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    <p>Instead of using clock() to measure time, you should Events:</p> <p>Using events you would have something like this: </p> <pre><code> cudaEvent_t start, stop; // variables that holds 2 events float time; // Variable that will hold the time cudaEventCreate(&amp;start); // creating the event 1 cudaEventCreate(&amp;stop); // creating the event 2 cudaEventRecord(start, 0); // start measuring the time // What you want to measure cudaMalloc((void**)&amp;dev_B, Nel*Nface*sizeof(int)); cudaMemcpy(dev_B, B, Nel*Nface*sizeof(int),cudaMemcpyHostToDevice); cudaEventRecord(stop, 0); // Stop time measuring cudaEventSynchronize(stop); // Wait until the completion of all device // work preceding the most recent call to cudaEventRecord() cudaEventElapsedTime(&amp;time, start, stop); // Saving the time measured </code></pre> <p><strong>EDIT</strong> : Additional information :</p> <p>"The kernel launch returns control to the CPU thread before it is finished. Therefore your timing construct is measuring both the kernel execution time as well as the 2nd memcpy. When timing the copy after the kernel, your timer code is being executed immediately, but the cudaMemcpy is waiting for the kernel to complete before it starts. This also explains why your timing measurement for the data return seems to vary based on kernel loop iterations. It also explains why the time spent on your kernel function is "negligible"". credits to <a href="https://stackoverflow.com/users/1695960/robert-crovella">Robert Crovella</a></p>
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    1. POcopy from GPU to CPU is slower than copying CPU to GPU
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    1. POcopy from GPU to CPU is slower than copying CPU to GPU
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    1. COYes you should use this method. The kernel launch returns control to the CPU thread *before it is finished*. Therefore your timing construct is measuring both the kernel execution time as well as the 2nd memcpy. When timing the copy after the kernel, your timer code is being executed immediately, but the cudaMemcpy is waiting for the kernel to complete before it starts. This also explains why your timing measurement for the data return seems to vary based on kernel loop iterations. It also explains why the time spent on your kernel function is "negligible".
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    2. CO@RobertCrovella Exactly.
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    3. COBtw RobertCrovella, can i put his information inside the answer as an side note, or do you prefer to give a answer your self?
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