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

[Shaohua Li]

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. 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.

Thanks,
Shaohua
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