Feedback-ID: id4a34324:Fastmail
Date: Wed, 20 Jul 2022 09:05:26 -0700
From: Andres Freund <andres@anarazel.de>
To: Greg Stark <stark@mit.edu>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	Melanie Plageman <melanieplageman@gmail.com>,
	Justin Pryzby <pryzby@telsasoft.com>,
	Thomas Munro <thomas.munro@gmail.com>,
	"David G. Johnston" <david.g.johnston@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: shared-memory based stats collector - v70
Message-ID: <20220720160526.3pyyq3ileaxzdkl2@alap3.anarazel.de>
References: <20220317073652.kz3uuh6le6vbfod4@alap3.anarazel.de>
 <CAAKRu_YsFzW3jsQUD7MVUY8c+87OiKW_Y93TsPAfLr3zi_jX9w@mail.gmail.com>
 <20220325.142256.1649716600998659084.horikyota.ntt@gmail.com>
 <20220325.172418.1055357550509939415.horikyota.ntt@gmail.com>
 <20220402082102.ojdd2e6cobsmljvh@alap3.anarazel.de>
 <20220404041516.cctrvpadhuriawlq@alap3.anarazel.de>
 <20220405030506.lfdhbu5zf4tzdpux@alap3.anarazel.de>
 <20220406030008.2qxipjxo776dwnqs@alap3.anarazel.de>
 <CAM-w4HN=4ss5dS2DH6Cof=zocJXeGOjRNKxEGgUZdJPMndyNzA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: 
 <CAM-w4HN=4ss5dS2DH6Cof=zocJXeGOjRNKxEGgUZdJPMndyNzA@mail.gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/20220720160526.3pyyq3ileaxzdkl2%40alap3.anarazel.de>
Precedence: bulk

Hi,

On 2022-07-20 11:35:13 -0400, Greg Stark wrote:
> Is it true that the shared memory allocation contains the hash table
> entry and body of every object in every database?

Yes. However, note that that was already the case with the old stats
collector - it also kept everything in memory. In addition every read
access to stats loaded a copy of the stats (well of the global stats and
the relevant per-database stats).

It might be worth doing something fancier at some point - the shared
memory stats was already a huge effort, cramming yet another change in
there would pretty much have guaranteed that it'd fail.

Greetings,

Andres Freund