Horton

Re: "as quickly as possible" (was: 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: 2020-11-18T22:48:40Z
Lists: pgsql-hackers 
Hi,

On 2020-11-18 18:41:27 -0300, Alvaro Herrera wrote:
> The amount of stuff that this is doing with ProcArrayLock held
> exclusively seems a bit excessive; it sounds like we could copy the
> values we need first, release the lock, and *then* do all that memory
> allocation and string printing -- it's a lock of code for such a
> contended lock.

Yea, that's a good point.


> Anytime a process sees itself as blocked by autovacuum
> and wants to signal it, there's a ProcArrayLock hiccup (I didn't
> actually measure it, but it's at least five function calls).

I'm a bit doubtful it's that important - it's limited in frequency
by deadlock_timeout. But worth improving anyway.


> We could make this more concurrent by copying lock->tag to a local
> variable, releasing the lock, then doing all the string formatting and
> printing.  See attached quickly.patch.

Sounds like a plan.


> Now, when this code was written (d7318d43d, 2012), this was a LOG
> message; it was demoted to DEBUG1 later (d8f15c95bec, 2015).  I think it
> would be fair to ... remove the message?  Or go back to Simon's original
> formulation from commit acac68b2bca, which had this message as DEBUG2
> without any string formatting.

I don't really have an opinion on this.

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