Nested Parallel.For() loops speed and performance

Question

I have a nested for loop. I have replaced the first For with a Parallel.For() and the speed of calculation increased.

My question is about replacing the second for (inside one) with a Parallel.For(). Will it increase the speed? or there is no difference? or it will be slower?

Edit:

Since the cores are not unlimited (usually there is 2 to 8 cores), the inside loop is running parallel. So, if I change the inside for with a Parallel.For(), again it runs parallel. But i'm not sure how it changes the performance and speed.

Answer 1

From "Too fine-grained, too coarse-grained" subsection, "Anti-patterns" section in "Patterns of parallel programming" book by .NET parallel computing team:

The answer is that the best balance is found through performance testing. If the overheads of parallelization are minimal as compared to the work being done, parallelize as much as possible: in this case, that would mean parallelizing both loops. If the overheads of parallelizing the inner loop would degrade performance on most systems, think twice before doing so, as it’ll likely be best only to parallelize the outer loop.

Take a look at that subsection, it is self-contained with detailed examples from parallel ray tracing application. And its suggestion of flattening the loops to have better degree of parallelism may be helpful for you too.

Answer 2

It depends a lot on the data and functions you use inside the for and the machine. I have been messing lately with the parallel.for and parallel.foreach and found out that they made my apps even slower... (on a 4 core machine, probably if you have a 24 core server is another story)

I think that managing the threads means too much overhead...

Even MS on their documentation (here is a very long pdf on msdn about it http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=19222) admits it doesnt make the apps run faster. You have to try every time, and if it works, great, and if not bad luck.

You should try with the external for and the internal, but at least on the apps i tried none of them made the app faster. External or internal didnt matter much i was just getting the same execution times or even worse.

Maybe if you use Concurrent collections too, you get better performance. But again, without trying there is no way to tell.

EDIT:

I just found a nice link on MSDN that proved to be very useful (in my case) to improve Parallel.foreach performance http://msdn.microsoft.com/en-us/library/dd560853.aspx

Answer 3

It again depends on many scenarios,

1. Number of parallel threads your cpu can run.
2. Number of iterations.

If your CPU is a single-core processor, you will not get any benefits.

If the number of iterations is greater, you will get some improvements.

If there are just a few iterations, it will be slow as it involves extra overload.