Linux – PostgreSQL won’t start because it “Cannot allocate memory”

I'm running PostgreSQL 8.4.5 on Ubuntu 10.04. I run an EC2 instance with a few sites running on nginx. Most of these sites run on Django and connect to this Postgres instance.

For some reason, at 8:45pm tonight, Postgres went down. I logged into the machine, and I see this error message:

 * Starting PostgreSQL 8.4 database server
 * The PostgreSQL server failed to start. Please check the log output:
2011-04-17 04:46:49 UTC FATAL:  could not create shared memory segment: Cannot allocate memory
2011-04-17 04:46:49 UTC DETAIL:  Failed system call was shmget(key=5432001, size=16211968, 03600).
2011-04-17 04:46:49 UTC HINT:  This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory or swap space. To reduce the request size (currently 16211968 bytes), reduce PostgreSQL's shared_buffers parameter (currently 1792) and/or its max_connections parameter (currently 53).
        The PostgreSQL documentation contains more information about shared memory configuration.
   ...fail!

The first thing I did was change the Linux shared memory allocation.

sysctl -w kernel.shmmax=367108864>
sysctl -p /etc/sysctl.conf

That didn't do it. So then I edited /etc/postgresql/8.4/main/postgresql.conf and lowered the max_connections value. That worked…for about 10 minutes.

Now I'm getting the same error, no matter how low max_connections is. I need to set it to at least 9 (since that's the number of Django projects that need access to this Postgres server).

Any thoughts on how I might go about fixing this?