Messages in this thread | | | Subject | Re: [PATCH v4 1/4] sched/fair: Introduce primitives for CFS bandwidth burst | From | changhuaixin <> | Date | Fri, 19 Mar 2021 20:51:59 +0800 |
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> On Mar 18, 2021, at 8:59 PM, Phil Auld <pauld@redhat.com> wrote: > > 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. >
Yeah, more quota is helpful for this workload. But that usually prevents us from improving the total CPU usage by putting more work onto a single machine.
> 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? >
I see. Maybe there should be some calculation on the probabilities of that, as Peter has replied.
> > Cheers, > Phil > > --
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