Reduce cost of async waiting on SemaphoreSlim with cancellation/timeout #83294
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If a WaitAsync on a SemaphoreSlim is unable to synchronously acquire the semaphore, it calls an async method that handles the waiting. That method awaits with a custom awaiter that avoids throwing in the case of cancellation or timeout. However, in using a custom awaiter, we get knocked off the highest-perf path that can avoid allocating an Action. This change puts it back onto the golden path by having the awaiter implement our internal interface that lets it backchannel with the async method builders. Eventually we want this awaiter to be available publicly as well, and it'll end up benefiting more than just semaphores.
ghost
assigned 
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Tagging subscribers to this area: @mangod9 Issue DetailsIf a WaitAsync on a SemaphoreSlim is unable to synchronously acquire the semaphore, it calls an async method that handles the waiting. That method awaits with a custom awaiter that avoids throwing in the case of cancellation or timeout. However, in using a custom awaiter, we get knocked off the highest-perf path that can avoid allocating an Action. This change puts it back onto the golden path by having the awaiter implement our internal interface that lets it backchannel with the async method builders. Eventually we want this awaiter to be available publicly as well, and it'll end up benefiting more than just semaphores.
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Threading;
using System.Threading.Tasks;
[MemoryDiagnoser]
public partial class Program
{
static void Main(string[] args) => BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args);
private CancellationToken _token = new CancellationTokenSource().Token;
private SemaphoreSlim _sem = new SemaphoreSlim(0);
private Task[] _tasks = new Task[100];
[Benchmark]
public Task WaitAsync()
{
for (int i = 0; i < _tasks.Length; i++)
{
_tasks[i] = _sem.WaitAsync(_token);
}
_sem.Release(_tasks.Length);
return Task.WhenAll(_tasks);
}
}
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If a WaitAsync on a SemaphoreSlim is unable to synchronously acquire the semaphore, it calls an async method that handles the waiting. That method awaits with a custom awaiter that avoids throwing in the case of cancellation or timeout. However, in using a custom awaiter, we get knocked off the highest-perf path that can avoid allocating an Action. This change puts it back onto the golden path by having the awaiter implement our internal interface that lets it backchannel with the async method builders. Eventually we want this awaiter to be available publicly as well, and it'll end up benefiting more than just semaphores.