Re: [PATCH] cfq: Make use of service count to estimate the rb_keyoffset

[Jens Axboe]

On Mon, Nov 30 2009, Gui Jianfeng wrote:
> Corrado Zoccolo wrote:
> > Hi Guy,
> > On Fri, Nov 27, 2009 at 2:42 AM, Gui Jianfeng
> > <guijianfeng@xxxxxxxxxxxxxx> wrote:
> >> Corrado Zoccolo wrote:
> >>> Hi Gui, Jens
> >>> On Thu, Nov 26, 2009 at 7:20 AM, Gui Jianfeng
> >>> <guijianfeng@xxxxxxxxxxxxxx> wrote:
> >>>> Hi Jens, Czoccolo
> >>>>
> >>>> For the moment, different workload cfq queues are put into different
> >>>> service trees. But CFQ still uses "busy_queues" to estimate rb_key
> >>>> offset when inserting a cfq queue into a service tree. I think this
> >>>> isn't appropriate, and it should make use of service tree count to do
> >>>> this estimation. This patch is for for-2.6.33 branch.
> >>> In cfq_choose_wl, we rely on consistency of rb_keys across service
> >>> trees to compute the next workload to be serviced.
> >>>         for (i = 0; i < 3; ++i) {
> >>>                 /* otherwise, select the one with lowest rb_key */
> >>>                 queue = cfq_rb_first(service_tree_for(prio, i, cfqd));
> >>>                 if (queue &&
> >>>                     (!key_valid || time_before(queue->rb_key, lowest_key))) {
> >>>                         lowest_key = queue->rb_key;
> >>>                       cur_best = i;
> >>>                       key_valid = true;
> >>>               }
> >>>         }
> >>>
> >>> If you change how the rb_key is computed (so it is no longer
> >>> consistent across service trees) without changing how it is used can
> >>> introduce problems.
> >>  Ok, I think I was missing this part. This part still behaves like old CFQ regardless
> >>  of workload type. I'm wondering why you prefer starting from sync no-idle only when
> >>  priorities switched, after that, you do it like old CFQ behavior?
> > 
> > When switching priorities (e.g. from RT to BE), we may come from a
> > long stall. In this case, I think it is better to run no-idle first.
> > During normal operation, instead, we want a fair, starvation free way
> > to switch between workloads, and I thought it was simpler to mimic old
> > CFQ behaviour, instead of cook up a different method.
> > The difference between new and old CFQ is that now, when we decide to
> > service one no-idle request, we will then service subsequent ones from
> > the same workload type.
> > This allows processing them optimally on NCQ hardware.
> > Moreover, when no more no-idle requests are available, but the
> > timeslice for this workload did not expire yet, we will wait for more.
> > This guarantees fairness for no-idle workload.
> > 
> >>  In order to improve
> >>  latency for sync no-idle workload, is it possible to take workload type into account,
> >>  not only rely on rb_keys across service trees?
> > When loading a program into memory, your process will go through
> > various phases w.r.t. disk access pattern: some are seeky, some others
> > are sequential.
> > 
> > If you just improve latency for one workload, penalizing the others,
> > you won't get an overall improvement of the system.
> > The new scheme improves overall system behaviour because grouping
> > no-idle requests together gives a better utilization of the disk, and
> > fairness allows also processes making seeky requests to progress.
> > Penalizing the idle service tree, instead, you will give you lower
> > overall throughput (forbidding progress to the processes that make
> > sequential requests), while penalizing writeback you will find
> > yourself waiting for freeing dirty pages more often, and maybe
> > incurring in OOM conditions.
> > 
> > Regarding the rb_key computation, I have done various experiments, and
> > found that the actual formula doesn't matter much on rotational
> > hardware, where the slice length has most importance.
> > But I think it is essential on NCQ SSDs, to obtain fairness.
> > Unfortunately, I don't have an NCQ SSD, so I can't test my improvement ideas.
> > 
> 
> Corrado, thanks for the detailed explanation.
> 
> Jens, I think Corrado is right, we still need consistency of rb_keys
> across service to compute next workload type. So would you revert this
> patch please?

Yes, I agree. I thought I had already reverted it, will do so now.

-- 
Jens Axboe

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