Question

Here the code on this concern:

while (true)
{
    Console.WriteLine("start " + DateTime.Now);
    ParallelOptions options = new ParallelOptions();
    options.MaxDegreeOfParallelism = -1;                
    Parallel.ForEach(hosts, item =>
    {
        using (Ping ping = new Ping())
        {
            PingReply pingReply = ping.Send(item.Value, 2000);  // timeout is 2 secs
            App.dict[item.Key].lastConnectTry = new KeyValuePair<bool, DateTime>((pingReply.Status == IPStatus.Success), DateTime.Now);
        }
    });        
    Console.WriteLine("end " + DateTime.Now);
    Thread.Sleep(15000);
}

But, then I run that app it gives slightly different results:

start 27.04.2012 10:12:32
end 27.04.2012 10:12:42
// it took 10 seconds
start 27.04.2012 10:12:57
end 27.04.2012 10:13:02
// this took 5 secs
start 27.04.2012 10:13:17
end 27.04.2012 10:13:22
//   5 secs
start 27.04.2012 10:13:37
end 27.04.2012 10:13:42
// 5 secs
start 27.04.2012 10:13:57
end 27.04.2012 10:14:01
start 27.04.2012 10:14:16
end 27.04.2012 10:14:19
start 27.04.2012 10:14:34
end 27.04.2012 10:14:36
start 27.04.2012 10:14:51
end 27.04.2012 10:14:54
start 27.04.2012 10:15:09
end 27.04.2012 10:15:11
start 27.04.2012 10:15:26
end 27.04.2012 10:15:29
start 27.04.2012 10:15:44
end 27.04.2012 10:15:46
start 27.04.2012 10:16:01
end 27.04.2012 10:16:06
start 27.04.2012 10:16:21
end 27.04.2012 10:16:24
start 27.04.2012 10:16:39
end 27.04.2012 10:16:41
start 27.04.2012 10:16:56
end 27.04.2012 10:16:59
start 27.04.2012 10:17:14
end 27.04.2012 10:17:16

Seems like it takes some time to bootstrap the threads, creating them and then Parallel execution time will arranged correctly.

So the question is why it takes more than 2 secs to handle all processing and how can I avoid it by bootstrap the threads before processing?

Update:

This loop is placed in separate background thread.

Answer 1

Yes, threadpool needs to grow number of threads based on load (unrelated to Parallel.ForEach).

Check Parallel.ForEach not spinning up new threads and Parallel.Foreach spawning way too many threads for some background.