Question

here’s a simple OpenCL Matrix Multiplication kernel which is driving me crazy:

By the way I am using pyopencl.

__kernel void matrixMul(  __global int* C,
                          __global int* A,
                          __global int* B,
                          int wA, int wB){

                int row = get_global_id(1); //2D Threas ID x
                int col = get_global_id(0); //2D Threas ID y

                //Perform dot-product accumulated into value
                int value = 0;
                for ( int k = 0; k < wA; k++ ){
                    value += A[row*wA + k] * B[k*wB+col];
                }
                C[row*wA+col] = value; //Write to the device memory
            }

Where (inputs)

A = [72 45
     75 61]
B = [26 53 
     46 76]
wA = wB = 2

Output I am getting:

Sometime I get:

C = [3942 0
     0 5472]

Else I get:

C = [3942 7236
     3312 5472]

But the output should be:

C = [3942 7236
     4756 8611]

I don’t know what mistake I am making here. I have spent the entire day with no luck.

Please help me with this

Here’s the full python code:

import pyopencl as cl
import numpy as np
import os

ORDER = 2
LEN = ORDER*ORDER
ctx = cl.create_some_context()

commandQueue = cl.CommandQueue( ctx )

A = np.array((72, 45, 75, 61), dtype = np.int32)
B = np.array((26, 53, 46, 76), dtype = np.int32)
C = np.empty_like(A)

in_buf1 = cl.Buffer( ctx, cl.mem_flags.READ_ONLY | cl.mem_flags.COPY_HOST_PTR,
                 hostbuf = A )
in_buf2 = cl.Buffer( ctx, cl.mem_flags.READ_ONLY | cl.mem_flags.COPY_HOST_PTR,
                 hostbuf = B )
out_buf = cl.Buffer( ctx, cl.mem_flags.WRITE_ONLY, C.nbytes )

kernelSrc1 = """__kernel void
            matrixMul(  /*const int Mdim,
                        const int Ndim,
                        const int Pdim,*/
                        __global int* C,
                        __global int* A,
                        __global int* B,
                        int wA, int wB)
           {
                int row = get_global_id(1); //2D Threas ID x
                int col = get_global_id(0); //2D Threas ID y                

                //Perform dot-product accumulated into value
                int value = 0;
                for ( int k = 0; k < wA; k++ ){
                    value += A[row*wA + k] * B[k*wB+col];
                }
                C[row*wA+col] = value; //Write to the device memory
            }"""

program1 = cl.Program(ctx, kernelSrc1 ).build()
event1 = program1.matrixMul( commandQueue, (LEN, ), None,
                     out_buf, in_buf1, in_buf2, np.int32(ORDER), np.int32(ORDER));
event1.wait()

cl.enqueue_copy(commandQueue, C, out_buf)
print C

I am using Python 2.7.x, pyopencl 2012.1, AMD APP SDK

Answer 1

You are setting your global size argument incorrectly. Since you are using two dimensions of global size in your kernel, you need to set your global size to (ORDER,ORDER). When you change it to that, you get:

[3942 7236
 4756 8611]