Re: testing HS/SR - 1 vs 2 performance

On Sun, 2010-04-25 at 19:18 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > [ v2 patch ]
> 
> I've been studying this some more while making notes for improved
> comments, and I've about come to the conclusion that having readers
> move the tail pointer (at the end of KnownAssignedXidsGetAndSetXmin)
> is overly tricky and probably not a performance improvement anyway.
> The code is in fact wrong as it stands: it's off-by-one about setting
> the new tail value.  And there's potential for contention with multiple
> readers all wanting to move the tail pointer at once.  

OK, since contention was my concern, I want to avoid that.

> And most
> importantly, KnownAssignedXidsSearch can't move the tail pointer so
> we might expend many inefficient searches while never moving the tail
> pointer.

> I think we should get rid of that and just have the two functions that
> can mark entries invalid (which they must do with exclusive lock)
> advance the tail pointer when they invalidate the current tail element.

OK

> Then we have the very simple rule that only the startup process ever
> changes this data structure.

-- Simon Riggs           www.2ndQuadrant.com