Horton

Re: remove spurious CREATE INDEX CONCURRENTLY wait

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Michael Paquier <michael@paquier.xyz>, Dmitry Dolgov <9erthalion6@gmail.com>, James Coleman <jtc331@gmail.com>, Pg Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2020-11-10T02:31:15Z
Lists: pgsql-hackers 
On 2020-Nov-09, Tom Lane wrote:

> Michael Paquier <michael@paquier.xyz> writes:
> > On Mon, Nov 09, 2020 at 08:32:13PM -0500, Tom Lane wrote:
> >> Do we really need exclusive lock on the ProcArray to make this flag
> >> change?  That seems pretty bad from a concurrency standpoint.
> 
> > Any place where we update vacuumFlags acquires an exclusive LWLock on
> > ProcArrayLock.  That's held for a very short time, so IMO it won't
> > matter much in practice, particularly if you compare that with the
> > potential gains related to the existing wait phases.
> 
> Not sure I believe that it doesn't matter much in practice.  If there's
> a steady stream of shared ProcArrayLock acquisitions (for snapshot
> acquisition) then somebody wanting exclusive lock will create a big
> hiccup, whether they hold it for a short time or not.

Yeah ... it would be much better if we can make it use atomics instead.
Currently it's an uint8, and in PGPROC itself it's probably not a big
deal to enlarge that, but I fear that quadrupling the size of the
mirroring array in PROC_HDR might be bad for performance.  However,
maybe if we use atomics to access it, then we don't need to mirror it
anymore?  That would need some benchmarking of GetSnapshotData.

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