Re: [PATCH] cfq-iosched: rework seeky detection

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Shaohua Li wrote:
> On Sat, Jan 09, 2010 at 11:59:17PM +0800, Corrado Zoccolo wrote:
>> Current seeky detection is based on average seek lenght.
>> This is suboptimal, since the average will not distinguish between:
>> * a process doing medium sized seeks
>> * a process doing some sequential requests interleaved with larger seeks
>> and even a medium seek can take lot of time, if the requested sector
>> happens to be behind the disk head in the rotation (50% probability).
>>
>> Therefore, we change the seeky queue detection to work as follows:
>> * each request can be classified as sequential if it is very close to
>>   the current head position, i.e. it is likely in the disk cache (disks
>>   usually read more data than requested, and put it in cache for
>>   subsequent reads). Otherwise, the request is classified as seeky.
>> * an history window of the last 32 requests is kept, storing the
>>   classification result.
>> * A queue is marked as seeky if more than 1/8 of the last 32 requests
>>   were seeky.
>>
>> This patch fixes a regression reported by Yanmin, on mmap 64k random
>> reads.
> Can we not count a big request (say the request data is >= 32k) as seeky
> regardless the seek distance? In this way we can also make a 64k random sync
> read not as seeky.

  Or maybe we can rely on *dynamic* CFQQ_SEEK_THR in terms of data lenght to 
  determine whether a request should be a seeky one.

> 
> Thanks,
> Shaohua
> 
> 
> 

-- 
Regards
Gui Jianfeng