I have read that await/async should not create/use new threads but the compiler creates a astate machien under the hood....so i tried this, and each call to F1 uses a thread from the ThreadPool.
class Program
{
static System.Threading.SemaphoreSlim sem =
new System.Threading.SemaphoreSlim(20, 20);
static async void F1()
{
await sem.WaitAsync();
await sem.WaitAsync();
await Task.Delay(6000);
var threadId = GetCurrentThreadId();
sem.Release();
sem.Release();
}
static void Main(string[] args)
{
var threadId = GetCurrentThreadId();
F1();
F1();
F1();
F1();
F1();
F1();
F1();
F1();
Task.Delay(30000).Wait();
}
[DllImport("kernel32.dll")]
static extern uint GetCurrentThreadId();
}
You've used await Task.Delay(6000)
, which is guaranteed to not complete synchronously, so: the completion will come back via whatever it can - typically the thread-pool (since there's no sync-context in play here). Thread pool threads are cheap as long as you don't tie them up for extended periods; if all it does is capture the current thread id and release, they should all be complete virtually immediately after they fire.
The code needs to run somewhere ; what thread would you expect it to run on? Your main thread is currently in a blocking Wait()
call, so it can't do anything. Note: you should almost never call .Wait()
on an incomplete Task
.
After some research, i found the answer i was looking for. https://blogs.msdn.microsoft.com/pfxteam/2012/01/20/await-synchronizationcontext-and-console-apps/
Actually Mrinal Kamboj added a comment, to my question, pointing to the right direction. I needed to create a synchronization context for the continuations to run on. This snippet also shows why you should not Wait() on incomplete task: if the continuation runs on the same thread (which happens for GUI threads or ASP.NET) , if this thread is locked waiting for continuation then we run in a dead-lock situation.
class Program
{
static System.Threading.SemaphoreSlim Semaphore = new System.Threading.SemaphoreSlim(3, 3);
static async Task F(int i)
{
Console.WriteLine($"F{i} starts on " + GetCurrentThreadId());
await Semaphore.WaitAsync();
Console.WriteLine($"F{i} continues on " + GetCurrentThreadId());
await Task.Delay(6000); //.ConfigureAwait(false); //<- uncomment to solve deadlock
Console.WriteLine($"F{i} ends on " + GetCurrentThreadId());
Semaphore.Release(1);
//DeadLock();
}
static void DeadLock()
{
F(70).Wait();
}
static void Main(string[] args)
{
var context = new SingleThreadSynchronizationContext();
SynchronizationContext.SetSynchronizationContext(context);
for (int i = 0; i < 10; i++)
{
int ind = i;
F(ind);
}
context.RunOnCurrentThread();
}
[DllImport("kernel32.dll")]
static extern uint GetCurrentThreadId();
}
sealed class SingleThreadSynchronizationContext : SynchronizationContext
{
private readonly Thread Thread = Thread.CurrentThread;
public override void Post(SendOrPostCallback d, object state)
{
if (d == null)
throw new ArgumentNullException("d");
WorkItemsQueue.Add(new KeyValuePair<SendOrPostCallback, object>(d, state));
}
public override void Send(SendOrPostCallback d, object state)
{
throw new NotSupportedException("Synchronously sending is not supported.");
}
public void RunOnCurrentThread()
{
foreach (var workItem in WorkItemsQueue.GetConsumingEnumerable())
workItem.Key(workItem.Value);
}
public void Complete() { WorkItemsQueue.CompleteAdding(); }
}
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