Re: [HACKERS] kqueue

On Sat, Jan 25, 2020 at 11:29:11AM +1300, Thomas Munro wrote:
> On Thu, Jan 23, 2020 at 9:38 AM Rui DeSousa  wrote:
> > On Jan 22, 2020, at 2:19 PM, Tom Lane  wrote:
> >> It's certainly possible that to see any benefit you need stress
> >> levels above what I can manage on the small box I've got these
> >> OSes on.  Still, it'd be nice if a performance patch could show
> >> some improved performance, before we take any portability risks
> >> for it.
> 
> You might need more than one CPU socket, or at least lots more cores
> so that you can create enough contention.  That was needed to see the
> regression caused by commit ac1d794 on Linux[1].
> 
> > Here is two charts comparing a patched and unpatched system.
> > These systems are very large and have just shy of thousand
> > connections each with averages of 20 to 30 active queries concurrently
> > running at times including hundreds if not thousand of queries hitting
> > the database in rapid succession.  The effect is the unpatched system
> > generates a lot of system load just handling idle connections where as
> > the patched version is not impacted by idle sessions or sessions that
> > have already received data.
> 
> Thanks.  I can reproduce something like this on an Azure 72-vCPU
> system, using pgbench -S -c800 -j32.  The point of those settings is
> to have many backends, but they're all alternating between work and
> sleep.  That creates a stream of poll() syscalls, and system time goes
> through the roof (all CPUs pegged, but it's ~half system).  Profiling
> the kernel with dtrace, I see the most common stack (by a long way) is
> in a poll-related lock, similar to a profile Rui sent me off-list from
> his production system.  Patched, there is very little system time and
> the TPS number goes from 539k to 781k.
> 
> [1] https://www.postgresql.org/message-id/flat/CAB-SwXZh44_2ybvS5Z67p_CDz%3DXFn4hNAD%3DCnMEF%2BQqkXwFrGg%40mail.gmail.com

Just to add some data...

I tried the kqueue v14 patch on a AWS EC2 m5a.24xlarge (96 vCPU) with
FreeBSD 12.1, driving from a m5.8xlarge (32 vCPU) CentOS 7 system.

I also use pgbench with a scale factor of 1000, with -S -c800 -j32.

Comparing pg 12.1 vs 13-devel (30012a04):

* TPS increased from ~93,000 to ~140,000, ~ 32% increase
* system time dropped from ~ 78% to ~ 70%, ~ 8% decrease
* user time increased from ~16% to ~ 23%, ~7% increase

I don't have any profile data, but I've attached a couple chart showing
the processor utilization over a 15 minute interval from the database
system.

Regards,
Mark
-- 
Mark Wong
2ndQuadrant - PostgreSQL Solutions for the Enterprise
https://www.2ndQuadrant.com/