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

Detecting sequential scan and increasing read-ahead is a standard OS
capability, and most/all do that already.  Solaris has code to detect
when a sequential scan is wiping the cache and adjusting the buffer
frees, called "free-behind."

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Ron Johnson wrote:
> On Mon, 2003-01-27 at 04:34, Curt Sampson wrote:
> > On Mon, 27 Jan 2003, Sean Chittenden wrote:
> >
> > >     The FreeBSD VM caching system does prevent one process from exhausting
> > >     another process's cached data due to a sequential access, but the
> > >     FreeBSD VM cache does not try to outsmart sequential data accesses to
> > >     datasets which are larger then available cache space because it's an
> > >     insanely difficult (impossible) problem to solve properly without
> > >     foreknowledge of what data elements will be accessed when.
> >
> > This is not impossible; Solaris does just this. I'm a little short of
>
> Quite.  One way to do it is:
> - the OS notices that process X has been sequentially reading thru
>   file Y for, say, 3 seconds.
> - the OS knows that X is currently at the mid-point of file Y
> - OS says, "Hey, I think I'll be a bit more agressive about, when I
>   have a bit of time, trying to read Y faster than X is requesting
>   it
>
> It wouldn't work well, though, in a client-server DB like Postgres,
> which, in a busy multi-user system, is constantly hitting different
> parts of different files.
>
> The algorithm, though, is used in the RDBMS Rdb.  It uses the algorithm
> above, substituting "process X" for "client X", and passes the agressive
> reads of Y on to the OS.  It's a big win when processing a complete
> table, like during a CREATE INDEX, or  "SELECT foo, COUNT(*)" where
> there's no index on foo.
>
> --
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> | Ron Johnson, Jr.        mailto:ron.l.johnson@cox.net          |
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