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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> [email protected]
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--
Jeff Squyres
[email protected]
For corporate legal information go to:
http://www.cisco.com/web/about/doing_business/legal/cri/
