Re: heap vacuum & cleanup locks

On Mon, Jun 6, 2011 at 11:30 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> But I think you've hit the important point here. The problem is not
> whether VACUUM waits for the pin, its that the pins can be held for
> extended periods.

Yes

> It makes more sense to try to limit pin hold times than it does to
> come up with pin avoidance techniques.

Well it's super-exclusive-vacuum-lock avoidance techniques. Why
shouldn't it make more sense to try to reduce the frequency and impact
of the single-purpose outlier in a non-critical-path instead of
burdening every other data reader with extra overhead?

I think Robert's plan is exactly right though I would phrase it
differently. We should get the exclusive lock, freeze/kill any xids
and line pointers, then if the pin-count is 1 do the compaction.

I'm really wishing we had more bits in the vm. It looks like we could use:
- contains not-all-visible tuples
- contains not-frozen xids
- in need of compaction

I'm sure we could find a use for one more page-level vm bit too.

--
greg