sheep94lion commented on issue #12159: C++ api executor->Forward(false) is much slower than MXPredForward? URL: https://github.com/apache/incubator-mxnet/issues/12159#issuecomment-506118494 > Hi @xiaojingxie > > The Executor->Forward() method performs more tasks as compared to MXPredForward(). It invokes following 2 C APIs internally > MXExecutorForward() > and > MXExecutorOutputs() > where as MXPredForward() only runs the forward pass and does not retrieve the outputs. > In addition, the outputs of the forward pass are copied to the 'output' array in the Executor object, for faster retrieval later. > > Therefore, we can not compare MXPredForward() with Executor->Forward(). For the correct comparison, we should find out the time required to invoke MXPredGetOutputShape() and MXPredGetOutput() after invoking the MXPredForward() > > I hope this answers the question. > @mxnet-label-bot add [Pending Requester Info] The running time of C API MXPredForward is much shorter than the running time of MXPredGetOutput: ``` auto start = std::chrono::high_resolution_clock::now(); MXPredForward(pred_hnd); auto stop = std::chrono::high_resolution_clock::now(); auto duration = std::chrono::duration_cast<std::chrono::microseconds>(stop - start); LOGI("MXPredForward: %d microseconds.", duration.count()); std::vector<float> data(size); start = std::chrono::high_resolution_clock::now(); MXPredGetOutput(pred_hnd, output_index, &(data[0]), static_cast<mx_uint>(size)); stop = std::chrono::high_resolution_clock::now(); duration = std::chrono::duration_cast<std::chrono::microseconds>(stop - start); LOGI("MXPredGetOutput: %d microseconds.", duration.count()); ``` The result is: ``` I/MXNET: MXPredForward: 106 microseconds. I/MXNET: MXPredGetOutput: 3748967 microseconds. ``` Why? Is it something related to lazy evaluation? The code runs on Pixel3 with Snapdragon 835.
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