为什么将数据从CPU传输到GPU比从GPU传输到CPU更快?

我注意到,将数据传输到最近的高端GPU比将数据收集回CPU要快。以下是使用mathworks技术支持向我提供的基准测试功能得出的结果,该功能运行在较旧的Nvidia K20和最新的Nvidia P100(带PCIE)上:

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

我在下面附上了基准函数以供参考。P100上不对称的原因是什么?这是依赖于系统还是最近高端GPU的标准?收集速度可以提高吗?

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))

编辑:我们现在知道它不依赖于系统(请参见注释)。我仍然想知道不对称的原因,或者是否可以改变。