On Tue, Nov 11, 2008 at 02:17:20PM -0300, Alvaro Herrera wrote: > Thom Brown escribió: > > I've noticed that if I set my SHMMAX to 256 * 1024 * 1024 (268435456) > > and my shared_buffers value in postgresql.conf to 256MB the server > > fails to start. I managed to find a tipping point: 249MB seems to be > > too much and 248MB seems to be okay. > > > > Could someone explain what I'm missing here? I thought the > > shared_buffers could be anything up to SHMMAX? > > shared_buffers is not the only factor to shared memory, so you need to > provide some extra SHMMAX slop. what is this "extra slop" needed for? I'd always ignored it before and just made SHMMAX a bit bigger until PG stopped complaining and started. But computers are supposed to be deterministic; what's taking up the extra space? I expect this knowledge to be most useful to people like Greg Smith who's making a config auto-tuner, it would be bad if it said to set SHMMAX to xMB and it really needs to be set to (x+12)MB. I've just found a table[1] in the docs that attempts to explain what's going on, but I get 10.5MB of extra shared memory being used (default config values, expect for shared_buffers going up to 256MB). Sam [1] http://www.postgresql.org/docs/current/static/kernel-resources.html#SHARED-MEMORY-PARAMETERS -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
