Re: [PATCHES] Proposed patch: synchronized_scanning GUC variable

On Mon, 2008-01-28 at 16:21 -0500, Tom Lane wrote:
> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> > Rather than having a boolean GUC, we should have a number and make the
> > parameter "synchronised_scan_threshold".
>
> This would open up a can of worms I'd prefer not to touch, having to do
> with whether the buffer-access-strategy behavior should track that or
> not. As the note in heapam.c says,
>
> * If the table is large relative to NBuffers, use a bulk-read access
> * strategy and enable synchronized scanning (see syncscan.c). Although
> * the thresholds for these features could be different, we make them the
> * same so that there are only two behaviors to tune rather than four.
>
> It's a bit late in the cycle to be revisiting that choice. Now we do
> already have three behaviors to worry about (BAS on and syncscan off)
> but throwing in a randomly settable knob will take it back to four,
> and we have no idea how that fourth case will behave. The other tack we
> could take (having the one GUC variable control both thresholds) is
> not good since it will result in pg_dump trashing the buffer cache.

OK, good points.

I'm still concerned that the thresholds gets higher as we increase
shared_buffers. We may be removing performance features as fast as we
gain performance when we set shared_buffers higher.

Might we agree that the threshold should be fixed at 8MB, rather than
varying upwards as we try to tune?

The objective of having a tuning hook would have been simply to
normalise the effects of increasing shared_buffers anyway, so fixing it
would solve the problem I see.

(8MB is the default, based upon 25% of 32MB).

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com