Re: Setting kernel.shmmax

From: Iñigo Martinez Lasala <imartinez@vectorsf.com>
Subject: Re: [ADMIN] Setting kernel.shmmax
To: "fatih ozturk" <ozturkfa@yahoo.com>
Cc: pgsql-admin@postgresql.org
Date: Friday, July 10, 2009, 9:21 AM

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@yahoo.com>
To: pgsql-admin@postgresql.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