linux-kernel - Re: fio posixaio performance problem

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Date:	Fri, 05 Aug 2011 08:56:21 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Shaohua Li <shli@...nel.org>, Jens Axboe <jaxboe@...ionio.com>,
	linux-kernel@...r.kernel.org
Subject: Re: 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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