Hi Mike, On Mon, Sep 28, 2009 at 8:53 PM, Mike Galbraith <efault@xxxxxx> 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@xxxxxxxxxx> >> 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@xxxxxxxxx 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 _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
