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Re: [ADMIN] shared_buffers and shmmax

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>, valiouk(at)yahoo(dot)co(dot)uk, pgsql-docs(at)postgresql(dot)org, dx k9 <bitsandbytes88(at)hotmail(dot)com>
Subject: Re: [ADMIN] shared_buffers and shmmax
Date: 2008-12-17 03:31:15
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Lists: pgsql-adminpgsql-docspgsql-hackers
Bruce Momjian <bruce(at)momjian(dot)us> writes:
> Tom Lane wrote:
>> If you're going to give a formula, why not just give a formula, eg

> You mean like this:

Yeah, more or less.  A couple thoughts now that I see it worked out:

* Combining the entries for max_connections and autovacuum_max_workers
is probably just making it look more complicated than it needs to.
How about two rows that just happen to have similar formulas, viz

max_connections		(1800 + 270 * max_locks_per_transaction) * max_connections
autovacuum_max_workers	(1800 + 270 * max_locks_per_transaction) * autovacuum_max_workers

* The right-hand column header should be something like "Approximate
shared memory bytes..." to avoid the impression that these formulas
are meant to be exact.

* If we do it like this then the left-hand column is really redundant,
not to say wrong because the right-hand formulas depend on more than
the single variable mentioned.  How about something like

	Table 17-2	PostgreSQL shared memory usage

Purpose				Approximate number of bytes required (as of 8.3)

Per-connection state		(1800 + 270 * max_locks_per_transaction) * max_connections
Autovacuum worker state		(1800 + 270 * max_locks_per_transaction) * autovacuum_max_workers
Prepared transaction state	...
Shared disk buffers		...
WAL buffers			...
Fixed space requirements	770kB

			regards, tom lane

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