Re: Inconsistent behavior of smart shutdown handling for queries with and without parallel workers

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Inconsistent behavior of smart shutdown handling for queries with and without parallel workers
Date: 2020-08-11 21:28:42
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Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> writes:
> In case of smart shutdown postmaster receives SIGTERM from the pg_ctl,
> it "disallows new connections, but lets existing sessions end their
> work normally". Which means that it doesn't abort any ongoing txns in
> any of the sessions and it lets the sessions to exit(on their own) and
> then the postmaster is shut down. Looks like this behavior is true
> only if the sessions are executing non-parallel queries. Parallel
> queries are getting aborted, see [1].

Hm. I kind of wonder why we're killing *anything* early in the
smart-shutdown case. postmaster.c has (in pmdie()):

/* autovac workers are told to shut down immediately */
/* and bgworkers too; does this need tweaking? */
/* and the autovac launcher too */
if (AutoVacPID != 0)
signal_child(AutoVacPID, SIGTERM);
/* and the bgwriter too */
if (BgWriterPID != 0)
signal_child(BgWriterPID, SIGTERM);
/* and the walwriter too */
if (WalWriterPID != 0)
signal_child(WalWriterPID, SIGTERM);

and it seems like every one of those actions is pretty damfool if we want
the existing sessions to continue to have normal performance, quite aside
from whether we're breaking parallel queries. Indeed, to the extent that
this is hurting performance, we can expect the existing sessions to take
*longer* to finish, making this pretty counterproductive.

So I'm thinking we should move all of these actions to happen only after
the regular children are all gone.

regards, tom lane

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