Messages in this thread | | | Date | Wed, 13 Jan 2010 11:45:58 +0800 | From | Shaohua Li <> | Subject | Re: [PATCH] cfq-iosched: rework seeky detection |
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On Tue, Jan 12, 2010 at 04:52:59PM +0800, Corrado Zoccolo wrote: > Hi > On Tue, Jan 12, 2010 at 2:49 AM, Shaohua Li <shaohua.li@intel.com> wrote: > > 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. > Assume we have two queues, one with 64k requests, and an other with 4k requests, > and that our ideal disk will service them with the same IOPS 'v'. > Then, servicing for 100ms the first, and then for 100ms the second, we > will have, averaging on the > 200ms period of the schedule: > first queue IOPS = v * 100/200 = v/2 > second queue IOPS = v * 100/200 = v/2 > Now the bandwidth will be simply IOPS * request size. > If instead, you service one request from one queue, and one from the > other (and keep switching for 200ms), > with v IOPS, each queue will obtain again v/2 IOPS, i.e. exactly the > same numbers. > > But, instead, if we have a 2-disk RAID 0, with stripe >= 64k, and the > 64k accesses are aligned (do not cross the stripe), we will have 50% > probability that the requests from the 2 queues are serviced in > parallel, thus increasing the total IOPS and bandwidth. This cannot > happen if you service for 100ms a single depth-1 seeky queue. > > > > >> 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. > CFQ in 2.6.33 works differently than it worked before. > Now, seeky queues have an aggregate time slice, and within this time > slice, you will switch > between seeky queues fairly. So it cannot happen that a seeky queue > loses its time slice. Sorry for my ignorance here, from the code, I know we have a forced slice for a domain and service tree, but for a queue, it appears we haven't an aggregate time slice. From my understanding, we don't add a queue's remaining slice to its next run, and queue might not even init its slice if it's non-timedout preempted before it finishes its first request, which is normal for a seeky queue with a ncq device.
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