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

[Shaohua Li]

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@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> 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.

Thanks,
Shaohua
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