On Thu, 2011-07-28 at 15:38 -0400, Tom Lane wrote: > Hannu Krosing <ha...@krosing.net> writes: > > So the basic design could be "a sparse snapshot", consisting of 'xmin, > > xmax, running_txids[numbackends] where each backend manages its own slot > > in running_txids - sets a txid when aquiring one and nulls it at commit, > > possibly advancing xmin if xmin==mytxid. > > How is that different from what we're doing now? Basically, what you're > describing is pulling the xids out of the ProcArray and moving them into > a separate data structure. That could be a win I guess if non-snapshot- > related reasons to take ProcArrayLock represent enough of the contention > to be worth separating out, but I suspect they don't.
the idea was to make the thid array small enough to be able to memcpy it to backend local memory fast. But I agree it takes testing to see if it is an overall win > In particular, > the data structure you describe above *cannot* be run lock-free, because > it doesn't provide any consistency guarantees without a lock. You need > everyone to have the same ideas about commit order, and random backends > independently changing array elements without locks won't guarantee > that. > > regards, tom lane > -- ------- Hannu Krosing PostgreSQL Infinite Scalability and Performance Consultant PG Admin Book: http://www.2ndQuadrant.com/books/ -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
