Nested Parallel.ForEach loops

Question

I have some code which I am currently optimizing for concurrency in multicore architectures. In one of my classes, I found a nested foreach loop. Basically the outer loop iterates through an array of NetworkInterface objects. The inner loop iterates though the network interfaces IP addresses.

It got me thinking, is having Nested Parallel.ForEach loops necessarily a good idea? After reading this article (Nested Parallel.ForEach Loops on the same list?) I am still unsure what applies where in terms of efficiency and parallel design. This example is taking about Parallel.Foreach statements being applied to a list where both loops are performing operations on that list.

In my example, the loops are doing different things, so, should I:

1. Use nested Parallel.ForEach loops?
2. User Parallel.ForEach on the parent loop and leave the inner loop as-is?

Solution 1

A Parallel.ForEach does not necessarily execute in parallel -- it is just a request to do so if possible. Therefore, if the execution environment does not have the CPU power to execute the loops in parallel, it will not do so.

If the actions on the loops are not related (i.e., if they are separate and do not influence each other), I see no problem using Parallel.ForEach both on inner and outer loops.

It really depends on the execution environment. You could do timing tests if your test environment is similar enough to the production environment, and then determine what to do. When in doubt, test ;-)

Good luck!

Solution 2

The answer will be, it depends;

1. What are you doing with the IP address once you have it?
2. How long does each step take?

Threads are not cheap, they take time to create, and memory to exist. If you're not doing something computationally expensive with those IP Addresses, and using the wrong type of collection for concurrent access, you're almost certainly slowing down your application.

Use StopWatch to help you answer these questions.