Re: track_planning causing performance regression

Hi,

On Mon, Jun 29, 2020 at 7:49 AM Tharakan, Robins <tharar@amazon.com> wrote:
>
> During fully-cached SELECT-only test using pgbench, Postgres v13Beta1 shows
> ~45% performance drop [2] at high DB connection counts (when compared with v12.3)
>
> Disabling pg_stat_statements.track_planning (which is 'On' by default)
> brings the TPS numbers up to v12.3 levels.
>
> The inflection point (in this test-case) is 128 Connections, beyond which the
> TPS numbers are consistently low. Looking at the mailing list [1], this issue
> didn't surface earlier possibly since the regression is trivial at low connection counts.
>
> It would be great if this could be optimized further, or track_planning
> disabled (by default) so as to not trip users upgrading from v12 with pg_stat_statement
> enabled (but otherwise not particularly interested in track_planning).
>
> These are some details around the above test:
>
> pgbench: scale - 100 / threads - 16
> test-duration - 30s each
> server - 96 vCPUs / 768GB - r5.24xl (AWS EC2 instance)
> client - 72 vCPUs / 144GB - c5.18xl (AWS EC2 instance) (co-located with the DB server - Same AZ)
> v12 - REL_12_STABLE (v12.3)
> v13Beta1 - REL_13_STABLE (v13Beta1)
> max_connections = 10000
> shared_preload_libraries = 'pg_stat_statements'
> shared_buffers 128MB

I can't reproduce this on my laptop, but I can certainly believe that
running the same 3 queries using more connections than available cores
will lead to extra overhead.

I disagree with the conclusion though.  It seems to me that if you
really have this workload that consists in these few queries and want
to get better performance, you'll anyway use a connection pooler
and/or use prepared statements, which will make this overhead
disappear entirely, and will also yield an even bigger performance
improvement.  A quick test using pgbench -M prepared, with
track_planning enabled, with still way too many connections already
shows a 25% improvement over the -M simple without track_planning.