From: Jeremy Harris <jgh@wizmail.org>
To: pgsql-performance@postgresql.org
Subject: Re: How does PG know if data is in memory?
Date: Mon, 04 Oct 2010 23:47:16 +0100
Message-ID: <4CAA5974.2050805@wizmail.org> (raw)
In-Reply-To: <4CA9488C.2080502@2ndquadrant.com>
References: <AANLkTin5G0jXUbeNdC+1s9LQpk1ZpvDuX-6n90oonen3@mail.gmail.com>
<4CA9488C.2080502@2ndquadrant.com>
On 10/04/2010 04:22 AM, Greg Smith wrote:
> I had a brain-storming session on this subject with a few of the hackers in the community in this area a while back I haven't had a chance to do something with yet (it exists only as a pile of scribbled notes so far). There's a couple of ways to collect data on what's in the database and OS cache, and a couple of ways to then expose that data to the optimizer. But that needs to be done very carefully, almost certainly as only a manual process at first, because something that's producing cache feedback all of the time will cause plans to change all the time, too. Where I suspect this is going is that we may end up tracking various statistics over time, then periodically providing a way to export a mass of "typical % cached" data back to the optimizer for use in plan cost estimation purposes. But the idea of monitoring continuously and always planning based on the most recent data available has some stability issues, both from a "too many unpredictable plan changes" and a "ba
d
> short-term feedback loop" perspective, as mentioned by Tom and Kevin already.
Why not monitor the distribution of response times, rather than "cached" vs. not?
That a) avoids the issue of discovering what was a cache hit b) deals neatly with
multilevel caching c) feeds directly into cost estimation.
Cheers,
Jeremy
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: jgh@wizmail.org
Subject: Re: How does PG know if data is in memory?
In-Reply-To: <4CAA5974.2050805@wizmail.org>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox