Shmax

http://www.postgresql.org/docs/9.1/static/runtime-config-resource.html

https://gist.github.com/1285137

#!/bin/bash
# http://archives.postgresql.org/pgsql-admin/2010-05/msg00285.php
# Output lines suitable for sysctl configuration based
# on total amount of RAM on the system. The output
# will allow up to 50% of physical memory to be allocated
# into shared memory.
# On Linux, you can use it as follows (as root):
#
# ./shmsetup >> /etc/sysctl.conf
# sysctl -p
# Early FreeBSD versions do not support the sysconf interface
# used here. The exact version where this works hasn't
# been confirmed yet.

page_size=`getconf PAGE_SIZE`
phys_pages=`getconf _PHYS_PAGES`

if [ -z "$page_size" ]; then
echo Error: cannot determine page size
exit 1
fi

if [ -z "$phys_pages" ]; then
echo Error: cannot determine number of memory pages
exit 2
fi

shmall=`expr $phys_pages / 2` 
shmmax=`expr $shmall \* $page_size`

echo \# Maximum shared segment size in bytes
echo kernel.shmmax = $shmmax 
echo \# Maximum number of shared memory segments in pages
echo kernel.shmall = $shmall