| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
| Cc: | Simon Riggs <simon(dot)riggs(at)2ndquadrant(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, Greg Stark <stark(at)mit(dot)edu>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com> |
| Subject: | Re: Turning off HOT/Cleanup sometimes |
| Date: | 2015-04-20 19:37:57 |
| Message-ID: | 20150420193757.GP11720@momjian.us |
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| Lists: | pgsql-hackers |
On Mon, Apr 20, 2015 at 04:19:22PM -0300, Alvaro Herrera wrote:
> Bruce Momjian wrote:
>
> > I think the limit has to be in terms of a percentage of the table size.
> > For example, if we do one SELECT on a table with all non-dirty pages, it
> > would be good to know that 5% of the pages were pruned --- that tells me
> > that another 19 SELECTs will totally prune the table, assuming no future
> > writes.
>
> This seems simple to implement: keep two counters, where the second one
> is pages we skipped cleanup in. Once that counter hits SOME_MAX_VALUE,
> reset the first counter so that further 5 pages will get HOT pruned. 5%
> seems a bit high though. (In Simon's design, SOME_MAX_VALUE is
> essentially +infinity.)
Oh, I pulled 5% out of the air. Thinking of a SELECT-only workload,
which would be our worse case, I was thinking how many SELECTS running
through HOT update chains would it take to be slower than generating the
WAL to prune the page. I see the percentage as something that we could
reasonably balance, while a fixed page count couldn't be analyzed in
that way.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ Everyone has their own god. +
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