Also note that recent versions of the Linux kernel have changed what sched_yield() does -- it no longer does essentially what Ralph describes below. Google around to find those discussions.
On Dec 9, 2010, at 4:07 PM, Ralph Castain wrote:
> Sorry for delay - am occupied with my day job.
> Yes, that is correct to an extent. When you yield the processor, all that happens is that you surrender the rest of your scheduled time slice back to the OS. The OS then cycles thru its scheduler and sequentially assigns the processor to the line of waiting processes. Eventually, the OS will cycle back to your process, and you'll begin cranking again.
> So if no other process wants or needs attention, then yes - it will cycle back around to you pretty quickly. In cases where only system processes are running (besides my MPI ones, of course), then I'll typically see cpu usage drop a few percentage points - down to like 95% - because most system tools are very courteous and call yield is they don't need to do something. If there is something out there that wants time, or is less courteous, then my cpu usage can change a great deal.
> Note, though, that top and ps are -very- coarse measuring tools. You'll probably see them reading more like 100% simply because, averaged out over their sampling periods, nobody else is using enough to measure the difference.
> On Dec 9, 2010, at 1:37 PM, Hicham Mouline wrote:
>>> -----Original Message-----
>>> From: users-bounces_at_[hidden] [mailto:users-bounces_at_[hidden]] On
>>> Behalf Of Eugene Loh
>>> Sent: 08 December 2010 16:19
>>> To: Open MPI Users
>>> Subject: Re: [OMPI users] curious behavior during wait for broadcast:
>>> 100% cpu
>>> I wouldn't mind some clarification here. Would CPU usage really
>>> decrease, or would other processes simply have an easier time getting
>>> cycles? My impression of yield was that if there were no one to yield
>>> to, the "yielding" process would still go hard. Conversely, turning on
>>> "yield" would still show 100% cpu, but it would be easier for other
>>> processes to get time.
>> Any clarifications?
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