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SubjectRe: fio posixaio performance problem
On 2011-8-4 22:12, Vivek Goyal wrote:
> On Thu, Aug 04, 2011 at 03:44:55PM +0800, Gui Jianfeng wrote:
>
> [..]
>>> oh, not related per your blktrace. so we have two problems here:
>>> 1. fio doesn't dispatch request in 8ms.
>>> 2. no close request preempt.
>>
>> Yes, these're actual factors why performance is so bad.
>>
>>> both looks quite wield. can you post a longer blktrace output, like
>>> for one second? the piece is too short.
>>
>> Attached.
>
> Gui, few observations from you log file.
>
> - preemption happened 1631 times and did not happen 527 times and idle
> timer fired.
>
> - In some cases where preemption did not happen, next request seems to
> be too far away (more than CFQQ_CLOSE_THR=8K sectors).
>
> - I noticed couple of cases where next request was with-in 8K distanace
> still preemption did not happen. This makes me curious. Can you please

Yes, I also noticed such case...

> put some trace messages in should_preempt() and rq_close() call and see
> what's going on?

Will dig into it.

>
> For example, following trace shows that next request is 5176 sector behind
> the previous one completed. I am wondering why did preemption not take
> place.
>
> 8,0 0 606 2.751892651 16420 D W 512146800 + 8 [fio]
> 8,0 2 579 2.752127950 0 C W 512146800 + 8 [0]
>
> 8,0 0 609 2.752235995 16421 Q WS 512141624 + 8 [fio]
> 8,0 0 610 2.752238859 16421 G WS 512141624 + 8 [fio]
> 8,0 0 612 2.752243818 16421 I W 512141624 + 8 [fio]
> 8,0 0 0 2.752246262 0 m N cfq16421S / insert_request
> 8,0 0 0 2.752247729 0 m N cfq16421S / add_to_rr
> 8,0 2 0 2.759710295 0 m N cfq idle timer fired
>
> Putting some extra trace messages in CFQ might help here. BTW, which
> kernel version are you using? 3.0?

Yes, latest upstream kernel.

Thanks,
Gui

>
> Thanks
> Vivek
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