Re: 7.3.1 New install, large queries are slow - Mailing list pgsql-performance

From Tom Lane
Subject Re: 7.3.1 New install, large queries are slow
Date
Msg-id 13372.1042826469@sss.pgh.pa.us
Whole thread Raw
In response to Re: 7.3.1 New install, large queries are slow  (Bruce Momjian <pgman@candle.pha.pa.us>)
Responses Re: 7.3.1 New install, large queries are slow  (Jeff <threshar@torgo.978.org>)
List pgsql-performance
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Just as a data point, I believe other database systems recommend very
> large shared memory areas if a lot of data is being accessed.  I seem to
> remember Informix doing that.

Yeah, but isn't that theory a hangover from pre-Unix operating systems?
In all modern Unixen, you can expect the kernel to make use of any spare
RAM for disk buffer cache --- and that behavior makes it pointless for
Postgres to try to do large amounts of its own buffering.

Having a page in our own buffer instead of kernel buffer saves a context
swap to access the page, but it doesn't save I/O, so the benefit is a
lot less than you might think.  I think there's seriously diminishing
returns in pushing shared_buffers beyond a few thousand, and once you
get to the point where it distorts the kernel's ability to manage
memory for processes, you're really shooting yourself in the foot.

            regards, tom lane

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