Set shared memory for postgres on a linux box to a constant fraction of ram

set_postgresql_shared_mem.sh

# If you want maximum shared memory of 1/5 of your RAM,
# leave FRACTION as 5
FRACTION=5
RAM=`cat /proc/meminfo|grep ^MemTotal|awk '{printf("%.f", $2*1024)}'`
SHMMAX=`echo "$RAM / $FRACTION" |bc`
PAGESIZE=`getconf PAGESIZE`
SHMALL=`echo "$SHMMAX / $PAGESIZE" |bc`

sudo sysctl -w kernel.shmmax=$SHMMAX
sudo sysctl -w kernel.shmall=$SHMALL