Messages in this thread | | | Date | Thu, 8 Sep 2022 16:25:47 +0800 | Subject | Re: [PATCH v4] ipc/msg: mitigate the lock contention with percpu counter | From | "Sun, Jiebin" <> |
| |
On 9/8/2022 5:34 AM, Andrew Morton wrote: > On Wed, 07 Sep 2022 09:01:53 -0700 Tim Chen <tim.c.chen@linux.intel.com> wrote: > >> On Thu, 2022-09-08 at 01:25 +0800, Jiebin Sun wrote: >>> The msg_bytes and msg_hdrs atomic counters are frequently >>> updated when IPC msg queue is in heavy use, causing heavy >>> cache bounce and overhead. Change them to percpu_counter >>> greatly improve the performance. Since there is one percpu >>> struct per namespace, additional memory cost is minimal. >>> Reading of the count done in msgctl call, which is infrequent. >>> So the need to sum up the counts in each CPU is infrequent. >>> >>> >>> Apply the patch and test the pts/stress-ng-1.4.0 >>> -- system v message passing (160 threads). >>> >>> Score gain: 3.17x >>> >>> >> ... >>> >>> +/* large batch size could reduce the times to sum up percpu counter */ >>> +#define MSG_PERCPU_COUNTER_BATCH 1024 >>> + >> Jiebin, >> >> 1024 is a small size (1/4 page). >> The local per cpu counter could overflow to the gloabal count quickly >> if it is limited to this size, since our count tracks msg size. >> >> I'll suggest something larger, say 8*1024*1024, about >> 8MB to accommodate about 2 large page worth of data. Maybe that >> will further improve throughput on stress-ng by reducing contention >> on adding to the global count. >> > I think this concept of a percpu_counter_add() which is massively > biased to the write side and with very rare reading is a legitimate > use-case. Perhaps it should become an addition to the formal interface. > Something like > > /* > * comment goes here > */ > static inline void percpu_counter_add_local(struct percpu_counter *fbc, > s64 amount) > { > percpu_counter_add_batch(fbc, amount, INT_MAX); > } > > and percpu_counter_sub_local(), I guess. > > The only instance I can see is > block/blk-cgroup-rwstat.h:blkg_rwstat_add() which is using INT_MAX/2 > because it always uses percpu_counter_sum_positive() on the read side. > > But that makes two!
Yes. Using INT_MAX or INT_MAX/2 could have a big improvement on the performance if heavy writing but rare reading. In our case, if the local percpu counter is near to INT_MAX and there comes a big msgsz, the overflow issue could happen. So I think INT_MAX/2, which is used in blkg_rwstat_add(), might be a better choice. /$ percpu_counter_add_batch(&ns->percpu_msg_bytes, msgsz, batch); /I will send the performance data and draft patch out for discussing.//Jiebin//
| |