Re: [HACKERS] ANALYZE getting dead tuple count hopelessly wrong

On Tue, 2008-04-01 at 10:22 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> > On Tue, 2008-04-01 at 13:07 +0530, Pavan Deolasee wrote:
> >> Please see the attached patch. One change I made is to hold the SHARE lock
> >> on the page while ANALYZE is reading tuples from it. I thought it would
> >> be a right thing to do instead of repeatedly acquiring/releasing the lock.
>
> > ANALYZE is a secondary task and so we shouldn't be speeding it up at the
> > possible expense of other primary tasks. So I think holding locks for
> > longer than minimum is not good in this case and I see no reason to make
> > the change described.
>
> I think Pavan's change is probably good. In the first place, it's only
> a shared buffer lock and besides ANALYZE isn't going to be holding it
> long (all it's doing at this point is counting tuples and copying some
> of them into memory). In the second place, repeated lock release and
> re-grab threatens cache line contention and a context swap storm if
> there is anyone else trying to access the page. In the third, whether
> there's contention or not the extra acquire/release work will cost CPU
> cycles. In the fourth, if we actually believed this was a problem we'd
> need to redesign VACUUM too, as it does the same thing.

VACUUM waits until nobody else has the buffer pinned, so lock contention
is much less of a consideration there. Plus it rearranges the block,
which is hard to do one tuple at a time even if we wanted to.

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

PostgreSQL UK 2008 Conference: http://www.postgresql.org.uk