Horton

Re: PG 15 (and to a smaller degree 14) regression due to ExprEvalStep size

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Peter Geoghegan <pg@bowt.ie>, PostgreSQL-development <pgsql-hackers@postgresql.org>, Nikita Glukhov <n.gluhov@postgrespro.ru>, Andrew Dunstan <andrew@dunslane.net>
Date: 2022-06-17T06:24:56Z
Lists: pgsql-hackers 
Hi,

On 2022-06-16 22:22:28 -0700, Andres Freund wrote:
> On 2022-06-17 16:53:31 +1200, David Rowley wrote:
> > On Fri, 17 Jun 2022 at 15:33, Peter Geoghegan <pg@bowt.ie> wrote:
> > > Have you tried this with the insert benchmark [1]?
> >
> > I was mostly focusing on the performance of the hashed saop feature
> > after having removed the additional fields that pushed ExprEvalStep
> > over 64 bytes in 14.
> >
> > I agree it would be good to do further benchmarking to see if there's
> > anything else that's snuck into 15 that's slowed that benchmark down,
> > but we can likely work on that after we get the ExprEvalStep size back
> > to 64 bytes again.
>
> I did reproduce a regression between 14 and 15, using both pgbench -Mprepared
> -S (scale 1) and TPC-H Q01 (scale 5). Between 7-10% - not good, particularly
> that that's not been found so far. Fixing the json size issue gets that down
> to ~2%. Not sure what that's caused by yet.

The remaining difference looks like it's largely caused by the
enable_timeout_after(IDLE_STATS_UPDATE_TIMEOUT, ...) introduced as part of the
pgstats patch. It's only really visible when I pin a single connection pgbench
to the same CPU core as the server (which gives a ~16% boost here).

It's not the timeout itself - that we amortize nicely (via 09cf1d522). It's
that enable_timeout_after() does a GetCurrentTimestamp().

Not sure yet what the best way to fix that is.

We could just leave the timer active and add some gating condition indicating
idleness to the IdleStatsUpdateTimeoutPending body in ProcessInterrupts()?

Or we could add a timeout.c API that specifies the timeout?
pgstat_report_stat() uses GetCurrentTransactionStopTimestamp(), it seems like
it'd make sense to use the same for arming the timeout?

- Andres

Commits

  1. JSON_TABLE: Add support for NESTED paths and columns

  2. Add basic JSON_TABLE() functionality

  3. Add SQL/JSON query functions

  4. Add soft error handling to some expression nodes

  5. Adjust populate_record_field() to handle errors softly

  6. Refactor code used by jsonpath executor to fetch variables

  7. Add more SQL/JSON constructor functions

  8. SQL/JSON: support the IS JSON predicate

  9. SQL/JSON: add standard JSON constructor functions

  10. Add static assertion ensuring sizeof(ExprEvalStep) <= 64 bytes

  11. Remove size increase in ExprEvalStep caused by hashed saops

  12. pgstat: reduce timer overhead by leaving timer running.

  13. expression eval: Fix EEOP_JSON_CONSTRUCTOR and EEOP_JSONEXPR size.

  14. SQL/JSON query functions

  15. Speedup ScalarArrayOpExpr evaluation