device.
*/

mwSize const * const A_size = mxGPUGetDimensions(A);
mwSize const * const B_size = mxGPUGetDimensions(B);
N = A_size[0];
M = B_size[1];
K = B_size[0];
mwSize * C_size = new mwSize[2];
C_size[0] = N;
C_size[1] = M;

/* Create a GPUArray to hold the result and get its underlying pointer. */
C = mxGPUCreateGPUArray(mxGPUGetNumberOfDimensions(A),
C_size,
mxGPUGetClassID(A),
mxGPUGetComplexity(A),
MX_GPU_INITIALIZE_VALUES);
//MX_GPU_DO_NOT_INITIALIZE);
d_C = (float *)(mxGPUGetData(C));

//N = (int)(mxGPUGetNumberOfElements(A));
MatMult<<<blocksPerGrid, threadsPerBlock>>>(d_A, d_B, d_C, N, M, K);

/* Wrap the result up as a MATLAB gpuArray for return. */
plhs[0] = mxGPUCreateMxArrayOnGPU(C);

mxGPUDestroyGPUArray(A);
mxGPUDestroyGPUArray(B);
mxGPUDestroyGPUArray(C);
}
``````

yeah, it’s due to better algorithm. google for “matrix multiplication algorithm”

I confirmed that cublassgemm (in cuBLAS) is as fast as Matlab’s gpu matrix multiplication. Both of them are about 30-40x faster than naive implementation using CUDA.

What should I look at if I want to learn a little bit about the algorithm used by cublassgemm?
Does the speedup mainly come from algorithm (O(n^3) -> O(n^2.8), etc.) or the implementation (better management of memory/io, etc.)?
Thanks.

There are a sequence of optimizations that are involved. I can’t list them out for you, but one of the first ones would be better usage of memory.

A straightforward optimization is the use of shared memory. An example of this is actually covered in the CUDA C Programming guide:

http://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#shared-memory

You might also be interested in this post by a GPU ninja:

https://github.com/NervanaSystems/maxas/wiki/SGEMM

Thank you very much, txbob!