Modify PostgreSQL max_connections

The default maximum number of connections in PostgreSQL is 200. If the number of connections exceeds 200, so many clients alreay will be reported. Of course, you can view the current maximum number of connections through show max_connections;

 

Modify the maximum number of connections. The most important thing is to modify the shmmax (maximum size of shared memory segment (bytes ))

Http://www.postgresql.org/docs/current/static/kernel-resources.html#SYSVIPC-PARAMETERS
Here are some explanations

What is the use of SHM, (shared memory segment)
Http://www.postgresql.org/docs/current/static/kernel-resources.html#SYSVIPC

Here is the description of Server start failed.
Http://www.postgresql.org/docs/current/static/server-start.html#SERVER-START-FAILURES

You can use sysctl to modify SHM in Linux (centos ).
1> sysctl-A: List all
Sysctl-A | grep shmmax
2> modify sysctl-W
Sysctl-W kernel. shmmax = 500000000 (Note that there cannot be spaces before and after the equal sign)