Re: Possible performance regression in version 10.1 with pgbench read-write tests.

On Fri, Jul 20, 2018 at 7:56 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
>> On 2018-Jul-19, Amit Kapila wrote:
>>> It appears so. I think we should do something about it as the
>>> regression is quite noticeable.
>
> 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.

--
Thomas Munro
http://www.enterprisedb.com