MATLAB: GPU enabled functions react slow on first call

gpu enabled functions

Dear community, I want to utilize the GPU of my system to accelerate matrix multiplication. From the list of GPU-enabled functions, I can see that "mtimes" is available, therefore it should be accessible using gpuArrays.
My system is:
Matlab R2016b
Visual Studio 2013
CUDA 9.0
GTX 1070
I start by defining two matrices as gpuArrays:
 >>gDexMat=gpuArray(DexMat);
>> gx_PD=gpuArray(x_PD);
>> whos gDexMat gx_PD
  Name              Size                Bytes  Class       Attributes
gDexMat      200000x1000                  4  gpuArray              
gx_PD          1000x200000                4  gpuArray
I then perform the multiplication, gather the data and stop the time how long it took:
 >> tic;gIQMat=gx_PD*gDexMat;IQMat=gather(gIQMat);toc %doing the demodulation
Elapsed time is 0.528126 seconds.
>> tic;gIQMat=gx_PD*gDexMat;IQMat=gather(gIQMat);toc
Elapsed time is 0.414626 seconds.
>> tic;gIQMat=gx_PD*gDexMat;IQMat=gather(gIQMat);toc
Elapsed time is 0.405255 seconds.
In this example, the time it took is pretty constant. However, when I start Matlab the first time, the execution may take extremely long, up to 45 seconds. Even the first call:
 >>gDexMat=gpuArray(DexMat);
Can sometimes take up to a minute.
Does any body have any ideas why this is happening? Best, Ludwig

Best Answer

MATLAB R2016b is built with CUDA 7.5. The first time you load the GPU libs they have to be just-in-time compiled to match your Pascal-architecture GPU. See the answer here for an explanation and some steps to prevent this compilation happening every time.
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