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

[Shaohua Li]

On Sat, Jan 09, 2010 at 04:59:48AM +0800, Vivek Goyal wrote:
> On Fri, Jan 08, 2010 at 09:35:33PM +0100, Corrado Zoccolo wrote:
> > On Fri, Jan 8, 2010 at 6:15 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> > > On Thu, Jan 07, 2010 at 10:44:27PM +0100, 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:
> > >> >>  cfq_slice_idle * ilog2(nr_dispatched+1)
> > >> >>  cfq_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.
> > >>
> > >
> > > But I guess the right comparison here would service times vary when we
> > > push queue depths from 4 to higher (as done by this patch).
> > 
> > I think here we want to determine the average cost of a request, when
> > there are many submitted.
> > 
> > > Were you
> > > running deep seeky queues or sequential queues. Curious to know whether
> > > service times reduced even in case of deep seeky queues on this single
> > > disk.
> > 
> > Seeky queues. Seeks where rather small (not more than 1/64 of the
> > whole disk), but already meaningful for comparison.
> > 
> > >
> > > I think this patch breaks the meaning of cfq_quantum? Now we can allow
> > > dispatch of more requests from the same queue. I had kind of liked the
> > > idea of respecting cfq_quantum. Especially it can help in testing. With
> > > this patch cfq_quantum will more or less loose its meaning.
> > cfq_quantum will still be enforced at the end of the slice, so its
> > meaning of how many requests can be still pending when you finish your
> > slice is preserved.
> 
> Not always and it will depend how accurate your approximation of service
> time is. If per request completion time is more than approximation (in
> this case slice_idle), than you will end up with more requests in dispatch
> queue from one cfqq at the time of slice expiry.
we use slice_idle for a long time and no complain. So assume the approximation
of service time is good.

> > 
> > One can argue, instead, that this reduces a bit the effectiveness of
> > preemption on ncq disks.
> > However, I don't think preemption is the solution for low latency,
> > while cfq_quantum reduction is.
> > With this change in place, we could change the default cfq_quantum to
> > a smaller number (ideally 1), to have lowest number of leftovers when
> > the slice finishes, while still driving deep queues at the beginning
> > of the slice.
> 
> I think using cfq_quantum as hard limit might be a better idea as it gives
> more predictable control. Instead of treating it as soft limit and trying
> to meet it at the end of slice expiry based on our approximation of
> predicted completion time.
Current patch has such hard limit too (100ms/8m = 12 for sync io and 40ms/8
 = 5 for async io).
 
> > This needs thorough testing, though. Maybe it is better to delay those
> > changes to 2.6.34...
> 
> Agreed. This should be tested more throughly and should be candidate for
> 2.6.34.
Sure, this needs a lot of test.

Thanks,
Shaohua
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