On Tue, 2008-03-25 at 17:26 -0300, Alvaro Herrera wrote: > I've finished (hopefully) the code to handle a current list of open > snapshots in a transaction. I'm now wondering how to put it to good use > ;-) I'm not posting it yet -- first I want to get some feedback on the > previous patch I posted, > http://archives.postgresql.org/pgsql-patches/2008-03/msg00245.php
As I said before, it looks fine. In your words, it "just moves code around", so there's not much to complain about. > I think the important change here is switching the semantics of > MyProc->xmin. Currently, it is "the minimum of Xmin and Xid, across all > backends, at the moment the current transaction fetches its serializable > snapshot". The first important bit is that it is computed only once: > when the serializable snapshot is taken. Yes, I see that as necessary. So the refactoring makes sense, since we'll be adding lots of stuff in that area and its good to keep it isolated. > So ISTM the important change is that we will have to update MyProc->xmin > more frequently than that. I'm thinking in keeping enough local state > so that we can detect at what time the earliest open snapshot is > unregistered; when that happens, we can recalculate MyProc->xmin based > on the snapshots we have and the Xid/Xmin of remote backends (which > could have also been updating their own xmins). > > There is one hole here: contention on ProcArrayLock. Basically, for > simple transactions we will need to update MyProc after every command. > It has been reported that ProcArrayLock is the most contended lock for > some loads; this would only add to that, and heavily I think. Perhaps > we could restructure the locking here somehow to avoid this problem, but > it is complex enough already that it may not even be possible. I don't see that this would be a contention problem. We are already careful to read the xmin just once during GetSnapshotData(). We advance it while holding only a LW_SHARED lock during a serializable snapshot, so not sure why we wouldn't advance it at other times also without contention issues. Why does anyone else know or care whether we're taking a serializable snapshot or not? The issue is whether we agree that is correct to do so. If we're advancing it in the circumstances you say, then yes I agree it is. -- Simon Riggs 2ndQuadrant http://www.2ndQuadrant.com PostgreSQL UK 2008 Conference: http://www.postgresql.org.uk -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
