Horton

Re: remove spurious CREATE INDEX CONCURRENTLY wait

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Michael Paquier <michael@paquier.xyz>, Dmitry Dolgov <9erthalion6@gmail.com>, James Coleman <jtc331@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2020-11-18T19:09:28Z
Lists: pgsql-hackers 
Hi,

On 2020-11-17 12:55:01 -0300, Alvaro Herrera wrote:
> ... ah, but I realize now that this means that we can use shared lock
> here, not exclusive, which is already an enormous improvement.  That's
> because ->pgxactoff can only be changed with exclusive lock held; so as
> long as we hold shared, the array item cannot move.

Uh, wait a second. The acquisition of this lock hasn't been affected by
the snapshot scalability changes, and therefore are unrelated to
->pgxactoff changing or not.

In 13 this is:
		LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
		MyPgXact->vacuumFlags |= PROC_IN_VACUUM;
		if (params->is_wraparound)
			MyPgXact->vacuumFlags |= PROC_VACUUM_FOR_WRAPAROUND;
		LWLockRelease(ProcArrayLock);

Lowering this to a shared lock doesn't seem right, at least without a
detailed comment explaining why it's safe. Because GetSnapshotData() etc
look at all procs with just an LW_SHARED ProcArrayLock, changing
vacuumFlags without a lock means that two concurrent horizon
computations could come to a different result.

I'm not saying it's definitely wrong to relax things here, but I'm not
sure we've evaluated it sufficiently.

Greetings,

Andres Freund

Commits

  1. Restore lock level to set vacuum flags

  2. Restore lock level to update statusFlags

  3. Avoid spurious waits in concurrent indexing

  4. Centralize logic for skipping useless ereport/elog calls.

  5. Don't hold ProcArrayLock longer than needed in rare cases

  6. Relax lock level for setting PGPROC->statusFlags

  7. Rename PGPROC->vacuumFlags to statusFlags

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

  9. Allow an autovacuum worker to be interrupted automatically when it is found