Messages in this thread | | | Date | Wed, 13 Jan 2010 09:09:26 +0100 | Subject | Re: [PATCH] cfq-iosched: rework seeky detection | From | Corrado Zoccolo <> |
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On Wed, Jan 13, 2010 at 9:00 AM, Shaohua Li <shaohua.li@intel.com> wrote: > On Wed, Jan 13, 2010 at 03:09:31PM +0800, Corrado Zoccolo wrote: >> On Wed, Jan 13, 2010 at 4:45 AM, Shaohua Li <shaohua.li@intel.com> wrote: >> > 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. >> By aggregate time slice for seeky queues, I mean the time slice >> assigned to the sync-noidle service tree. >> >> > 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. >> >> Exactly for this reason, a seeky queue has no private time slice (it >> is meaningless, since we want multiple seeky queues working in >> parallel), but it participates fairly to the service tree's slice. The >> service tree's slice is computed proportionally to the number of seeky >> queues w.r.t. all queues in the domain, so you also have that seeky >> queues are serviced fairly w.r.t. other queues as well. > Ok, I got your point. An off topic issue: > For a queue with iodepth 1 and a queue with iodepth 32, looks this mechanism can't > guanantee fairness. the queue with big iodepth can submit more requests > in every switch.
Yes. In fact, a queue that reaches large I/O depths will be marked as SYNC_IDLE, and have its dedicated time slice. Your testcase about cfq_quantum falls in this category.
> > Thanks, > Shaohua >
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