Messages in this thread | | | Date | Fri, 14 Jan 2011 08:38:12 +0100 | From | Jens Axboe <> | Subject | Re: [PATCH 1/2]block cfq: make queue preempt work for queues from different workload |
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On 2011-01-14 05:44, Shaohua Li wrote: > 2011/1/12 Shaohua Li <shaohua.li@intel.com>: >> Hi, >> On Wed, Jan 12, 2011 at 05:07:47AM +0800, Corrado Zoccolo wrote: >>> Hi Shaohua, >>> On Tue, Jan 11, 2011 at 9:51 AM, Shaohua Li <shaohua.li@intel.com> wrote: >>>> I got this: >>>> fio-874 [007] 2157.724514: 8,32 m N cfq874 preempt >>>> fio-874 [007] 2157.724519: 8,32 m N cfq830 slice expired t=1 >>>> fio-874 [007] 2157.724520: 8,32 m N cfq830 sl_used=1 disp=0 charge=1 iops=0 sect=0 >>>> fio-874 [007] 2157.724521: 8,32 m N cfq830 set_active wl_prio:0 wl_type:0 >>>> fio-874 [007] 2157.724522: 8,32 m N cfq830 Not idling. st->count:1 >>>> cfq830 is an async queue, and preempted by a sync queue cfq874. But since we >>>> have cfqg->saved_workload_slice mechanism, the preempt is a nop. >>>> Looks currently our preempt is totally broken if the two queues are not from >>>> the same workload type. >>>> Below patch fixes it. This will might make async queue starvation, but it's >>>> what our old code does before cgroup is added. >>> have you measured latency improvements by un-breaking preemption? >>> AFAIK, preemption behaviour changed since 2.6.33, before cgroups were >>> added, and the latency before the changes that weakened preemption in >>> 2.6.33 was far worse. >> Yes. I'm testing a SD card for MeeGo. The random write is very slow (~12k/s) but >> random read is relatively fast > 1M/s. >> >> Without patch: >> write: (groupid=0, jobs=1): err= 0: pid=3876 >> write: io=966656 B, bw=8054 B/s, iops=1 , runt=120008msec >> clat (usec): min=5 , max=1716.3K, avg=88637.38, stdev=207100.44 >> lat (usec): min=5 , max=1716.3K, avg=88637.69, stdev=207100.41 >> bw (KB/s) : min= 0, max= 52, per=168.17%, avg=11.77, stdev= 8.85 >> read: (groupid=0, jobs=1): err= 0: pid=3877 >> read : io=52516KB, bw=448084 B/s, iops=109 , runt=120014msec >> slat (usec): min=7 , max=1918.5K, avg=519.78, stdev=25777.85 >> clat (msec): min=1 , max=2728 , avg=71.17, stdev=216.92 >> lat (msec): min=1 , max=2756 , avg=71.69, stdev=219.52 >> bw (KB/s) : min= 1, max= 1413, per=66.42%, avg=567.22, stdev=461.50 >> >> With patch: >> write: (groupid=0, jobs=1): err= 0: pid=4884 >> write: io=81920 B, bw=677 B/s, iops=0 , runt=120983msec >> clat (usec): min=13 , max=742976 , avg=155694.10, stdev=244610.02 >> lat (usec): min=13 , max=742976 , avg=155694.50, stdev=244609.89 >> bw (KB/s) : min= 0, max= 31, per=inf%, avg= 8.40, stdev=12.78 >> read: (groupid=0, jobs=1): err= 0: pid=4885 >> read : io=133008KB, bw=1108.3KB/s, iops=277 , runt=120022msec >> slat (usec): min=8 , max=1159.1K, avg=164.24, stdev=9116.65 >> clat (msec): min=1 , max=1988 , avg=28.34, stdev=55.81 >> lat (msec): min=1 , max=1989 , avg=28.51, stdev=57.51 >> bw (KB/s) : min= 2, max= 1808, per=51.10%, avg=1133.42, stdev=275.59 >> >> Both read latency/throughput has big difference with the patch, but write >> gets starvation. > Hi Jens and others, > How do you think about the patch?
I think the patch is good. If preemption in some cases makes things worse, then we need to look into those. That's a separate issue.
-- Jens Axboe
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