Messages in this thread | | | Date | Mon, 10 Jul 2023 20:44:15 -0400 | Subject | Re: Expensive memory.stat + cpu.stat reads | From | Waiman Long <> |
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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.
Cheers, Longman
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