Re: On the usefulness of hint bits

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