Re: scan_recycle_buffers

On Fri, 2007-03-09 at 20:08 +0000, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > Patch to implement buffer cache recycling for scans, as being discussed
> > on pgsql-hackers.
>
> A few questions come to mind:

Good questions. I don't expect this will go through easily, so we need
to examine these thoughts thoroughly.

> How does it behave with Jeff's synchronized seq scans patch?

I've offered Jeff lots of support throughout that patch's development
and its a feature I'd like to see. The current synch scan patch relies
upon the cache spoiling effect to gain its benefit. I think that can be
tightened up, so that we can make both work.

Currently synch scans help DSS apps but not OLTP. This patch reduces the
negative effects of VACUUM on OLTP workloads, as well as helping DSS.

> I wonder if calling RelationGetNumberOfBlocks on every seq scan becomes
> a performance issue for tiny tables with for example just 1 page. It
> performs an lseek, which isn't free.

Jeff's patch does this also, for similar reasons.

> What happens if multiple backends choose overlapping sets of buffers to
> recycle?

They won't. If a buffer is pinned, it will fall out of the the list of
buffers being recycled and not be reused. So they will each tend towards
a unique list of buffers.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com