On Sun, 2010-04-25 at 19:18 -0400, Tom Lane wrote: > Simon Riggs <si...@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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
