c# - Does Parallel.ForEach limit the number of active threads?

Question

Given this code:

var arrayStrings = new string[1000];
Parallel.ForEach<string>(arrayStrings, someString =>
{
    DoSomething(someString);
});

Will all 1000 threads spawn almost simultaneously?

Answer 1

No, it won't start 1000 threads - yes, it will limit how many threads are used. Parallel Extensions uses an appropriate number of cores, based on how many you physically have and how many are already busy. It allocates work for each core and then uses a technique called work stealing to let each thread process its own queue efficiently and only need to do any expensive cross-thread access when it really needs to.

Have a look at the PFX Team Blog for loads of information about how it allocates work and all kinds of other topics.

Note that in some cases you can specify the degree of parallelism you want, too.