shared memory settings

Hi all!

We have at present the following parameters related to shared memory:

*postgres*

shared_buffers = 7GB
max_connections = 1500
max_locks_per_transaction = 1024
max_prepared_transactions = 0 (not set)

*system*

SHMALL = 2097152
SHMMAX = 17 670 512 640
SHMMNI = 4096

The amount of RAM is 24 693 176k

We need to increase max_connections to 3000. When we tried to do this we
got an error

*[1-1] FATAL: could not create shared memory segment: No space left on
device*
*[2-1] DETAIL: Failed system call was shmget(key=5432001, size=8964661248,
03600).*
*[3-1] HINT: This error does *not* mean that you have run out of disk
space. It occurs either if all available shared memory IDs have been
taken, in which case you need to raise the SHMMNI parameter in your kernel,
or because the system's overall limit for shared memory has been reached.
If you cannot increase the shared memory limit, reduce PostgreSQL's shared
memory request (currently 8964661248 bytes), perhaps by reducing
shared_buffers or max_connections.*
* The PostgreSQL documentation contains more information about shared
memory configuration.*

The hint proposes to increase SHMMNI kernel parameter, but I'm not sure
about how much to add :) Also, do we need to change any other parameters
accordingly?

Thanks in advance,
Alexander