From: | "CK Tan" <cktan(at)greenplum(dot)com> |
---|---|
To: | "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com> |
Cc: | "Zeugswetter Andreas ADI SD" <ZeugswetterA(at)spardat(dot)at>, "Luke Lonergan" <LLonergan(at)greenplum(dot)com>, "PostgreSQL-development" <pgsql-hackers(at)postgresql(dot)org>, "Jeff Davis" <pgsql(at)j-davis(dot)com>, "Simon Riggs" <simon(at)enterprisedb(dot)com> |
Subject: | Re: Seq scans roadmap |
Date: | 2007-05-10 18:12:23 |
Message-ID: | AD9630BA-9A0F-408C-A7E1-D799722986AC@greenplum.com |
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Thread: | |
Lists: | pgsql-hackers |
The patch has no effect on scans that do updates. The
KillAndReadBuffer routine does not force out a buffer if the dirty
bit is set. So updated pages revert to the current performance
characteristics.
-cktan
GreenPlum, Inc.
On May 10, 2007, at 5:22 AM, Heikki Linnakangas wrote:
> Zeugswetter Andreas ADI SD wrote:
>>> In reference to the seq scans roadmap, I have just submitted a
>>> patch that addresses some of the concerns.
>>>
>>> The patch does this:
>>>
>>> 1. for small relation (smaller than 60% of bufferpool), use the
>>> current logic 2. for big relation:
>>> - use a ring buffer in heap scan
>>> - pin first 12 pages when scan starts
>>> - on consumption of every 4-page, read and pin the next 4-page
>>> - invalidate used pages of in the scan so they do not force out
>>> other useful pages
>> A few comments regarding the effects:
>> I do not see how this speedup could be caused by readahead, so
>> what are
>> the effects ?
>
> I was wondering that as well. We'd really need to test all the
> changes separately to see where the improvements are really coming
> from.
>
> Also, that patch doesn't address the VACUUM issue at all. And using
> a small fixed size ring with scans that do updates can be
> devastating. I'm experimenting with different ring sizes for COPY
> at the moment. Too small ring leads to a lot of WAL flushes, it's
> basically the same problem we have with VACUUM in CVS HEAD.
>
> --
> Heikki Linnakangas
> EnterpriseDB http://www.enterprisedb.com
>
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