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

[Vivek Goyal]

On Mon, Jan 11, 2010 at 10:34:09AM +0800, Shaohua Li wrote:
> 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.

slice_idle is a variable and user can easily change it to 1ms and even 0.
In that case you will be theoritically be ready to dispatch 100/1 requests
from the cfqq?

> 
> > > 
> > > 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 is software logic driven and not cfq_quantum driven. We can always
keep on changing how to approximate service time completions. So a user
first needs to read the code, derive internal limits and then do testing?

I think than tunable looses its significance. That's why I am advocating
of treating cfq_quantum as hard limit and derive an internal soft limit
based on certain % of hard limit and use that as default max queue depth
for cfqq.

In this case user knows no matter what, you are not dispatching more than
cfq_quantum requests from a queue at a time.

>  
> > > 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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