Server crash... trying to figure it out

This has happened twice over the last couple of nights:

2011-05-30 02:08:27 PDT LOG: server process (PID 29979) was terminated by signal 9: Killed
2011-05-30 02:08:27 PDT LOG: terminating any other active server processes
2011-05-30 02:08:31 PDT LOG: all server processes terminated; reinitializing
2011-05-30 02:08:31 PDT FATAL: could not create shared memory segment: Cannot allocate memory
2011-05-30 02:08:31 PDT DETAIL: Failed system call was shmget(key=5432001, size=6595420160, 03600).
2011-05-30 02:08:31 PDT 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 6595420160 bytes), reduce PostgreSQL's shared_buffers parameter (currently 786432) and/or its max_connections parameter (currently 203).
The PostgreSQL documentation contains more information about shared memory configuration.

There isn't much server load at the time: the only messages in the log are the annoying autovacuum finding orphan temp tables b/c people haven't closed out their sessions. I'm guessing it's unrelated, but I'm not entirely sure.

The value of shared_buffers is set to 6GB, work_mem is 64MB, maintenance_work_mem is 128MB, effective_cache_size is 4GB.

max_connections is set to 200.

Any ideas here? The system has 12GB; raising the shared_buffers seems reasonable but 6GB also feels like it should be sufficient.

Any thoughts appreciated. Thanks!

--
Wells Oliver
Architect, Baseball Systems
619-795-5359
San Diego Padres | 100 Park Boulevard | San Diego CA 92101