Re: Expensive memory.stat + cpu.stat reads

[Waiman Long]

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.

Cheers,
Longman