Re: Showing /sys/fs/cgroup/memory/memory.stat very slow on some machines

[Michal Hocko <mhocko@kernel.org>]

On Thu 26-07-18 14:35:34, Bruce Merry wrote:
> On 24 July 2018 at 12:05, Bruce Merry <bmerry@ska.ac.za> wrote:
> > To reproduce:
> > 1. Start cadvisor running. I use the 0.30.2 binary from Github, and
> > run it with sudo ./cadvisor-0.30.2 --logtostderr=true
> > 2. Run the Python 3 script below, which repeatedly creates a cgroup,
> > enters it, stats some files in it, and leaves it again (and removes
> > it). It takes a few minutes to run.
> > 3. time cat /sys/fs/cgroup/memory/memory.stat. It now takes about 20ms for me.
> > 4. sudo sysctl vm.drop_caches=2
> > 5. time cat /sys/fs/cgroup/memory/memory.stat. It is back to 1-2ms.
> >
> > I've also added some code to memcg_stat_show to report the number of
> > cgroups in the hierarchy (iterations in for_each_mem_cgroup_tree).
> > Running the script increases it from ~700 to ~41000. The script
> > iterates 250,000 times, so only some fraction of the cgroups become
> > zombies.
> 
> I've discovered that I'd messed up that instrumentation code (it was
> incrementing inside a loop so counted 5x too many cgroups), so some of
> the things I said turn out to be wrong. Let me try again:
> - Running the script generates about 8000 zombies (not 40000), with or
> without Shakeel's patch (for 250,000 cgroups created/destroyed - so
> possibly there is some timing condition that makes them into zombies.
> I've only measured it with 4.17, but based on timing results I have no
> particular reason to think it's wildly different to older kernels.
> - After running the script 5 times (to generate 40K zombies), getting
> the stats takes 20ms with Shakeel's patch and 80ms without it (on
> 4.17.9) - which is a speedup of the same order of magnitude as Shakeel
> observed with non-zombies.
> - 4.17.9 already seems to be an improvement over 4.15: with 40K
> (non-zombie) cgroups, memory.stat time decreases from 200ms to 75ms.
> 
> So with 4.15 -> 4.17.9 plus Shakeel's patch, the effects are reduced
> by an order of magnitude, which is good news. Of course, that doesn't
> solve the fundamental issue of why the zombies get generated in the
> first place. I'm not a kernel developer and I very much doubt I'll
> have the time to try to debug what may turn out to be a race
> condition, but let me know if I can help with testing things.

As already explained. This is not a race. We just simply keep pages
charged to a memcg we are removing and rely on the memory reclaim to
free them when we need that memory for something else. The problem you
are seeing is a side effect of this because a large number of zombies
adds up when we need to get cumulative stats for their parent.
-- 
Michal Hocko
SUSE Labs