Re: fio posixaio performance problem

[Gui Jianfeng]

On 2011-8-5 8:56, Gui Jianfeng wrote:
> 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.

Vivek,

BTW, do you have any idea why fio doesn't issue any io in 8ms?

Thanks,
Gui

> 
> Thanks,
> Gui
> 
>>
>> Thanks
>> Vivek
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> 
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