Re: [PATCH v2 3/5] mm: memcg: make stats flushing threshold per-memcg

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Oct 10, 2023 at 2:02 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Tue, Oct 10, 2023 at 1:45 PM Shakeel Butt <shakeelb@google.com> wrote:
> >
> > On Mon, Oct 9, 2023 at 8:21 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> > >
> > > A global counter for the magnitude of memcg stats update is maintained
> > > on the memcg side to avoid invoking rstat flushes when the pending
> > > updates are not significant. This avoids unnecessary flushes, which are
> > > not very cheap even if there isn't a lot of stats to flush. It also
> > > avoids unnecessary lock contention on the underlying global rstat lock.
> > >
> > > Make this threshold per-memcg. The scheme is followed where percpu (now
> > > also per-memcg) counters are incremented in the update path, and only
> > > propagated to per-memcg atomics when they exceed a certain threshold.
> > >
> > > This provides two benefits:
> > > (a) On large machines with a lot of memcgs, the global threshold can be
> > > reached relatively fast, so guarding the underlying lock becomes less
> > > effective. Making the threshold per-memcg avoids this.
> > >
> > > (b) Having a global threshold makes it hard to do subtree flushes, as we
> > > cannot reset the global counter except for a full flush. Per-memcg
> > > counters removes this as a blocker from doing subtree flushes, which
> > > helps avoid unnecessary work when the stats of a small subtree are
> > > needed.
> > >
> > > Nothing is free, of course. This comes at a cost:
> > > (a) A new per-cpu counter per memcg, consuming NR_CPUS * NR_MEMCGS * 4
> > > bytes. The extra memory usage is insigificant.
> > >
> > > (b) More work on the update side, although in the common case it will
> > > only be percpu counter updates. The amount of work scales with the
> > > number of ancestors (i.e. tree depth). This is not a new concept, adding
> > > a cgroup to the rstat tree involves a parent loop, so is charging.
> > > Testing results below show no significant regressions.
> > >
> > > (c) The error margin in the stats for the system as a whole increases
> > > from NR_CPUS * MEMCG_CHARGE_BATCH to NR_CPUS * MEMCG_CHARGE_BATCH *
> > > NR_MEMCGS. This is probably fine because we have a similar per-memcg
> > > error in charges coming from percpu stocks, and we have a periodic
> > > flusher that makes sure we always flush all the stats every 2s anyway.
> > >
> > > This patch was tested to make sure no significant regressions are
> > > introduced on the update path as follows. The following benchmarks were
> > > ran in a cgroup that is 4 levels deep (/sys/fs/cgroup/a/b/c/d), which is
> > > deeper than a usual setup:
> > >
> > > (a) neper [1] with 1000 flows and 100 threads (single machine). The
> > > values in the table are the average of server and client throughputs in
> > > mbps after 30 iterations, each running for 30s:
> > >
> > >                                 tcp_rr          tcp_stream
> > > Base                            9504218.56      357366.84
> > > Patched                         9656205.68      356978.39
> > > Delta                           +1.6%           -0.1%
> > > Standard Deviation              0.95%           1.03%
> > >
> > > An increase in the performance of tcp_rr doesn't really make sense, but
> > > it's probably in the noise. The same tests were ran with 1 flow and 1
> > > thread but the throughput was too noisy to make any conclusions (the
> > > averages did not show regressions nonetheless).
> > >
> > > Looking at perf for one iteration of the above test, __mod_memcg_state()
> > > (which is where memcg_rstat_updated() is called) does not show up at all
> > > without this patch, but it shows up with this patch as 1.06% for tcp_rr
> > > and 0.36% for tcp_stream.
> > >
> > > (b) "stress-ng --vm 0 -t 1m --times --perf". I don't understand
> > > stress-ng very well, so I am not sure that's the best way to test this,
> > > but it spawns 384 workers and spits a lot of metrics which looks nice :)
> > > I picked a few ones that seem to be relevant to the stats update path. I
> > > also included cache misses as this patch introduce more atomics that may
> > > bounce between cpu caches:
> > >
> > > Metric                  Base            Patched         Delta
> > > Cache Misses            3.394 B/sec     3.433 B/sec     +1.14%
> > > Cache L1D Read          0.148 T/sec     0.154 T/sec     +4.05%
> > > Cache L1D Read Miss     20.430 B/sec    21.820 B/sec    +6.8%
> > > Page Faults Total       4.304 M/sec     4.535 M/sec     +5.4%
> > > Page Faults Minor       4.304 M/sec     4.535 M/sec     +5.4%
> > > Page Faults Major       18.794 /sec     0.000 /sec
> > > Kmalloc                 0.153 M/sec     0.152 M/sec     -0.65%
> > > Kfree                   0.152 M/sec     0.153 M/sec     +0.65%
> > > MM Page Alloc           4.640 M/sec     4.898 M/sec     +5.56%
> > > MM Page Free            4.639 M/sec     4.897 M/sec     +5.56%
> > > Lock Contention Begin   0.362 M/sec     0.479 M/sec     +32.32%
> > > Lock Contention End     0.362 M/sec     0.479 M/sec     +32.32%
> > > page-cache add          238.057 /sec    0.000 /sec
> > > page-cache del          6.265 /sec      6.267 /sec      -0.03%
> > >
> > > This is only using a single run in each case. I am not sure what to
> > > make out of most of these numbers, but they mostly seem in the noise
> > > (some better, some worse). The lock contention numbers are interesting.
> > > I am not sure if higher is better or worse here. No new locks or lock
> > > sections are introduced by this patch either way.
> > >
> > > Looking at perf, __mod_memcg_state() shows up as 0.00% with and without
> > > this patch. This is suspicious, but I verified while stress-ng is
> > > running that all the threads are in the right cgroup.
> > >
> > > (3) will-it-scale page_fault tests. These tests (specifically
> > > per_process_ops in page_fault3 test) detected a 25.9% regression before
> > > for a change in the stats update path [2]. These are the
> > > numbers from 30 runs (+ is good):
> > >
> > >              LABEL            |     MEAN    |   MEDIAN    |   STDDEV   |
> > > ------------------------------+-------------+-------------+-------------
> > >   page_fault1_per_process_ops |             |             |            |
> > >   (A) base                    | 265207.738  | 262941.000  | 12112.379  |
> > >   (B) patched                 | 249249.191  | 248781.000  | 8767.457   |
> > >                               | -6.02%      | -5.39%      |            |
> > >   page_fault1_per_thread_ops  |             |             |            |
> > >   (A) base                    | 241618.484  | 240209.000  | 10162.207  |
> > >   (B) patched                 | 229820.671  | 229108.000  | 7506.582   |
> > >                               | -4.88%      | -4.62%      |            |
> > >   page_fault1_scalability     |             |             |
> > >   (A) base                    | 0.03545     | 0.035705    | 0.0015837  |
> > >   (B) patched                 | 0.029952    | 0.029957    | 0.0013551  |
> > >                               | -9.29%      | -9.35%      |            |
> >
> > This much regression is not acceptable.
> >
> > In addition, I ran netperf with the same 4 level hierarchy as you have
> > run and I am seeing ~11% regression.
>
> Interesting, I thought neper and netperf should be similar. Let me try
> to reproduce this.
>
> Thanks for testing!
>
> >
> > More specifically on a machine with 44 CPUs (HT disabled ixion machine):
> >
> > # for server
> > $ netserver -6
> >
> > # 22 instances of netperf clients
> > $ netperf -6 -H ::1 -l 60 -t TCP_SENDFILE -- -m 10K
> >
> > (averaged over 4 runs)
> >
> > base (next-20231009): 33081 MBPS
> > patched: 29267 MBPS
> >
> > So, this series is not acceptable unless this regression is resolved.

