How to configure a maximum number of threads in a Parallel.For

Question

This is the example microsoft presents for the parallel for, and I'd like to know how configure a maximum number of threads for this code.

     // A basic matrix multiplication.
     // Parallelize the outer loop to partition the source array by rows.
     System.Threading.Tasks.Parallel.For(0, matARows, i =>
     {
        for (int j = 0; j < matBCols; j++)
        {
           // Use a temporary to improve parallel performance.
           double temp = 0;
           for (int k = 0; k < matACols; k++)
           {
              temp += matA[i, k] * matB[k, j];
           }
           result[i, j] = temp;
        }
     }); // Parallel.For

Is that *really* the example presented? Because it's really invalid at the moment. Could you show where that's presented? — Jon Skeet, Apr 10 '13 at 16:27
Yeah. Well, here I only presented the relevant fragment. The whole example is here: http://msdn.microsoft.com/en-us/library/dd460713.aspx — Alex. S., Apr 10 '13 at 16:34
You presented a fragment which is syntactically incorrect, by sticking "System.Threading.Tasks.TaskCreationOptions" in the middle of the argument list for no reason. — Jon Skeet, Apr 10 '13 at 16:48
possible duplicate of [Limit the number of parallel threads in C#](http://stackoverflow.com/questions/8853907/limit-the-number-of-parallel-threads-in-c-sharp) — Sergey Malyutin, Oct 16 '14 at 13:47

score 50 · Accepted Answer · answered Apr 10 '13 at 16:29

50

You need to specify a ParallelOptions value with a MaxDegreeOfParallelism:

For example:

Parallel.For(0, 10, new ParallelOptions { MaxDegreeOfParallelism = 4 }, count =>
{
    Console.WriteLine(count);
});

answered Apr 10 '13 at 16:29

Jon Skeet

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2

for Parallel for each use : Parallel.ForEach(lines,new ParallelOptions { MaxDegreeOfParallelism = 4 }, line => { Console.WriteLine(line); }); – Somnath Kadam Jul 27 '19 at 09:14

Sasha · Answer 2 · 2013-04-10T16:39:05.127

4

Use MaxDegreeOfParalelism property for running the loop

Parallel.For(0, 1000, new ParallelOptions { MaxDegreeOfParallelism = 2 }, ...);

edited Apr 10 '13 at 16:39

answered Apr 10 '13 at 16:28

Sasha

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score 4 · Answer 3 · answered Apr 10 '13 at 16:28

4

I'd suggest you take a look at the ParallelOption.MaxDegreesofParellelism and pass it into the For method

answered Apr 10 '13 at 16:28

Tyler Lee

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How to configure a maximum number of threads in a Parallel.For

3 Answers3

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