matlab - Why gpu slower than cpu? -

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i have gpu

>> d = gpudevice

d =     cudadevice properties:                    name: 'geforce 800m'                  index: 1      computecapability: '2.1'         supportsdouble: 1          driverversion: 6         toolkitversion: 5     maxthreadsperblock: 1024       maxshmemperblock: 49152     maxthreadblocksize: [1024 1024 64]            maxgridsize: [65535 65535 65535]              simdwidth: 32            totalmemory: 2.1475e+09             freememory: 1.9886e+09    multiprocessorcount: 1           clockratekhz: 1475000            computemode: 'default'   gpuoverlapstransfers: 1 kernelexecutiontimeout: 1       canmaphostmemory: 1        devicesupported: 1         deviceselected: 1`

i try use gpu in neuralnetwork train, gpu slower cpu. if try use gpuarray, gpu faster cpu, haven't speed acceleration in neural network training. example

>> a1 = rand(1000); b1 = rand(1000); tic; c1 = a1 * b1; toc; elapsed time 0.044095 seconds.

>> a2 = gpuarray(rand(1000)); b2 = gpuarray(rand(1000)); tic; c2 = a2 * b2; toc; elapsed time 0.000416 seconds.

but in code

 net = newff(h, f, layers, { 'tansig' 'tansig'}, 'traingdx', 'learngdm', 'mse');  net.trainparam.epochs = epochs; net.trainparam.show = 500; net.trainparam.time = 495; net.trainparam.goal = 2.0000e-11; net.trainparam.max_fail = 200000; net.trainparam.min_grad = 1.0000e-050; net.performparam.regularization = 0.05;  net.divideparam.trainratio = 1; net.divideparam.valratio = 0; net.divideparam.testratio = 0;  net.trainparam.showwindow = 0; net.trainparam.showcommandline = 0;  if gpu1 == 1     net = train(net, h, f, 'usegpu', 'yes', 'showresources','yes'); else     net = train(net, h, f, 'showresources','yes'); end;     tic; net = net_example(300, [23, 9], rand(100, 1000), rand(1, 1000), 1); toc;    computing resources: gpu device #1, geforce 800m

works slower than

 tic; net = net_example(300, [23, 9], rand(100, 1000), rand(1, 1000), 0); toc;   computing resources: mex2


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