Re: Expensive memory.stat + cpu.stat reads

[Ivan Babrou <ivan@cloudflare.com>]

On Mon, Jul 10, 2023 at 5:44 PM Waiman Long <longman@redhat.com> wrote:
>
> On 7/10/23 19:21, Ivan Babrou wrote:
> > On Wed, Jul 5, 2023 at 11:20 PM Shakeel Butt <shakeelb@google.com> 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

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.