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From: PG Doc comments form <[email protected]>
To: [email protected]
Cc: [email protected]
Subject: effective_cache_size
Date: Thu, 13 Sep 2018 15:14:08 +0000
Message-ID: <[email protected]> (raw)

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/runtime-config-query.html
Description:

Hi,

The effective_cache_size documentation (section 19.7) is: "When setting this
parameter you should consider both PostgreSQL's shared buffers and the
portion of the kernel's disk cache that will be used for PostgreSQL data
files.".

This description seems essentially stable since v8.1.

As far as I understand it means that the value associated to
effective_cache_size is the value of the shared_buffers plus the amount of
memory often used by the kernel for the buffercache (in Linux's parlance).

This explanation is offered by many published documents, for example
https://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server#effective_cache_size

Alas, this seems different to what B. Momjian declared in 2012 (
https://www.enterprisedb.com/node/3374 ): "The value doesn't have to be
perfect, but just a rough estimate of how much kernel memory is acting as
secondary cache for the shared buffers.", which (as far as I understand)
means "effective_cache_size = (approx of the often-observed buffercache
size)".

In the sourcecode v11b3) the module src/backend/utils/misc/guc.c contains
"Sets the planner's assumption about the size of the disk cache. That is,
the portion of the kernel's disk cache that will be used for PostgreSQL data
files." (so effective_cache_size is more or less the "buffercache" size).
However in the module src/backend/optimizer/path/costsize.c a comment states
that 'We also use a rough estimate "effective_cache_size" of the number of
disk pages in Postgres + OS-level disk cache." (so effective_cache_size is
more or less the size of the shared_buffers + the size of the
"buffercache").
This seems contradictory to me.

Thank you!

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