linux-kernel - Re: [patch] CFS scheduler, -v12

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Date:	Thu, 24 May 2007 17:43:58 +1000
From:	Peter Williams <pwil3058@...pond.net.au>
To:	Ingo Molnar <mingo@...e.hu>, colpatch@...ibm.com,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Con Kolivas <kernel@...ivas.org>,
	Christoph Lameter <clameter@....com>
CC:	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch] CFS scheduler, -v12

Peter Williams wrote:
> Peter Williams wrote:
>> Peter Williams wrote:
>>> Dmitry Adamushko wrote:
>>>> On 18/05/07, Peter Williams <pwil3058@...pond.net.au> wrote:
>>>> [...]
>>>>> One thing that might work is to jitter the load balancing interval a
>>>>> bit.  The reason I say this is that one of the characteristics of top
>>>>> and gkrellm is that they run at a more or less constant interval (and,
>>>>> in this case, X would also be following this pattern as it's doing
>>>>> screen updates for top and gkrellm) and this means that it's possible
>>>>> for the load balancing interval to synchronize with their intervals
>>>>> which in turn causes the observed problem.
>>>>
>>>> Hum.. I guess, a 0/4 scenario wouldn't fit well in this explanation..
>>>
>>> No, and I haven't seen one.
>>>
>>>> all 4 spinners "tend" to be on CPU0 (and as I understand each gets
>>>> ~25% approx.?), so there must be plenty of moments for
>>>> *idle_balance()* to be called on CPU1 - as gkrellm, top and X consume
>>>> together just a few % of CPU. Hence, we should not be that dependent
>>>> on the load balancing interval here..
>>>
>>> The split that I see is 3/1 and neither CPU seems to be favoured with 
>>> respect to getting the majority.  However, top, gkrellm and X seem to 
>>> be always on the CPU with the single spinner.  The CPU% reported by 
>>> top is approx. 33%, 33%, 33% and 100% for the spinners.
>>>
>>> If I renice the spinners to -10 (so that there load weights dominate 
>>> the run queue load calculations) the problem goes away and the 
>>> spinner to CPU allocation is 2/2 and top reports them all getting 
>>> approx. 50% each.
>>
>> For no good reason other than curiosity, I tried a variation of this 
>> experiment where I reniced the spinners to 10 instead of -10 and, to 
>> my surprise, they were allocated 2/2 to the CPUs on average.  I say on 
>> average because the allocations were a little more volatile and 
>> occasionally 0/4 splits would occur but these would last for less than 
>> one top cycle before the 2/2 was re-established.  The quickness of 
>> these recoveries would indicate that it was most likely the idle 
>> balance mechanism that restored the balance.
>>
>> This may point the finger at the tick based load balance mechanism 
>> being too conservative
> 
> The relevant code, find_busiest_group() and find_busiest_queue(), has a 
> lot of code that is ifdefed by CONFIG_SCHED_MC and CONFIG_SCHED_SMT and, 
> as these macros were defined in the kernels I was testing with, I built 
> a kernel with these macros undefined and reran my tests.  The 
> problems/anomalies were not present in 10 consecutive tests on this new 
> kernel.  Even better on the few occasions that a 3/1 split did occur it 
> was quickly corrected to 2/2 and top was reporting approx 49% of CPU for 
> all spinners throughout each of the ten tests.
> 
> So all that is required now is an analysis of the code inside the ifdefs 
> to see why it is causing a problem.

Further testing indicates that CONFIG_SCHED_MC is not implicated and
it's CONFIG_SCHED_SMT that's causing the problem.  This rules out the
code in find_busiest_group() as it is common to both macros.

I think this makes the scheduling domain parameter values the most
likely cause of the problem.  I'm not very familiar with this code so 
I've added those who've modified this code in the last year or so to the 
address of this e-mail.

Peter
-- 
Peter Williams                                   pwil3058@...pond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce

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