Re: [PATCH v4 1/4] sched/fair: Introduce primitives for CFS bandwidth burst

Phil Auld
2021-03-18T13:00:40Z

On Thu, Mar 18, 2021 at 09:26:58AM +0800 changhuaixin wrote:
>
>
> > On Mar 17, 2021, at 4:06 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Mar 17, 2021 at 03:16:18PM +0800, changhuaixin wrote:
> >
> >>> Why do you allow such a large burst? I would expect something like:
> >>>
> >>> if (burst > quote)
> >>> return -EINVAL;
> >>>
> >>> That limits the variance in the system. Allowing super long bursts seems
> >>> to defeat the entire purpose of bandwidth control.
> >>
> >> I understand your concern. Surely large burst value might allow super
> >> long bursts thus preventing bandwidth control entirely for a long
> >> time.
> >>
> >> However, I am afraid it is hard to decide what the maximum burst
> >> should be from the bandwidth control mechanism itself. Allowing some
> >> burst to the maximum of quota is helpful, but not enough. There are
> >> cases where workloads are bursty that they need many times more than
> >> quota in a single period. In such cases, limiting burst to the maximum
> >> of quota fails to meet the needs.
> >>
> >> Thus, I wonder whether is it acceptable to leave the maximum burst to
> >> users. If the desired behavior is to allow some burst, configure burst
> >> accordingly. If that is causing variance, use share or other fairness
> >> mechanism. And if fairness mechanism still fails to coordinate, do not
> >> use burst maybe.
> >
> > It's not fairness, bandwidth control is about isolation, and burst
> > introduces interference.
> >
> >> In this way, cfs_b->buffer can be removed while cfs_b->max_overrun is
> >> still needed maybe.
> >
> > So what is the typical avg,stdev,max and mode for the workloads where you find
> > you need this?
> >
> > I would really like to put a limit on the burst. IMO a workload that has
> > a burst many times longer than the quota is plain broken.
>
> I see. Then the problem comes down to how large the limit on burst shall be.
>
> I have sampled the CPU usage of a bursty container in 100ms periods. The statistics are:
> average : 42.2%
> stddev : 81.5%
> max : 844.5%
> P95 : 183.3%
> P99 : 437.0%
>
> If quota is 100000ms, burst buffer needs to be 8 times more in order for this workload not to be throttled.
> I can't say this is typical, but these workloads exist. On a machine running Kubernetes containers,
> where there is often room for such burst and the interference is hard to notice, users would prefer
> allowing such burst to being throttled occasionally.
>

I admit to not having followed all the history of this patch set. That said, when I see the above I just
think your quota is too low for your workload.

The burst (mis?)feature seems to be a way to bypass the quota. And it sort of assumes cooperative
containers that will only burst when they need it and then go back to normal.

> In this sense, I suggest limit burst buffer to 16 times of quota or around. That should be enough for users to
> improve tail latency caused by throttling. And users might choose a smaller one or even none, if the interference
> is unacceptable. What do you think?
>

Having quotas that can regularly be exceeded by 16 times seems to make the concept of a quota
meaningless. I'd have thought a burst would be some small percentage.

What if several such containers burst at the same time? Can't that lead to overcommit that can effect
other well-behaved containers?

Cheers,
Phil

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