Re: Expensive memory.stat + cpu.stat reads

[Ivan Babrou]

On Fri, Jul 14, 2023 at 10:23 AM Waiman Long <longman@xxxxxxxxxx> wrote:
>
> On 7/13/23 19:25, Ivan Babrou wrote:
> > On Mon, Jul 10, 2023 at 5:44 PM Waiman Long <longman@xxxxxxxxxx> wrote:
> >> On 7/10/23 19:21, Ivan Babrou wrote:
> >>> On Wed, Jul 5, 2023 at 11:20 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> >>>> On Fri, Jun 30, 2023 at 04:22:28PM -0700, Ivan Babrou wrote:
> >>>>> Hello,
> >>>>>
> >>>>> We're seeing CPU load issues with cgroup stats retrieval. I made a
> >>>>> public gist with all the details, including the repro code (which
> >>>>> unfortunately requires heavily loaded hardware) and some flamegraphs:
> >>>>>
> >>>>> * https://gist.github.com/bobrik/5ba58fb75a48620a1965026ad30a0a13
> >>>>>
> >>>>> I'll repeat the gist of that gist here. Our repro has the following
> >>>>> output after a warm-up run:
> >>>>>
> >>>>> completed:  5.17s [manual / mem-stat + cpu-stat]
> >>>>> completed:  5.59s [manual / cpu-stat + mem-stat]
> >>>>> completed:  0.52s [manual / mem-stat]
> >>>>> completed:  0.04s [manual / cpu-stat]
> >>>>>
> >>>>> The first two lines do effectively the following:
> >>>>>
> >>>>> for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/memory.stat
> >>>>> /sys/fs/cgroup/system.slice/cpu.stat > /dev/null
> >>>>>
> >>>>> The latter two are the same thing, but via two loops:
> >>>>>
> >>>>> for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/cpu.stat >
> >>>>> /dev/null; done
> >>>>> for _ in $(seq 1 1000); do cat /sys/fs/cgroup/system.slice/memory.stat
> >>>>>> /dev/null; done
> >>>>> As you might've noticed from the output, splitting the loop into two
> >>>>> makes the code run 10x faster. This isn't great, because most
> >>>>> monitoring software likes to get all stats for one service before
> >>>>> reading the stats for the next one, which maps to the slow and
> >>>>> expensive way of doing this.
> >>>>>
> >>>>> We're running Linux v6.1 (the output is from v6.1.25) with no patches
> >>>>> that touch the cgroup or mm subsystems, so you can assume vanilla
> >>>>> kernel.
> >>>>>
> >>>>>   From the flamegraph it just looks like rstat flushing takes longer. I
> >>>>> used the following flags on an AMD EPYC 7642 system (our usual pick
> >>>>> cpu-clock was blaming spinlock irqrestore, which was questionable):
> >>>>>
> >>>>> perf -e cycles -g --call-graph fp -F 999 -- /tmp/repro
> >>>>>
> >>>>> Naturally, there are two questions that arise:
> >>>>>
> >>>>> * Is this expected (I guess not, but good to be sure)?
> >>>>> * What can we do to make this better?
> >>>>>
> >>>>> I am happy to try out patches or to do some tracing to help understand
> >>>>> this better.
> >>>> Hi Ivan,
> >>>>
> >>>> Thanks a lot, as always, for reporting this. This is not expected and
> >>>> should be fixed. Is the issue easy to repro or some specific workload or
> >>>> high load/traffic is required? Can you repro this with the latest linus
> >>>> tree? Also do you see any difference of root's cgroup.stat where this
> >>>> issue happens vs good state?
> >>> I'm afraid there's no easy way to reproduce. We see it from time to
> >>> time in different locations. The one that I was looking at for the
> >>> initial email does not reproduce it anymore:
> >> My understanding of mem-stat and cpu-stat is that they are independent
> >> of each other. In theory, reading one shouldn't affect the performance
> >> of reading the others. Since you are doing mem-stat and cpu-stat reading
> >> repetitively in a loop, it is likely that all the data are in the cache
> >> most of the time resulting in very fast processing time. If it happens
> >> that the specific memory location of mem-stat and cpu-stat data are such
> >> that reading one will cause the other data to be flushed out of the
> >> cache and have to be re-read from memory again, you could see
> >> significant performance regression.
> >>
> >> It is one of the possible causes, but I may be wrong.
> > Do you think it's somewhat similar to how iterating a matrix in rows
> > is faster than in columns due to sequential vs random memory reads?
> >
> > * https://stackoverflow.com/q/9936132
> > * https://en.wikipedia.org/wiki/Row-_and_column-major_order
> > * https://en.wikipedia.org/wiki/Loop_interchange
>
> Yes, it is similar to what is being described in those articles.
>
>
> >
> > I've had a similar suspicion and it would be good to confirm whether
> > it's that or something else. I can probably collect perf counters for
> > different runs, but I'm not sure which ones I'll need.
> >
> > In a similar vein, if we could come up with a tracepoint that would
> > tell us the amount of work done (or any other relevant metric that
> > would help) during rstat flushing, I can certainly collect that
> > information as well for every reading combination.
>
> The perf-c2c tool may be able to help. The data to look for is how often
> the data is from caches vs direct memory load/store.

It looks like c2c only works for the whole system, not individual
treads. There's a lot of noise from the rest of the system.