[Devel] Re: IO scheduler based IO controller V10

[Mike Galbraith]

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 :-/

> > 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 at oracle.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

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