How Can I Limit Parallelism When Processing Messages in C#?

Limiting Maximum Parallelism in Message Processing with C#

When processing a large collection of items in parallel, it's often desirable to control the maximum number of tasks that can run simultaneously. This helps optimize resource utilization and prevent overwhelming the system.

Using the provided sample code, which loops through a collection of 1,000 input messages and starts a task for each, it's difficult to predict the exact number of messages that will be processed in parallel. However, on a typical quad-core processor, multiple messages will likely be processed simultaneously.

Limiting Maximum Parallelism

To limit the maximum number of messages processed at a time, the Parallel.ForEach method with the MaxDegreeOfParallelism option can be employed. This option specifies the maximum number of threads that can be used for parallel execution. For instance, the code below limits the number of parallel tasks to 10:

Parallel.ForEach(messages, new ParallelOptions { MaxDegreeOfParallelism = 10 },
msg =>
{
    // logic
    Process(msg);
});

Maintaining Sequence

In the above code, the messages may not necessarily be processed in the same order in which they appear in the collection. To ensure strict sequence, the AsParallel extension method can be used:

var orderedMessages = messages.AsParallel();
Parallel.For(0, orderedMessages.Count(), i =>
{
    Process(orderedMessages[i]);
});

In this code, the Parallel.For method creates a loop that iterates over the collection in parallel, but within each iteration, the message being processed is accessed sequentially.

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