On Tue, Oct 10, 2023 at 1:45 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > On Mon, Oct 9, 2023 at 8:21 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> 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.
