PG 8.1.1 Cannot allocate shared_buffers memory error

I just installed postgres 8.1.1 on a dual optron Linux box running Linux
4 advanced server, with 32GB of RAM and 2GB of swap.
I'm trying to allocate 24 GB as shared_bufferes. I'm getting an error
that I can not allocate memory (see below). If I change my
shared_buffers to 16GB, postgres starts up with no issue. I'm trying to
understand why I'm getting the shared memory error at sizes above 16GB
whilst I have 32GB with appropriate (I think) settings of kernel
parameters. Any help is appreciated. Thanks...

My sysctl.conf has :
kernel.shmmax = 34359738368
kernel.shmall = 34359738368 (32GB).

And here's some of related postgresql.conf parameters:

shared_buffers = 2359296
work_mem = 16384
maintenance_work_mem = 2097152
max_connections = 100
effective_cache_size = 2621440

---
Error:

FATAL: could not create shared memory segment: Cannot allocate memory
[2005-12-14 13:56:03 PST|||] DETAIL: Failed system call was
shmget(key=5432001, size=19760111616, 03600).
[2005-12-14 13:56:03 PST|||] 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 19760111616
bytes), reduce PostgreSQL's shared_buffers parameter (currently 2359296)
and/or its max_connections parameter (currently 100).

---

Regards,

Husam Tomeh

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