I tried this on a machine with 72 cpus (also ixion), running both
netserver and netperf in /sys/fs/cgroup/a/b/c/d as follows:
# echo "+memory" > /sys/fs/cgroup/cgroup.subtree_control
# mkdir /sys/fs/cgroup/a
# echo "+memory" > /sys/fs/cgroup/a/cgroup.subtree_control
# mkdir /sys/fs/cgroup/a/b
# echo "+memory" > /sys/fs/cgroup/a/b/cgroup.subtree_control
# mkdir /sys/fs/cgroup/a/b/c
# echo "+memory" > /sys/fs/cgroup/a/b/c/cgroup.subtree_control
# mkdir /sys/fs/cgroup/a/b/c/d
# echo 0 > /sys/fs/cgroup/a/b/c/d/cgroup.procs
# ./netserver -6

# echo 0 > /sys/fs/cgroup/a/b/c/d/cgroup.procs
# for i in $(seq 10); do ./netperf -6 -H ::1 -l 60 -t TCP_SENDFILE --
-m 10K; done

Base:
540000 262144 10240 60.00 54613.89
540000 262144 10240 60.00 54940.52
540000 262144 10240 60.00 55168.86
540000 262144 10240 60.00 54800.15
540000 262144 10240 60.00 54452.55
540000 262144 10240 60.00 54501.60
540000 262144 10240 60.00 55036.11
540000 262144 10240 60.00 52018.91
540000 262144 10240 60.00 54877.78
540000 262144 10240 60.00 55342.38

Average: 54575.275

Patched:
540000 262144 10240 60.00 53694.86
540000 262144 10240 60.00 54807.68
540000 262144 10240 60.00 54782.89
540000 262144 10240 60.00 51404.91
540000 262144 10240 60.00 55024.00
540000 262144 10240 60.00 54725.84
540000 262144 10240 60.00 51400.40
540000 262144 10240 60.00 54212.63
540000 262144 10240 60.00 51951.47
540000 262144 10240 60.00 51978.27

Average: 53398.295

That's ~2% regression. Did I do anything incorrectly?