From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Leonardo Francalanci <m_lists(at)yahoo(dot)it>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: On the usefulness of hint bits |
Date: | 2010-10-11 16:03:19 |
Message-ID: | 24913.1286812999@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Oct 11, 2010 at 10:14 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Reduction of contention for pg_clog access, for one thing. If you read
>> the archives, you'll find that pg_clog access contention has been shown
>> to be one cause of "context swap storms".
> I wonder if we could improve this with some sort of process-local
> cache - not to get rid of hint bits, just to reduce pg_clog
> contention. We might easily end up testing the same XID many times
> during the same table scan.
There already is a one-entry cache --- see TransactionLogFetch. Not
sure if making it bigger would be a win in current usage, although
you'd likely have to if you were trying to not set hint bits.
> Another idea that's been discussed before is to avoid writing out
> pages when only the hit bints have changed.
Yeah.
regards, tom lane
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