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c#multithreadingtask-parallel-library.net-6.0parallel.foreachasync
MarkusParker
MarkusParker

Reputation: 1596

Can .NET 6's Parallel.ForEachAsync be nested?

I want to do calculations with an outer and an inner loop which I can do in parallel. Furthermore, I want to use the async/await-based programming model. In the outer loop there is a place where a resource is needed which can only be used by one thread.

I thought of implementing the loops using ForEachAsync and restrict the access to the resource using SemaphoreSlim:

using System.Linq;
using System.Threading;
using System.Threading.Tasks;
                    
public class Program {
    private static Dictionary<int,IReadOnlyList<int>> resource = new();
    private static SemaphoreSlim semaphore = new(1);
    
    public static async Task Main() {       
        var outerLoopSource = Enumerable.Range(0,10);
        await Parallel.ForEachAsync(outerLoopSource, OuterLoopFunction);
        foreach(var (key, list) in resource)
            Console.WriteLine(key+": "+string.Join(',', list));
    }
                                    
    public static async ValueTask OuterLoopFunction(int i, CancellationToken cancel) {
        // some time consuming calculation ...      
        var key = i%3;
        const int listSize = 10;
        IReadOnlyList<int> list;
        await semaphore.WaitAsync();
        try {
            if(!resource.TryGetValue(key, out list)) {
                var newList = new int[listSize];
                list = newList;
                resource.Add(key, list);
                await Parallel.ForEachAsync(Enumerable.Range(0,listSize), InnerLoopFunction);
                ValueTask InnerLoopFunction(int j, CancellationToken cancel) {
                    // some time consuming calculation ...
                    newList[j] = 42+i;
                    return ValueTask.CompletedTask;
                }
            }
        } finally {
            semaphore.Release();
        }           
        // do something with list
    }   
}

Example in fiddle

Can ForEachAsync be used in nested loops like this and is the number of operations in parallel still restricted by System.Environment.ProcessorCount?

Update

In the comments people suggested to use dataflow components from the Task Parallel Library. That might be the better approach if I wrote the code from scratch. However, in my case there is quite a lot of legacy code doing the calculations and it seems to me I would have to restructure it significantly in order to apply the concept, since I would have to lift what is currently the inner loop on the same level as the outer loop. Hence I wonder if using annother SemaphoreSlim to restrict the number of parallel executions as described here avoids running to many tasks/threads in parallel without too much performance penalty.

Upvotes: 1

Views: 2633

Answers (1)

Theodor Zoulias
Theodor Zoulias

Reputation: 43951

No, the ParallelOptions.MaxDegreeOfParallelism affects only the degree of parallelism of the configured Parallel.ForEachAsync loop. It's not an ambient property that affects all other parallel loops that might be nested inside the outer parallel loop. For example if you configure the outer parallel loop with MaxDegreeOfParallelism = 5, and the inner parallel loop with MaxDegreeOfParallelism = 3, the delegate of the inner parallel loop might be invoked concurrently 15 times (5 * 3) at any given moment.

This assuming that the inner parallel loop is unrestricted. In your example you have enclosed the inner parallel loop in a protected region, by using a SemaphoreSlim(1). So only one inner parallel loop can be active at any given moment. The maximum number of concurrent invocations of the inner loop's delegate is Environment.ProcessorCount (the default MaxDegreeOfParallelism for the Parallel.ForEachAsync API).

Upvotes: 1

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