Re: [RFC]cfq-iosched: quantum check tweak

[Corrado Zoccolo]

On Fri, Jan 8, 2010 at 1:57 AM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
> On Fri, Jan 08, 2010 at 05:44:27AM +0800, Corrado Zoccolo wrote:
>> Hi Shahoua,
>>
>> On Thu, Jan 7, 2010 at 3:04 AM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
>> > On Mon, 2009-12-28 at 17:02 +0800, Corrado Zoccolo wrote:
>> >> Hi Shaohua,
>> >> On Mon, Dec 28, 2009 at 4:35 AM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
>> >> > On Fri, Dec 25, 2009 at 05:44:40PM +0800, Corrado Zoccolo wrote:
>> >> >> On Fri, Dec 25, 2009 at 10:10 AM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
>> >> >> > Currently a queue can only dispatch up to 4 requests if there are other queues.
>> >> >> > This isn't optimal, device can handle more requests, for example, AHCI can
>> >> >> > handle 31 requests. I can understand the limit is for fairness, but we could
>> >> >> > do some tweaks:
>> >> >> > 1. if the queue still has a lot of slice left, sounds we could ignore the limit
>> >> >> ok. You can even scale the limit proportionally to the remaining slice
>> >> >> (see below).
>> >> > I can't understand the meaning of below scale. cfq_slice_used_soon() means
>> >> > dispatched requests can finish before slice is used, so other queues will not be
>> >> > impacted. I thought/hope a cfq_slice_idle time is enough to finish the
>> >> > dispatched requests.
>> >> cfq_slice_idle is 8ms, that is the average time to complete 1 request
>> >> on most disks. If you have more requests dispatched on a
>> >> NCQ-rotational disk (non-RAID), it will take more time. Probably a
>> >> linear formula is not the most accurate, but still more accurate than
>> >> taking just 1 cfq_slice_idle. If you can experiment a bit, you could
>> >> also try:
>> >> Âcfq_slice_idle * ilog2(nr_dispatched+1)
>> >> Âcfq_slice_idle * (1<<(ilog2(nr_dispatched+1)>>1))
>> >>
>> >> >
>> >> >> > 2. we could keep the check only when cfq_latency is on. For uses who don't care
>> >> >> > about latency should be happy to have device fully piped on.
>> >> >> I wouldn't overload low_latency with this meaning. You can obtain the
>> >> >> same by setting the quantum to 32.
>> >> > As this impact fairness, so natually thought we could use low_latency. I'll remove
>> >> > the check in next post.
>> >> Great.
>> >> >> > I have a test of random direct io of two threads, each has 32 requests one time
>> >> >> > without patch: 78m/s
>> >> >> > with tweak 1: 138m/s
>> >> >> > with two tweaks and disable latency: 156m/s
>> >> >>
>> >> >> Please, test also with competing seq/random(depth1)/async workloads,
>> >> >> and measure also introduced latencies.
>> >> > depth1 should be ok, as if device can only send one request, it should not require
>> >> > more requests from ioscheduler.
>> >> I mean have a run with, at the same time:
>> >> * one seq reader,
>> >> * h random readers with depth 1 (non-aio)
>> >> * one async seq writer
>> >> * k random readers with large depth.
>> >> In this way, you can see if the changes you introduce to boost your
>> >> workload affect more realistic scenarios, in which various workloads
>> >> are mixed.
>> >> I explicitly add the depth1 random readers, since they are sceduled
>> >> differently than the large (>4) depth ones.
>> > I tried a fio script which does like your description, but the data
>> > isn't stable, especially the write speed, other kind of io speed is
>> > stable. Apply below patch doesn't make things worse (still write speed
>> > isn't stable, other io is stable), so I can't say if the patch passes
>> > the test, but it appears latency reported by fio hasn't change. I adopt
>> > the slice_idle * dispatched approach, which I thought should be safe.
>>
>> I'm doing some tests right now on a single ncq rotational disk, and
>> the average service time when submitting with a high depth is halved
>> w.r.t. depth 1, so I think you could test also with the formula :
>> slice_idle * dispatched / 2. It should give a performance boost,
>> without noticeable impact on latency
> Thanks for looking at it. can you forward your tests to me so I can
> check here?
Sure, you can find them at:
* simple seek time: http://dl.dropbox.com/u/3525644/stride.c
* avg seek time with NCQ: http://dl.dropbox.com/u/3525644/ncq.c
> I'll do more aggressive formula next. It shouldn't impact
> my SSD which is very fast, each request takes less than 1ms. We'd better
> have a mechanism to measure device speed, but jiffies isn't good. I'm thinking
> using sched_clock() which has its issue too like having drift between CPUs.
I'm using ktime_get() successfully. Measuring is the easy part. The
difficult one is decide what to do with the value :)

Thanks
Corrado

>
> Thanks,
> Shaohua
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@xxxxxxxxx
PhD - Department of Computer Science - University of Pisa, Italy
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The self-confidence of a warrior is not the self-confidence of the average
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that self-confidence. The warrior seeks impeccability in his own eyes and
calls that humbleness.
                               Tales of Power - C. Castaneda
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