How to Limit Parallelism in Parallel.ForEach?

Controlling concurrency in Parallel.ForEach

In parallel programming, it can be beneficial to run loops in parallel, allowing multiple threads to perform independent tasks simultaneously. However, when dealing with constrained resources, it becomes critical to limit the number of parallel tasks to avoid overloading the system. This article demonstrates how to limit parallelism in a Parallel.ForEach loop.

Consider a situation where you have a Parallel.ForEach loop downloading a web page. To ensure your bandwidth isn't overwhelmed, you'll want to limit the number of simultaneous downloads. Traditional web crawling solutions are not enough as your tasks may involve different types of operations.

Limit parallelism

To control the number of threads used in a Parallel.ForEach loop, you can use the MaxDegreeOfParallelism parameter of the ParallelOptions class. This parameter specifies the maximum number of threads that can execute the loop in parallel.

<code class="language-csharp">Parallel.ForEach(
    listOfWebpages,
    new ParallelOptions { MaxDegreeOfParallelism = 4 },
    webpage => { Download(webpage); }
);</code>

In this example, MaxDegreeOfParallelism is set to 4, which means that up to four threads will be used to download the web page simultaneously. This ensures that your bandwidth does not exceed limits and prevents undue pressure on resources.

More resources

For more information about Parallel.ForEach, see the MSDN documentation:

• Parallel.ForEach
• ParallelOptions.MaxDegreeOfParallelism

By controlling the degree of parallelism in a Parallel.ForEach loop, you can effectively manage resource usage and ensure optimal performance for your application.

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