Horton

Re: Improving connection scalability: GetSnapshotData()

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Daniel Gustafsson <daniel@yesql.se>, "Jonathan S. Katz" <jkatz@postgresql.org>, Alexander Korotkov <a.korotkov@postgrespro.ru>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Peter Geoghegan <pg@bowt.ie>, Bruce Momjian <bruce@momjian.us>, David Rowley <dgrowleyml@gmail.com>
Date: 2020-08-16T18:30:24Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> writes:
> 690 successful runs later, it didn't trigger for me :(. Seems pretty
> clear that there's another variable than pure chance, otherwise it seems
> like that number of runs should have hit the issue, given the number of
> bf hits vs bf runs.

It seems entirely likely that there's a timing component in this, for
instance autovacuum coming along at just the right time.  It's not too
surprising that some machines would be more prone to show that than
others.  (Note peripatus is FreeBSD, which we've already learned has
significantly different kernel scheduler behavior than Linux.)

> My current plan would is to push a bit of additional instrumentation to
> help narrow down the issue.

+1

			regards, tom lane

Commits

  1. Try to unbreak 021_row_visibility.pl on mingw.

  2. Fix and test snapshot behavior on standby.

  3. Fix race condition in snapshot caching when 2PC is used.

  4. snapshot scalability: cache snapshots using a xact completion counter.

  5. Fix use of wrong index in ComputeXidHorizons().

  6. Make vacuum a bit more verbose to debug BF failure.

  7. snapshot scalability: Introduce dense array of in-progress xids.

  8. snapshot scalability: Move PGXACT->vacuumFlags to ProcGlobal->vacuumFlags.

  9. snapshot scalability: Move subxact info to ProcGlobal, remove PGXACT.

  10. snapshot scalability: Move PGXACT->xmin back to PGPROC.

  11. snapshot scalability: Don't compute global horizons while building snapshots.

  12. BRIN: Handle concurrent desummarization properly

  13. Track latest completed xid as a FullTransactionId.

  14. Rename VariableCacheData.nextFullXid to nextXid.

  15. snapshot scalability: Move delayChkpt from PGXACT to PGPROC.

  16. Report progress of CREATE INDEX operations