Horton

Re: Atomics for heap_parallelscan_nextpage()

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>, Heikki Linnakangas <hlinnaka@iki.fi>, David Rowley <david.rowley@2ndquadrant.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>, Amit Khandekar <amitdkhan.pg@gmail.com>, Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
Date: 2017-08-16T17:49:42Z
Lists: pgsql-hackers 
On Wed, Aug 16, 2017 at 1:44 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I was feeling a bit uncomfortable with the BUFFERALIGN_DOWN() for a
> different reason: if the caller has specified the exact amount of space it
> needs, having shm_toc_create discard some could lead to an unexpected
> failure.

Well, that's why Heikki also patched shm_toc_estimate.  With the
patch, if the size being used in shm_toc_create comes from
shm_toc_estimate, it will always be aligned and nothing bad will
happen.  If the user invents another size out of whole cloth, then
they might get a few bytes less than they expect, but that's what you
get for not using shm_toc_estimate().

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Fix pg_atomic_u64 initialization.

  2. Fix shm_toc.c to always return buffer-aligned memory.

  3. Use atomic ops to hand out pages to scan in parallel scan.

  4. Improve 64bit atomics support.