IO scheduler based IO controller V10
efault at gmx.de
Mon Sep 28 11:53:54 PDT 2009
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).
> 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
> 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..
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