Why PostgreSQL doesn't start with shared_buffers=6GB ?

[Yuri]

When I am setting shared_buffers=6GB in postgresql.conf it fails to start:
DETAIL:  Failed system call was shmget(key=5432001, size=6612361216, 03600).
even though kern.ipc.shmmax is set to ~7GB:
$ sysctl -a | grep shm
kern.ipc.shm_allow_removed: 0
kern.ipc.shm_use_phys: 0
kern.ipc.shmall: 1310720
kern.ipc.shmseg: 128
kern.ipc.shmmni: 192
kern.ipc.shmmin: 1
kern.ipc.shmmax: 7000000000
kern.features.sysv_shm: 1
kern.features.posix_shm: 1

There are 17GB free memory as reported by top(1).

Why shmget fails despite kern.ipc.shmmax is being high enough?
Experimentally I found that shared_buffers=5GB also fails but 4GB 
succeeds. Is there another system limit on shmem besides kern.ipc.shmmax ?

9.1-RC3 and64

Yuri