I think there *was* a decision and it effectively changed how sched_yield()
effectively operates, and that it may not do what we expect any more.
See this thread (the discussion of Linux/sched_yield() comes in the later
messages):
http://www.open-mpi.org/community/lists/users/2010/07/13729.php
I believe there's similar threads in the MPICH mailing list archives; that's
why Dave posted on the OMPI list about it.
We briefly discussed replacing OMPI's sched_yield() with a usleep(1), but it
was shot down.
On Dec 13, 2010, at 10:47 AM, Ralph Castain wrote:
> Thanks for the link!
>
> Just to clarify for the list, my original statement is essentially correct.
> When calling sched_yield, we give up the remaining portion of our time slice.
>
> The issue in the kernel world centers around where to put you in the
> scheduling cycle once you have called sched_yield. Do you go to the end of
> the schedule for your priority? Do you go to the end of the schedule for all
> priorities? Or...where?
>
> Looks like they decided to not decide, and left several options available.
> Not entirely clear of the default, and they recommend we not use sched_yield
> and release the time some other method. We'll take this up on the developer
> list to see what (if anything) we want to do about it.
>
> Bottom line for users: the results remain the same. If no other process wants
> time, you'll continue to see near 100% utilization even if we yield because
> we will always poll for some time before deciding to yield.
>
>
> On Dec 13, 2010, at 7:52 AM, Jeff Squyres wrote:
>
>> See the discussion on kerneltrap:
>>
>> http://kerneltrap.org/Linux/CFS_and_sched_yield
>>
>> Looks like the change came in somewhere around 2.6.23 or so...?
>>
>>
>>
>> On Dec 13, 2010, at 9:38 AM, Ralph Castain wrote:
>>
>>> Could you at least provide a one-line explanation of that statement?
>>>
>>>
>>> On Dec 13, 2010, at 7:31 AM, Jeff Squyres wrote:
>>>
>>>> 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: [email protected] [mailto:[email protected]] 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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>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>
>>>>
>>>> --
>>>> Jeff Squyres
>>>> [email protected]
>>>> For corporate legal information go to:
>>>> http://www.cisco.com/web/about/doing_business/legal/cri/
>>>>
>>>>
>>>> _______________________________________________
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>>>
>>>
>>> _______________________________________________
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>>
>>
>> --
>> Jeff Squyres
>> [email protected]
>> For corporate legal information go to:
>> http://www.cisco.com/web/about/doing_business/legal/cri/
>>
>>
>> _______________________________________________
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>> [email protected]
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
>
> _______________________________________________
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> [email protected]
> http://www.open-mpi.org/mailman/listinfo.cgi/users
--
Jeff Squyres
[email protected]
For corporate legal information go to:
http://www.cisco.com/web/about/doing_business/legal/cri/
