Re: Hint Bits and Write I/O

On Wed, 2008-05-28 at 06:08 -0400, Gregory Stark wrote:
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>
> > (Although that argument might not hold water for a bulk seqscan: you'll
> > have hinted all the tuples and then very possibly throw the page away
> > immediately.
>
> That seems like precisely the case where we don't want to dirty the buffer.

(1)

> > So counting the hints and eventually deciding we did
> > enough to justify dirtying the page might be worth doing.)
>
> What if we counted how many hint bits were *not* set? I feel like the goal
> should be to dirty the buffer precisely once when all the bits can be set.

(2)

Agreed. I think the difficulty is that (1) and (2) are contradictory
goals, and since those conditions frequently occur together, cause
conflict.

When we fully scan a buffer this will result in 1 or more actual clog
lookups, L. L is often less than the number of tuples on the page
because of the single-item xid cache. If L = 1 then there is a high
probability that when we do a seq scan the clog blocks will be cached
also, so although we do a 1 clog lookup per table block we would seldom
do clog I/O during a SeqScan. So what I tried to say in a previous post
was that if L > 1 then we should dirty the buffer because the
single-item cache becomes less-effective and we may need to access other
clog blocks, that may result in clog I/O.

--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support