Horton

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

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Masahiko Sawada <masahiko.sawada@2ndquadrant.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: 2020-02-14T14:42:40Z
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().

Amit Kapila <amit.kapila16@gmail.com> writes:
> I think MaxBackends will generally limit the number of different
> relations that can simultaneously extend, but maybe tables with many
> partitions might change the situation.  You are right that some tests
> might suggest a good number, let Mahendra write a patch and then we
> can test it.  Do you have any better idea?

In the first place, there certainly isn't more than one extension
happening at a time per backend, else the entire premise of this
thread is wrong.  Handwaving about partitions won't change that.

In the second place, it's ludicrous to expect that the underlying
platform/filesystem can support an infinite number of concurrent
file-extension operations.  At some level (e.g. where disk blocks
are handed out, or where a record of the operation is written to
a filesystem journal) it's quite likely that things are bottlenecked
down to *one* such operation at a time per filesystem.  So I'm not
that concerned about occasional false-sharing limiting our ability
to issue concurrent requests.  There are probably worse restrictions
at lower levels.

			regards, tom lane