Under Mono and MonoTouch, I am seeing an approx 500 millisecond delay between when I call:
StartNew(Action<object> action, object state, CancellationToken cancellationToken,
TaskCreationOptions creationOptions, TaskScheduler scheduler);
and when the worker code actually starts executing.
I created a test to show this:
public static class TestTaskFactory
{
private class TaskInfo
{
public int Number;
}
private static int NUM_TASKS = 5;
private static int NumFinished = 0;
public static void Run()
{
for (int n = 1; n <= NUM_TASKS; n++)
{
Log("Starting task #" + n + " ...");
var task_info = new TaskInfo { Number = n };
var task = Task.Factory.StartNew(Worker, task_info, CancellationToken.None, TaskCreationOptions.None, TaskScheduler.Default);
Thread.Sleep(0);
}
Log("Waiting for tasks to finish ...");
while (NumFinished < NUM_TASKS)
{
Thread.Sleep(1);
}
Log("All done");
}
private static void Worker(object state)
{
var task_info = (TaskInfo)state;
Log("Task #" + task_info.Number + " running");
// Do something
Thread.Sleep(2000);
// Done
++NumFinished;
}
private static void Log(string msg)
{
Console.WriteLine(DateTime.Now.ToString("HH.mm.ss.fff") + ": Thread " + Thread.CurrentThread.ManagedThreadId + ": " + msg);
}
}
Output under Mono on Mac:
16.57.31.420: Thread 1: Starting task #1 ...
16.57.31.508: Thread 1: Starting task #2 ...
16.57.31.508: Thread 1: Starting task #3 ...
16.57.31.508: Thread 1: Starting task #4 ...
16.57.31.508: Thread 1: Starting task #5 ...
16.57.31.508: Thread 1: Waiting for tasks to finish ...
16.57.31.510: Thread 5: Task #1 running
16.57.32.009: Thread 6: Task #2 running <-- Approx 500 msec later
16.57.32.511: Thread 7: Task #3 running <-- Approx 500 msec later
16.57.33.012: Thread 8: Task #4 running <-- Approx 500 msec later
16.57.33.513: Thread 9: Task #5 running <-- Approx 500 msec later
16.57.35.515: Thread 1: All done
It is as if Mono wants to wait up to 500 msec to reuse an existing thread before spawing a new one. If I decrease the worker time below 500 msec, the delay decreases. For example, changing the worker Thread.Sleep(2000) to Thread.Sleep(50):
...
17.13.20.262: Thread 5: Task #1 running
17.13.20.314: Thread 5: Task #2 running <-- approx 50 msec later
17.13.20.365: Thread 5: Task #3 running <-- approx 50 msec later
17.13.20.416: Thread 5: Task #4 running <-- approx 50 msec later
17.13.20.466: Thread 5: Task #5 running <-- approx 50 msec later
But under MS Framework 4.0, no large delay before worker code starts:
...
17.05.42.238: Thread 9: Waiting for tasks to finish ...
17.05.42.256: Thread 11: Task #1 running
17.05.42.256: Thread 12: Task #3 running <-- little delay
17.05.42.256: Thread 13: Task #4 running <-- little delay
17.05.42.257: Thread 10: Task #2 running <-- little delay
17.05.43.264: Thread 14: Task #5 running <-- little delay
Before I submit a bug report on Mono, I wanted to sanity check that I am not missing some tweak I need to make on Mono or using Task.Factory wrong. I am actually using a max concurrency scheduler in my real app.
So my question: is this a bug in Mono/MonoTouch?
Update: I've switched from using ThreadPool under Mono* to Ami Bar's Smart Thread Pool (github; Code Project article). GSerjo's Extended Thread Pool also looked nice, but had alot of dependencies that I was trying to avoid on mobile. I wrote up some of my simple testing at a Xamarim thread. I probably missed 100 other thread pool implementations, but I've been happy with SmartThreadPool so far. Use WINDOWS_PHONE mode to compile under MonoTouch.
