Should we use MaxDegreeOfParallelism option when use ParallelForEachAsync

Question

I have a list of objects and for each item on the list I have to consume an API. I used ParallelForEachAsync, but I was unsure whether to change the maxDegreeOfParallelism setting.

await listObject.ParallelForEachAsync(
async item =>
{

                await CallAPI();


            }, maxDegreeOfParallelism: Convert.ToInt32(Math.Ceiling((Environment.ProcessorCount * 0.75) * 2.0)));

Answer 1

Unless you have a special reason, you don’t have to explicitly set it

Some explanations are provided in the documentation:

By default, For and ForEach will utilize however many threads the underlying scheduler provides, so changing MaxDegreeOfParallelism from the default only limits how many concurrent tasks will be used.

Generally, you do not need to modify this setting. However, you may choose to set it explicitly in advanced usage scenarios such as these:

When you know that a particular algorithm you're using won't scale beyond a certain number of cores. You can set the property to avoid wasting cycles on additional cores.

When you're running multiple algorithms concurrently and want to manually define how much of the system each algorithm can utilize. You can set a MaxDegreeOfParallelism value for each.

When the thread pool's heuristics is unable to determine the right number of threads to use and could end up injecting too many threads. For example, in long-running loop body iterations, the thread pool might not be able to tell the difference between reasonable progress or livelock or deadlock, and might not be able to reclaim threads that were added to improve performance. In this case, you can set the property to ensure that you don't use more than a reasonable number of threads.

It limits the upper limit of parallel operations, which may reduce efficiency at certain times.

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