shared_buffers > 284263 on OS X

I'm trying to optimize a PostgreSQL 8.1.5 database running on an
Apple G5 Xserve (dual G5 2.3 GHz w/ 8GB of RAM), running Mac OS X
10.4.8 Server.

The queries on the database are mostly reads, and I know a larger
shared memory allocation will help performance (also by comparing it
to the performance of the same database running on a SUSE Linux box,
which has a higher shared_buffers setting).

When I set shared_buffers above 284263 (~ 2.17 GB) in the
postgresql.conf file, I get the standard error message when trying to
start the db:

FATAL:  could not create shared memory segment: Cannot allocate memory
DETAIL:  Failed system call was shmget(key=5432001, size=3289776128,
03600).

shmmax and shmall are set to 4GB, as can be seen by the output from
sysctl:
hw.physmem = 2147483648
hw.usermem = 1885794304
hw.memsize = 8589934592
kern.sysv.shmmax: 4294967296
kern.sysv.shmmin: 1
kern.sysv.shmmni: 32
kern.sysv.shmseg: 8
kern.sysv.shmall: 1048576

Has anyone else noticed this limitation on OS X? Any ideas on how I
might get shared_buffers higher than 284263?

Brian Wipf
<brian@clickspace.com>