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Signaling Constructs and Performance
Waiting or signaling an AutoResetEvent or ManualResetEvent takes about one microsecond
(assuming no blocking).
ManualResetEventSlim and CountdownEvent can be up to 50 times faster in shortwait
scenarios, because of their nonreliance on the operating system and judicious
use of spinning constructs.
In most scenarios, however, the overhead of the signaling classes themselves
doesn’t create a bottleneck, and so is rarely a consideration. An exception is with
highly concurrent code, which we’ll discuss in the following chapter.
A ManualResetEvent is useful in allowing one thread to unblock many other threads.
The reverse scenario is covered by CountdownEvent.
CountdownEvent
CountdownEvent lets you wait on more than one thread. The class is new to Framework
4.0 and has an efficient, fully managed implementation. To use the class, instantiate
it with the number of threads or “counts” that you want to wait on:
var countdown = new CountdownEvent (3); // Initialize with "count" of 3.
Calling Signal decrements the “count”; calling Wait blocks until the count goes
down to zero. For example:
static CountdownEvent _countdown = new CountdownEvent (3);
static void Main()
{
new Thread (SaySomething).Start ("I am thread 1");
new Thread (SaySomething).Start ("I am thread 2");
new Thread (SaySomething).Start ("I am thread 3");
_countdown.Wait(); // Blocks until Signal has been called 3 times
Console.WriteLine ("All threads have finished speaking!");
}
static void SaySomething (object thing)
{
Thread.Sleep (1000);
Console.WriteLine (thing);
_countdown.Signal();
}
Problems for which CountdownEvent is effective can sometimes
be solved more easily using the structured parallelism constructs
that we’ll cover in the following chapter (PLINQ and the
Parallel class).
Signaling with Event Wait Handles | 837
Threading