Edit file /etc/sysctl.conf

Add the following lines

kernel.shmmax=2147483648
vm.overcommit_memory=2

kernel.shmmax
1. Around 1/2 of the total system memory
e.g. 4GB system memory, kernel.shmmax =  2 x 1024 x 1024 x 1024

Modify postgresql server effective cache settings
edit /var/lib/pgsql/data/postgresql.conf

effective_cache_size = 1800MB