Re: Why INFINITE loop in a thread occupy so much CPU time??
On Sat, 17 Nov 2007 23:59:56 GMT, Geoff <geoff@invalid.invalid> wrote:
The empty loop: for(;;); assembles to:
here: jmp short here
and this will generate 100% cpu utilization but won't degrade the
system at all.
You will of course observe slowdowns WRT other CPU-intensive threads in
proportion to the number of instances of the for(;;) program you run. Try
running multiple such programs on an SMP system and watch the system slow
down big time. IIRC, it has a worse effect on the UI than on a uniprocessor
system, which I remember finding quite strange. (This would have been on
Win2K or WinXP; I'm pretty sure I wasn't imagining things when I observed
it, but it has been several years.)
<snip>
In summary, if you MUST poll or write infinite loops, call Sleep(1) to
yield back to idle and keep CPU utilization under control.
I dunno, if I'm running a CPU-intensive task, I normally want it to
complete as quickly as possible, and I don't want it sleeping on the job.
I'm not sure what the "infinite loop" concern is, since a loop that is just
spinning for long periods of time and isn't accomplishing useful work has
no reason to exist. As for polling, it can usually be replaced by WFSO or
similar function, using condition variables, etc, which eliminates the need
for calling Sleep. Explicitly yielding to avoid hogging the CPU is mainly a
technique to help with cooperative multitasking, and it isn't much needed
when running under a preemptive scheduler.
--
Doug Harrison
Visual C++ MVP