From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: Expensive memory.stat + cpu.stat reads Date: Fri, 14 Jul 2023 13:23:42 -0400 Message-ID: References: <20230706062045.xwmwns7cm4fxd7iu@google.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1689355428; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=IjSCUEVQqiObQodmnZyPQvDEZ31gPn7XJ/bLLSzzo2c=; b=G7EHlYsN9BxuOKBhudcioQVvtVk46H0MzLZGw6w7hoE5FhlJvFM2WRR5q99DQAG1H9PG2K fPYuG2YwRsLY+lMZKD0ViW+8HiyB+X2pz1zir2PSJdq7X0S0lI9r/RjZ/3rfrxWs7hlkG+ OsEiIixGVrDXb+Gd1x1qkDfndnlzz7M= Content-Language: en-US In-Reply-To: List-ID: Content-Type: text/plain; charset="utf-8"; format="flowed" To: Ivan Babrou Cc: Shakeel Butt , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Linux MM , kernel-team , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Andrew Morton , linux-kernel On 7/13/23 19:25, Ivan Babrou wrote: > On Mon, Jul 10, 2023 at 5:44 PM Waiman Long wrote: >> On 7/10/23 19:21, Ivan Babrou wrote: >>> On Wed, Jul 5, 2023 at 11:20 PM Shakeel Butt 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. Cheers, Longman