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Re: Prereading using posix_fadvise (was Re: Commitfest patches)

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
Cc: Zeugswetter Andreas OSB SD <Andreas(dot)Zeugswetter(at)s-itsolutions(dot)at>, Gregory Stark <stark(at)enterprisedb(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Prereading using posix_fadvise (was Re: Commitfest patches)
Date: 2008-03-28 15:41:58
Message-ID: 200803281541.m2SFfwk06208@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Heikki Linnakangas wrote:
> > So it has nothing to do with table size. The fadvise calls need to be
> > (and are) 
> > limited by what can be used in the near future, and not for the whole
> > statement.
> 
> Right, I was sloppy. Instead of table size, what matters is the amount 
> of data the scan needs to access. The point remains that if the data is 
> already in OS cache, the posix_fadvise calls are a waste of time, 
> regardless of how many pages ahead you advise.

I now understand what posix_fadvise() is allowing us to do. 
posix_fadvise(POSIX_FADV_WILLNEED) allows us to tell the kernel we will
need a certain block in the future --- this seems much cheaper than a
background reader.

We know we will need the blocks, and telling the kernel can't hurt,
except that there is overhead in telling the kernel.  Has anyone
measured how much overhead?  I would be interested in a test program
that read the same page over and over again from the kernel, with and
without a posix_fadvise() call.

Should we consider only telling the kernel X pages ahead, meaning when
we are on page 10 we tell it about page 16?

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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