Shared memory segment error

Hi, sometimes ill get this error:

FATAL:  could not create shared memory segment: Cannot allocate memory
DETAIL:  Failed system call was shmget(key=2, size=4153344, 03600).
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 4153344 bytes), reduce PostgreSQL's
shared_buffers parameter (currently 400) and/or its max_connections
parameter (currently 20).
    The PostgreSQL documentation contains more information about shared
memory configuration.

I was what exactly this shared memory space is used for and what size
is good to set it to. I need to be able to start up many backends
(postgres) without using the postmaster.

thanks!

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Alexander Cohen
http://www.toomuchspace.com
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