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Date:      Fri, 09 Nov 2012 09:28:10 +0100
From:      Frank Broniewski <brfr@metrico.lu>
To:        freebsd-questions@freebsd.org
Subject:   Postgresql related memory question
Message-ID:  <509CBE9A.5020007@metrico.lu>

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Hi,

I recently had a discussion of PostgreSQLs memory usage on FreeBSD, 
notably the display of the different memory types in top, on the 
PostgreSQL mailing list [1]. My server has 32GB ram, of which approx. 
8GB vanish from tops display after a while. My question was wether there 
is a memory problem on FreeBSD or if top is not capable of dealing well 
with PostgreSQLs memory handling, eg. the shared_buffers.

We used a perl script [2] to get a better understanding of the current 
memory usage on FreeBSD and a line from the scripts output caught my eye:
 > mem_gap_vm:  +   8812892160 (   8404MB) [ 26%] Memory gap: UNKNOWN

I don't have a good understanding of the different types of system 
memory, so the question is, in relation with PostgreSQL, if the 
mem_gap_vm value can be considered normal or if it is a sign of memory 
leak. Since this question is more of a FreeBSD question, we decided to 
move the discussion here in order to get more insight on the system 
internals :-)
This value equals more or less the shared buffers (shared_buffers=8GB) 
setting of PostgreSQL and is stable, which means, it doesn't change 
anymore after reaching its peak value.

I did post already some system configuration information in the thread 
on the PostgreSQL list, but I am happy to provide all the information 
necessary to shed some light on this matter.


Many thanks,

Frank


[1] 
http://postgresql.1045698.n5.nabble.com/Memory-issue-on-FreeBSD-td5730651.html
[2] http://www.cyberciti.biz/faq/freebsd-command-to-get-ram-information/
-- 
Frank BRONIEWSKI

METRICO s.à r.l.
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rue des Romains 36
L-5433 NIEDERDONVEN

tél.: +352 26 74 94 - 28
fax.: +352 26 74 94 99
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