Re: Setting kernel.shmmax

Free memory is going to be used by operating system as cache memory. If
operating system does not need to use this memory, it will remain as
free.
How big is your database? If it is smaller that your RAM you will always
get free memory since it won't be used by operating system in order to
cache database files.
And, although your database is bigger, only frequently accessed database
files will be in cache.

You only need to modify kernel.shmmax in order to accommodate postgres
process only, and memory usage by postgres process is mainly controlled
by shared_buffers, workmem, etc parameters from postgresql.conf.
Remaining memory will be used by operating system as cache memory.

-----Original Message-----
From: fatih ozturk <ozturkfa(at)yahoo(dot)com>
To: pgsql-admin(at)postgresql(dot)org
Subject: [ADMIN] Setting kernel.shmmax
Date: Fri, 10 Jul 2009 00:57:09 -0700 (PDT)

Hi

My question is about shmmax kernel parameter and its importance in
tuning postgresql database.

Our database version:

postgres=# select version();
version
----------------------------------------------------------------------------------------------------------
PostgreSQL 8.3.3 on x86_64-unknown-linux-gnu, compiled by GCC gcc (GCC)
3.4.6 20060404 (Red Hat 3.4.6-3)
(1 row)

"uname -a" output:

Linux vt02.deneme.net 2.6.9-42.0.3.ELsmp #1 SMP Fri Oct 6 06:28:26 CDT
2006 x86_64 x86_64 x86_64 GNU/Linux

And some of our kernel parameters are

kernel.shmmni = 4096
kernel.shmall = 3279547
kernel.shmmax = 2147483648

"free" output

total used free shared buffers
cached
Mem: 16415276 14906444 1508832 0 139192
13743608
-/+ buffers/cache: 1023644 15391632
Swap: 4192924 208 4192716

As you see we have 16 GB RAM and we use about 14 GB of it.
But kernel.shmmax parameter 2 GB.

In most of tuning guides setting shmmax parameter to %90 of total memory
is advised.
But even though we didn't do such shmmax tuning "free" command shows
that we are using 16 GB of RAM effectively.

How postgresql using RAM effectively without tuned shmmax?
Should we tune shmmax in our stuation?
If so, what is shmmax's proper value for our stuation?

Thanks for your interest