asp.net - How does parallel.for works

Question

How does parallel.for works. Does it invoke threads for each loop/ divide the loops in parts and execute them in parallel? If it does then can we ensure the same result as normal for loop? I tested for performance and it really uses multi core processors. But i want to know the internal working as to how this works

Answer 1

Parallel.For partitions the work for a number of concurrent iterations. Per default it uses the default task scheduler to schedule the iterations, which essentially uses the current thread as well as a number of thread pool threads. There are overloads that will allow you to change this behavior.

A parallel loop may look very similar to a regular loop, but there are actually a number of important differences. First of all the order of the iterations is not guaranteed. I.e. the code cannot assume any specific order. Doing so will lead to unpredictable results.

Also, since the code may be run on multiple threads exception handling is completely different from a regular for loop. Parallel.For will catch exceptions for the threads and marshal these back to the calling thread as inner exceptions in an instance of AggregateException.

For additional details please check Parallel Programming with Microsoft .NET by Microsoft patterns and practices.

Answer 2

parallel.for loops run iterations of the loop in different processes in parallel. You can only use this if iterations are independent of one another. Only if the iterations are independent, you can assume the same results will be produced by a parallel and non-parallel for loop.