On Fri, Jul 20, 2018 at 10:52 AM, Thomas Munro < thomas.mu...@enterprisedb.com> wrote:
> On Fri, Jul 20, 2018 at 7:56 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > > > It's not *that* noticeable, as I failed to demonstrate any performance > > difference before committing the patch. I think some more investigation > > is warranted to find out why some other people are getting different > > results > Maybe false sharing is a factor, since sizeof(sem_t) is 32 bytes on > Linux/amd64 and we're probably hitting elements clustered at one end > of the array? Let's see... I tried sticking padding into > PGSemaphoreData and I got ~8% more TPS (72 client on multi socket > box, pgbench scale 100, only running for a minute but otherwise the > same settings that Mithun showed). > > --- a/src/backend/port/posix_sema.c > +++ b/src/backend/port/posix_sema.c > @@ -45,6 +45,7 @@ > typedef struct PGSemaphoreData > { > sem_t pgsem; > + char padding[PG_CACHE_LINE_SIZE - sizeof(sem_t)]; > } PGSemaphoreData; > > That's probably not the right idiom and my tests probably weren't long > enough, but there seems to be some effect here. > I did a quick test applying the patch with same settings as initial mail I have reported (On postgresql 10 latest code) 72 clients CASE 1: Without Patch : TPS 29269.823540 With Patch : TPS 36005.544960. --- 23% jump Just Disabling using unnamed POSIX semaphores: TPS 34481.207959 So it seems that is the issue as the test is being run on 8 node numa machine. I also came across a presentation [1] : slide 20 which says one of those futex architecture is bad for NUMA machine. I am not sure the new fix for same is included as part of Linux version 3.10.0-693.5.2.el7.x86_64 which is on my test machine. [1] https://www.slideshare.net/davidlohr/futex-scaling-for-multicore-systems -- Thanks and Regards Mithun C Y EnterpriseDB: http://www.enterprisedb.com