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-boun...@open-mpi.org [mailto:users-boun...@open-mpi.org] 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? >> >> _______________________________________________ >> users mailing list >> us...@open-mpi.org >> http://www.open-mpi.org/mailman/listinfo.cgi/users > > > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users -- Jeff Squyres jsquy...@cisco.com For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/
