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Re: adding support for posix_fadvise()

From: Neil Conway <neilc(at)samurai(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Hannu Krosing <hannu(at)tm(dot)ee>,PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: adding support for posix_fadvise()
Date: 2003-11-03 15:44:43
Message-ID: 1067874283.3089.241.camel@tokyo (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, 2003-11-03 at 10:01, Tom Lane wrote:
> Neil Conway <neilc(at)samurai(dot)com> writes:
> > POSIX_FADV_RANDOM doesn't effect the page cache, it just determines how
> > aggressive the kernel is when doing readahead (at least on Linux, but
> > I'd expect to see other kernels implement similar behavior).
> 
> I would expect POSIX_FADV_SEQUENTIAL to reduce the chance that a page
> will be kept in buffer cache after it's been used.

I don't think that can be reasonably implied from the POSIX text, which
is merely:

POSIX_FADV_SEQUENTIAL
        Specifies that the application expects to access the specified
        data sequentially from lower offsets to higher offsets.

The present Linux implementation doesn't do this, AFAICS -- all it does
it increase the readahead for this file:

	http://lxr.linux.no/source/mm/fadvise.c?v=2.6.0-test7

-Neil



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