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Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

Masahiko Sawada <masahiko.sawada@2ndquadrant.com>

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From: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, Michael Paquier <michael@paquier.xyz>, Mithun Cy <mithun.cy@enterprisedb.com>, Thomas Munro <thomas.munro@enterprisedb.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2020-02-12T02:05:48Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Allow page lock to conflict among parallel group members.

  2. Allow relation extension lock to conflict among parallel group members.

  3. Add assert to ensure that page locks don't participate in deadlock cycle.

  4. Assert that we don't acquire a heavyweight lock on another object after

  5. Fix unsafe usage of strerror(errno) within ereport().

On Wed, 12 Feb 2020 at 00:43, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> I took a brief look through this patch.  I agree with the fundamental
> idea that we shouldn't need to use the heavyweight lock manager for
> relation extension, since deadlock is not a concern and no backend
> should ever need to hold more than one such lock at once.  But it feels
> to me like this particular solution is rather seriously overengineered.
> I would like to suggest that we do something similar to Robert Haas'
> excellent hack (daa7527af) for the !HAVE_SPINLOCK case in lmgr/spin.c,
> that is,
>
> * Create some predetermined number N of LWLocks for relation extension.

My original proposal used LWLocks and hash tables for relation
extension but there was a discussion that using LWLocks is not good
because it's not interruptible[1]. Because of this reason and that we
don't need to have two lock level (shared, exclusive) for relation
extension lock we ended up with implementing dedicated lock manager
for extension lock. I think we will have that problem if we use LWLocks.

Regards,

[1] https://www.postgresql.org/message-id/CA%2BTgmoZnWYQvmeqeGyY%2B0j-Tfmx8cTzRadfxJQwK9A-nCQ7GkA%40mail.gmail.com



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Masahiko Sawada            http://www.2ndQuadrant.com/
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