From: | Neha Khatri <nehakhatri5(at)gmail(dot)com> |
---|---|
To: | Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: strange parallel query behavior after OOM crashes |
Date: | 2017-04-06 00:17:17 |
Message-ID: | CAFO0U+8P+5i0rRBya2n-qPLWMoJaaKMYV-PRbPZKGUbWUfMKgA@mail.gmail.com |
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On Wed, Apr 5, 2017 at 5:34 PM, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>
wrote:
> On Tue, Apr 4, 2017 at 12:16 PM, Neha Khatri <nehakhatri5(at)gmail(dot)com>
> wrote:
>
> > I feel there should be an assert if
> > (BackgroundWorkerData->parallel_register_count -
> > BackgroundWorkerData->parallel_terminate_count) > max_parallel_workers)
> >
> Backend 1 > SET max_parallel_worker = 8;
> Backend 1 > Execute a long running parallel query q1 with number of
> parallel worker spawned is say, 4.
> Backend 2> SET max_parallel_worker = 3;
> Backend 2 > Execute a long running parallel query q2 with number of
> parallel worker spawned > 0.
>
> The above assert statement will bring down the server unnecessarily
> while executing q2.
Right, with multiple backends trying to fiddle with max_parallel_workers,
that might bring the server down with the said assert:
Assert(parallel_register_count - parallel_terminate_count <=
max_parallel_workers)
The problem here seem to be the change in the max_parallel_workers value while
the parallel workers are still under execution. So this poses two questions:
1. From usecase point of view, why could there be a need to tweak the
max_parallel_workers exactly at the time when the parallel workers are at
play.
2. Could there be a restriction on tweaking of max_parallel_workers while
the parallel workers are at play? At least do not allow setting the
max_parallel_workers less than the current # of active parallel workers.
Regards,
Neha
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