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player999
Journeyman III

Huge error in floating point computation

Comrads, I'm geting frustrated about math accuracy in OpenCL

Here is the problem, simple multiplication:

0.647059 * -1.148511 = -1.040416

WHY IT IS SO?

I suppose it is some compiler configs, but i don't know which o_O

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4 Replies
eugenek
Journeyman III

Huge error in floating point computation

You need to post some code that demonstrates this problem.

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player999
Journeyman III

Huge error in floating point computation

Here it is. Part of pattern recognition kernel. Performs matrix-vector multiplication. Printf outputs results of multiplication vector on last row of matrix.

/*Kernelzzzz*/

#define lL1 130
#define lL2 100
#define lL3 36
#pragma OPENCL EXTENSION cl_amd_printf:enable


__kernel void recKernel(__global  float * output, __global  uchar * input,
                         __global  float * IW, __global  float * LW1,
                        __global  float * LW2, __global  float * B1,
                        __global  float * B2, __global  float * B3, uint L1,
                        uint L2, uint L3)
{
    uint x = get_global_id(0);
    __local float A1[lL1];
    __local float fin[lL1];
    uint i;
    float A;
    fin = (float)input / (float)255;
    if(x < lL1){       
        IW += x;
        A = 0;//B1;
        #pragma unroll 2
        for(i = 0; i < lL1; i++){

            A = fin * *IW;
            //A += fin
* *IW;
            if(x==129)
                printf("%f * %f = %f\n", fin, *IW, fin * *IW);       
            IW  += lL1;
        }
        A1 = native_recip(1 + native_exp(-A));
    }


UPD: I have triple-checked my source. Found there a couple of errors that affected accuracy. Now all is OK. Thanks for your attention!

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thesmileman
Journeyman III

Huge error in floating point computation

Originally posted by: player999 Comrads, I'm geting frustrated about math accuracy in OpenCL

Here is the problem, simple multiplication:

0.647059 * -1.148511 = -1.040416

WHY IT IS SO?

That ISN"T a precision issue unless somthign has gone horribly horribly wrong. Plus unless you have a fast relaxed math enabled you shouldn't have concerns with precision at least not at these levels. To be absolutely sure use the flag "-cl-opt-disable" to disable all optimizations.

Some inital thoughts You seem to be trying to unroll the loop with a pragma call 2 times for a look with 130 iterations. Why? Also are you sure you are not overridding some of your local variables with calculations from the surrounding calls?

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player999
Journeyman III

Huge error in floating point computation

OK, I've understood it already. And already disabled math optimiztions.

Why am i trying to unroll? I don't know-- profiler showed, that unrolling adds few nanoseconds 🙂 Actually all three sequential matrix-vector multiplications performing 92 microseconds. About overriding -- no i do not. Now my calculations precise enough and code is not buggy.

 

Nevertheless -- i've one more issue. I've sequentially enqued writing to GPU memory (loveland, AMD Fusion E-350) and kernel execution. When i measuring separatly time of execution of copy and kernel procedures, i get 35us and 92us respectively, but when i measure overall time of execution (from start of copying operation to the end of kernel execution), i get 1.4 ms. As mentioned before, all measurments are carried out by profiler function calls. Please, help me to understand why.

UPD: Oh, almost forgotten. The source!

status =     clEnqueueWriteBuffer(
                NN->commandQueue,
                NN->devInput,
                CL_TRUE,
                0,
                LAYER1 * sizeof(char),
                NN->hostInput,
                0,
                NULL,
                &events[0]);
    #ifdef DEBUG
    if(status != CL_SUCCESS)
    {
        printf(
            "Error: clEnqueueReadBuffer failed. \
             (clEnqueueReadBuffer)\n");
        return;
    }   
    #endif

    globalThreads[0] = 130;
    status = clEnqueueNDRangeKernel(
                 NN->commandQueue,
                 NN->recKernel,
                 1,
                 NULL,
                 globalThreads,
                 NULL,
                 1,
                 events,
                 &events[1]);
    #ifdef DEBUG
    if(status != CL_SUCCESS)
    {
        printf(
            "Error: Enqueueing kernel recKernel onto command queue. \
            (clEnqueueNDRangeKernel)\n");
            printf("Error code: %d\n", status);
        return;
    }
    #endif

    status = clWaitForEvents(1, &events[1]);
    #ifdef DEBUG
    if(status != CL_SUCCESS)
    {
        printf(
            "Error: Waiting for kernel run to finish. \
            (clWaitForEvents)\n");
        return;
    }
    #endif
   
    #ifdef PROFILING
    cl_ulong startTime, endTime;
   
    clGetEventProfilingInfo(events[0], CL_PROFILING_COMMAND_START,
    sizeof(cl_ulong), &startTime, NULL);
    clGetEventProfilingInfo(events[0], CL_PROFILING_COMMAND_END,
    sizeof(cl_ulong), &endTime, NULL);
    printf("Copy input: %d ns\n", endTime - startTime);
   
    clGetEventProfilingInfo(events[1], CL_PROFILING_COMMAND_START,
    sizeof(cl_ulong), &startTime, NULL);
    clGetEventProfilingInfo(events[1], CL_PROFILING_COMMAND_END,
    sizeof(cl_ulong), &endTime, NULL);
    printf("Kernel: %d ns\n", endTime - startTime);
    clGetEventProfilingInfo(events[0], CL_PROFILING_COMMAND_START,
    sizeof(cl_ulong), &startTime, NULL);
    clGetEventProfilingInfo(events[1], CL_PROFILING_COMMAND_END,
    sizeof(cl_ulong), &endTime, NULL);
    printf("Summary: %d ns\n", endTime - startTime);
    #endif

 

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