为什么ThreadPool仅启动一个线程？

Question

I start threads exactly like book says:

for (int i = 1; i <= 4; i++) {
  ThreadPool.QueueUserWorkItem(new WaitCallback(ThreadMethod), i);
}

ThreadMethod looks like:

static void ThreadMethod(object input) {
  Console.WriteLine(input + " thread started");
  //do some stuff for, like, 400 milliseconds
  Console.WriteLine(input + " thread completed");
}

In some reason 2 thread starts only after 1 is completed (in this moment all work is already done and 2-4 thread just start and stop doing nothing).

What could be wrong? Ask anything what could help solve this problem. I don't use any synchronization classes.

If it's matter, i have 2 core processor.

Answer 1

Your ThreadMethod just runs too fast. Everything is right with your code except if possible you should switch from ThreadPool to new abstractions like Task.Run .

Answer 2

Actually, as i made ThreadMethod to do much longer stuff, another threads started after some time but it don't worked in one time but just switches from thread to thread. Looks like i have to use another tool.

Answer 3

A TreadPool has a maximum number of threads that it can use. See ThreadPool.GetMaxThread and SetMaxThread. It is probably equal to the number of available cores by default.

For CPU intensive work it makes sense since you would actually lower performance by using more threads than you have cores. However, for slow jobs such as I/O intensive jobs, many threads can run in parallel to avoid blocking and wait until the I/O is complete. Example: Grabing several files at a time from various FTP servers.