Re: Utilizing Duo-Core Processors

"Doug Harrison [MVP]" <>
Tue, 12 May 2009 23:52:20 -0500
On Tue, 12 May 2009 20:20:16 -0700 (PDT), hamishd <>


I have an application which calls a function (X) to process a
computationally heavy task. When the application is running, task-
manager shows it to be running at 50% of the CPU.

If I run 2 of the applications simultaneously, then each use 50% of
the CPU. As such, I can process 2x as much data in the same time.

You're running a dual core system, right? What you've described in
consistent with SMP with two CPUs.

However, I do not want to have to run 2 applications. I want to,
within a single application, call X twice (start 2 threads). Will my
application run at 50% (and as such, each thread 25%) or will each
thread run at 50%?

How do I achieve this?

Threads don't normally run at 50% or 25% or whatever. They run flat out
100% unless they do something to slow themselves down, like entering an
efficient wait state by calling a blocking GetMessage or
WaitForSingleObject. I believe you're observing 50% CPU usage because you
have a single thread running full out on a dual core system. Windows cannot
parallelize your thread to make it run simultaneously in parts on multiple
CPUs or cores, so a single thread will never have more than 50% CPU usage
on a dual core machine; the 50% of two CPUs represents 100% of a single
CPU. You should configure Task Manager to show "One graph per CPU" to get a
better picture of things.

Certainly, you can run as many CPU-intensive threads as you have CPUs. You
can run more, but you're not going to get better performance, because the
threads will compete for the CPUs. So, for three threads on a dual core,
you could expect something less than 66% full speed for each of the
threads, with the "something less" being due to scheduler overhead. It
could become "significantly less" if the threads have to do a lot of
synchronization with each other or are so memory-intensive they cause
paging. (NB: By "full speed", I mean the speed a single thread would have
on a single CPU at the same clock speed as your CPU(s).)

Doug Harrison
Visual C++ MVP

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