shared_buffer optimization

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Hi, I have a question with shared_buffer.

Ok, I have a server with 4GB of RAM
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# cat /proc/meminfo
MemTotal:      4086484 kB
[...]
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So, if I want to, for example, shared_buffer to take 3 GB of RAM then
shared_buffer would be 393216 (3 * 1024 * 1024 / 8)

Postmaster dont start.
Error: FATAL:  shmat(id=360448) failed: Invalid argument


I can set a less value, but not higher than 3 GB.

Am I doing something wrong?
Any idea?
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