matlabgpunvidiatesla

Why is it faster to transfer data from CPU to GPU rather than GPU to CPU?

I've noticed that transferring data to recent high end GPUs is faster than gathering it back to the CPU. Here are the results using a benchmarking function provided to me by mathworks tech-support running on an older Nvidia K20 and a recent Nvidia P100 with PCIE:

Using a Tesla P100-PCIE-12GB GPU.
Achieved peak send speed of 11.042 GB/s
Achieved peak gather speed of 4.20609 GB/s

Using a Tesla K20m GPU.
Achieved peak send speed of 2.5269 GB/s
Achieved peak gather speed of 2.52399 GB/s

I've attached the benchmark function below for reference. What is the reason for the asymmetry on the P100? Is this system dependent or is it the norm on recent high end GPUs? Can the gather speed be increased? 

gpu = gpuDevice();
fprintf('Using a %s GPU.\n', gpu.Name)
sizeOfDouble = 8; % Each double-precision number needs 8 bytes of storage
sizes = power(2, 14:28);

sendTimes = inf(size(sizes));
gatherTimes = inf(size(sizes));
for ii=1:numel(sizes)
    numElements = sizes(ii)/sizeOfDouble;
    hostData = randi([0 9], numElements, 1);
    gpuData = randi([0 9], numElements, 1, 'gpuArray');
    % Time sending to GPU
    sendFcn = @() gpuArray(hostData);
    sendTimes(ii) = gputimeit(sendFcn);
    % Time gathering back from GPU
    gatherFcn = @() gather(gpuData);
    gatherTimes(ii) = gputimeit(gatherFcn);
end
sendBandwidth = (sizes./sendTimes)/1e9;
[maxSendBandwidth,maxSendIdx] = max(sendBandwidth);
fprintf('Achieved peak send speed of %g GB/s\n',maxSendBandwidth)
gatherBandwidth = (sizes./gatherTimes)/1e9;
[maxGatherBandwidth,maxGatherIdx] = max(gatherBandwidth);
fprintf('Achieved peak gather speed of %g GB/s\n',max(gatherBandwidth))

Edit: we now know it is not system dependent (see comments) . I still want to know the reason for the assymetry or if it can be changed.

Solution

  • This is a CW for anybody interested in posting benchmarks from their machine. Contributors are encouraged to leave their details in case some future question arises regarding their results.


    System: Win10, 32GB DDR4-2400Mhz RAM, i7 6700K. MATLAB: R2018a.

    Using a GeForce GTX 660 GPU.
Achieved peak send speed of 7.04747 GB/s
Achieved peak gather speed of 3.11048 GB/s


    Warning: The measured time for F may be inaccurate because it is running too fast. Try measuring something that takes
longer.

    Contributor: Dev-iL


    System: Win7, 32GB RAM, i7 4790K. MATLAB: R2018a.

    Using a Quadro P6000 GPU.
Achieved peak send speed of 1.43346 GB/s
Achieved peak gather speed of 1.32355 GB/s

    Contributor: Dev-iL