MS并发::: parallel_for（）的性能，可进行单次迭代

Question

I suppose there have been numerous questions regarding performance issues while trying to achieve concurrency with parallel_for . Even I have noted a performance drop while trying to parallelize memory-access extensive for loops using parallel_for . The application area that I am working on is image processing.

Surprisingly this performance drop is seen even if I create a loop with a single iteration to be processed by parallel_for !!

What I mean is that I have a code block as under, which executes in 7 sec without any parallelisation

<code block without parallelisation>   //(Executes in 7 seconds)

If I enclose the above code within a parallel_for loop as under, the performance increases to 18 seconds.

parallel_for(0,1,[&](int random_var){   //(Executes in 18 seconds)
<code block without parallelisation> 
});

I completely fail to understand such a behaviour. What could cause such a huge overhead for the processing. In such a case I assume there should not be any memory bandwidth related issues?

Let me know in case you require more information for this specific problem that I am facing.

Answer 1

because even for one iteration inside parallel_for its going to execute your code in a thread. so there will be preemption with main thread. also there are other thread related book keeping works, that will take time.