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

[Shaohua Li]

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.

Thanks,
Shaohua
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