On Fri, Jul 20, 2018 at 7:56 AM, Tom Lane <tgl@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).
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.