Tomas Vondra wrote:
Dne 2.5.2011 06:53, Adarsh Sharma napsal(a):
I am also in need of a proper documentation that explains how to set
SHMAX and SHMALL variables in Postgres.
What things need to be taken in consideration before doing that ?
What is the value of SHMAX & SHMALL if u have 16 GB RAM for Postgres
Server ?
Well, those two values actually define kernel limits for shared memory
segments (i.e. memory shared by multiple processes, in this case the
postmaster proces and backends). So it's rather a question of tuning
shared_buffers (because that's the shared memory segment) and then
setting those two values.
When I was tuning Postgresql for best Performance, I set my shared_buffers= 4096 MB as I set 25% of RAM ( 1/4 )
So Do I need to set my SHMMAX =4096 MB.
What is the SHMALL size now ?
SHMMAX - max. size of a single shared segment (in bytes)
SHMALL - total size of shared segments (in pages, page is usually 4kB)
So if you decide you want 1GB shared buffers, you'll need at least this
SHMMAX = 1024 * 1024 * 1024 (i.e. 1GB)
SHMALL = 1024 * 256 (1GB in 4kB pages)
(althouth the SHMALL should be higher, as there will be other processes
that need shared memory).
There's a lot of docs about this, e.g. this one (it's mostly for Oracle,
but it describes the shared memory quite nicely):
http://www.puschitz.com/TuningLinuxForOracle.shtml#SettingSharedMemory
regards
Tomas