Re: performance tuning: shared_buffers, sort_mem; swap

In article <tfo-549650.13201613082002@news.hub.org>,
 Thomas O'Connell <tfo@monsterlabs.com> wrote:

> "IpcMemoryCreate: shmget(key=5432001, size=72368128, 03600) failed:
> Invalid argument
> This error usually means that PostgreSQL's request for a shared memory
> segment exceeded your kernel's SHMMAX parameter. You can either reduce
> the request size or reconfigure the kernel with larger SHMMAX. To reduce
> the request size (currently 72368128 bytes), reduce PostgreSQL's
> shared_buffers parameter (currently 8192) and/or its max_connections
> parameter (currently 256). If the request size is already small, it's
> possible that it is less than your kernel's SHMMIN parameter, in which
> case raising the request size or reconfiguring SHMMIN is called for.

also, is there a way to figure out the request size the postmaster will
use when starting up? it doesn't seem to come directly from the
shared_buffers value.

for instance, if i set shared_buffers to 4096, this should mean
postmaster will require 4096 * 8192 = 33554432 bytes, right? but when i
start it, i get the error above with a request size of 38371328. what's
the extra padding from? sort_mem, etc?

-tfo