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Date: Tue, 29 Sep 2009 09:14:48 +0200
From: Corrado Zoccolo <czoccolo@...il.com>
To: Mike Galbraith <efault@....de>
Cc: Vivek Goyal <vgoyal@...hat.com>,
Ulrich Lukas <stellplatz-nr.13a@...enparkplatz.de>,
linux-kernel@...r.kernel.org,
containers@...ts.linux-foundation.org, dm-devel@...hat.com,
nauman@...gle.com, dpshah@...gle.com, lizf@...fujitsu.com,
mikew@...gle.com, fchecconi@...il.com, paolo.valente@...more.it,
ryov@...inux.co.jp, fernando@....ntt.co.jp, jmoyer@...hat.com,
dhaval@...ux.vnet.ibm.com, balbir@...ux.vnet.ibm.com,
righi.andrea@...il.com, m-ikeda@...jp.nec.com, agk@...hat.com,
akpm@...ux-foundation.org, peterz@...radead.org,
jmarchan@...hat.com, torvalds@...ux-foundation.org, mingo@...e.hu,
riel@...hat.com, jens.axboe@...cle.com,
Tobias Oetiker <tobi@...iker.ch>
Subject: Re: IO scheduler based IO controller V10
Hi Mike,
On Mon, Sep 28, 2009 at 8:53 PM, Mike Galbraith <efault@....de> wrote:
> On Mon, 2009-09-28 at 14:18 -0400, Vivek Goyal wrote:
>> On Mon, Sep 28, 2009 at 07:51:14PM +0200, Mike Galbraith wrote:
>
>> I guess changing class to IDLE should have helped a bit as now this is
>> equivalent to setting the quantum to 1 and after dispatching one request
>> to disk, CFQ will always expire the writer once. So it might happen that
>> by the the reader preempted writer, we have less number of requests in
>> disk and lesser latency for this reader.
>
> I expected SCHED_IDLE to be better than setting quantum to 1, because
> max is quantum*4 if you aren't IDLE. But that's not what happened. I
> just retested with all knobs set back to stock, fairness off, and
> quantum set to 1 with everything running nice 0. 2.8 seconds avg :-/
Idle doesn't work very well for async writes, since the writer process
will just send its writes to the page cache.
The real writeback will happen in the context of a kernel thread, with
best effort scheduling class.
>
>> > I saw
>> > the reference to Vivek's patch, and gave it a shot. Makes a large
>> > difference.
>> > Avg
>> > perf stat 12.82 7.19 8.49 5.76 9.32 8.7 anticipatory
>> > 16.24 175.82 154.38 228.97 147.16 144.5 noop
>> > 43.23 57.39 96.13 148.25 180.09 105.0 deadline
>> > 9.15 14.51 9.39 15.06 9.90 11.6 cfq fairness=0 dd=nice 0
>> > 12.22 9.85 12.55 9.88 15.06 11.9 cfq fairness=0 dd=nice 19
>> > 9.77 13.19 11.78 17.40 9.51 11.9 cfq fairness=0 dd=SCHED_IDLE
>> > 4.59 2.74 4.70 3.45 4.69 4.0 cfq fairness=1 dd=nice 0
>> > 3.79 4.66 2.66 5.15 3.03 3.8 cfq fairness=1 dd=nice 19
>> > 2.79 4.73 2.79 4.02 2.50 3.3 cfq fairness=1 dd=SCHED_IDLE
>> >
>>
>> Hmm.., looks like average latency went down only in case of fairness=1
>> and not in case of fairness=0. (Looking at previous mail, average vanilla
>> cfq latencies were around 12 seconds).
>
> Yup.
>
>> Are you running all this in root group or have you put writers and readers
>> into separate cgroups?
>
> No cgroups here.
>
>> If everything is running in root group, then I am curious why latency went
>> down in case of fairness=1. The only thing fairness=1 parameter does is
>> that it lets complete all the requests from previous queue before start
>> dispatching from next queue. On top of this is valid only if no preemption
>> took place. In your test case, konsole should preempt the writer so
>> practically fairness=1 might not make much difference.
>
> fairness=1 very definitely makes a very large difference. All of those
> cfq numbers were logged in back to back runs.
>
>> In fact now Jens has committed a patch which achieves the similar effect as
>> fairness=1 for async queues.
>
> Yeah, I was there yesterday. I speculated that that would hurt my
> reader, but rearranging things didn't help one bit. Playing with merge,
> I managed to give dd ~7% more throughput, and injured poor reader even
> more. (problem analysis via hammer/axe not always most effective;)
>
>> commit 5ad531db6e0f3c3c985666e83d3c1c4d53acccf9
>> Author: Jens Axboe <jens.axboe@...cle.com>
>> Date: Fri Jul 3 12:57:48 2009 +0200
>>
>> cfq-iosched: drain device queue before switching to a sync queue
>>
>> To lessen the impact of async IO on sync IO, let the device drain of
>> any async IO in progress when switching to a sync cfqq that has idling
>> enabled.
>>
>>
>> If everything is in separate cgroups, then we should have seen latency
>> improvements in case of fairness=0 case also. I am little perplexed here..
>>
>> Thanks
>> Vivek
>
>
Thanks,
Corrado
--
__________________________________________________________________________
dott. Corrado Zoccolo mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
--------------------------------------------------------------------------
The self-confidence of a warrior is not the self-confidence of the average
man. The average man seeks certainty in the eyes of the onlooker and calls
that self-confidence. The warrior seeks impeccability in his own eyes and
calls that humbleness.
Tales of Power - C. Castaneda
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