Re: Do we need so many hint bits?

On 2012-11-16 17:19:23 -0800, Jeff Davis wrote:
> On Fri, 2012-11-16 at 16:09 +0100, Andres Freund wrote:
> > As far as I understand the code the crash-safety aspects of the
> > visibilitymap currently rely on on having the knowledge that ALL_VISIBLE
> > has been cleared during a heap_(insert|update|delete). That allows
> > management of the visibilitymap without it being xlogged itself which
> > seems pretty important to me.
>
> It looks like the xlrec does contain a "cleared all visible" flag in it,
> and it uses that to clear the VM as well as PD_ALL_VISIBLE.

I think the point is that to check whether the visibilitymap bit needs
to be unset - and thus locked exlusively - no locks have to be acquired
but those heap_* already has. Given that in a the large amount of cases
ALL_VISIBLE does *not* need to be reset I think that this is a very
important property for concurrency purposes. If you consider the large
amount of pages that are covered by a single visibilitymap page we don't
want more locking in that path...

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services