linux-kernel - Re: [PATCH 1/2]block cfq: make queue preempt work for queues from different workload

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Date:	Mon, 17 Jan 2011 11:41:51 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Shaohua Li <shli@...nel.org>
CC:	Corrado Zoccolo <czoccolo@...il.com>,
	Jens Axboe <jaxboe@...ionio.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>
Subject: Re: [PATCH 1/2]block cfq: make queue preempt work for queues from
 different workload

Shaohua Li wrote:
> 2011/1/12 Shaohua Li <shaohua.li@...el.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@...el.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?

Further more, Consider the following piece code.

2132         /*
2133          * For RT and BE, we have to choose also the type
2134          * (SYNC, SYNC_NOIDLE, ASYNC), and to compute a workload
2135          * expiration time
2136          */
2137         st = service_tree_for(cfqg, cfqd->serving_prio, cfqd->serving_type);
2138         count = st->count;
2139 
2140         /*
2141          * check workload expiration, and that we still have other queues ready
2142          */
2143         if (count && !time_after(jiffies, cfqd->workload_expires))
2144                 return;

here, cfqd->serving_prio might be changed. But we continue to check workload expire
to decide whether let the old workload run. I don't think this makes too much sence.
I think if cfqd->serving_prio gets changed, we should recalculate workload type.
Am i missing something?

Thanks
Gui

> 
> Thanks,
> Shaohua
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