Re: Do we need so many hint bits?

On Thu, 2012-11-15 at 22:21 -0500, Tom Lane wrote:
> We aren't pressed for flag bits particularly.  I think the main
> attraction of this idea is precisely to reduce unnecessary page dirties,
> and so that leads me to suggest a variant: keep the four bits defined as
> now, but do not attempt to set XMIN_INVALID or XMAX_COMMITTED unless the
> page is already dirty.  This would make it a straight-up trade of more
> clog consultation for fewer page dirties.

Sounds reasonable.

> > Also, I am wondering about PD_ALL_VISIBLE. It was originally introduced
> > in the visibility map patch, apparently as a way to know when to clear
> > the VM bit when doing an update. It was then also used for scans, which
> > showed a significant speedup. But I wonder: why not just use the
> > visibilitymap directly from those places?
> 
> Added contention for access to the visibility map pages would be a
> problem.

Can you think of a reasonable (or worst-case) test scenario that would
illustrate this problem? Would I need a many-core box to actually test
this?

Also, what locks do you expect to be contended? The buffer header lock
while pinning the VM page?

Regards,
	Jeff Davis