Messages in this thread | | | Date | Tue, 12 Jan 2010 09:49:53 +0800 | From | Shaohua Li <> | Subject | Re: [PATCH] cfq-iosched: rework seeky detection |
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On Mon, Jan 11, 2010 at 10:46:23PM +0800, Corrado Zoccolo wrote: > Hi, > On Mon, Jan 11, 2010 at 2:47 AM, Shaohua Li <shaohua.li@intel.com> 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. > I think I understand what you are proposing, but I don't think request > size should > matter at all for rotational disk. randread a 32k bs definitely has better throughput than a 4k bs. So the request size does matter. From iops point of view, 64k and 4k might not have difference in device, but from performance point of view, they have big difference.
> Usually, the disk firmware will load a big chunk of data in its cache even when > requested to read a single sector, and will provide following ones > from the cache > if you read them sequentially. > > Now, in CFQ, what we really mean by saying that a queue is seeky is that > waiting a bit in order to serve an other request from this queue doesn't > give any benefit w.r.t. switching to an other queue. If no idle, we might switch to a random 4k access or any kind of queues. Compared to continue big request access and switch to other queue with small block, no switching does give benefit.
> So, if you read a single 64k block from disk and then seek, then you can service > any other request without losing bandwidth. But the 64k bs queue loses its slice, which might means device serves more 4k access. As a result, reduce bandwidth.
> Instead, if you are reading 4k, then the next ones (and so on up to 64k, as it > happens with mmap when you fault in a single page at a time), then it > is convenient > to wait for the next request, since it has 3/4 of changes to be > sequential, so be > serviced by cache. > > I'm currently testing a patch to consider request size in SSDs, instead. > In SSDs, the location of the request doesn't mean anything, but the > size is meaningful. > Therefore, submitting together many small requests from different > queues can improve > the overall performance. Agree.
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