Hi, For the logical decoding patch I added support for pegging RecentGlobalXmin (and GetOldestXmin) to a lower value. To avoid causing undue bloat & cpu overhead (hot pruning is friggin expensive) I split RecentGlobalXmin into RecentGlobalXmin and RecentGlobalDataXmin where the latter is the the xmin horizon used for non-shared, non-catalog tables. That removed almost all overhead I could measure.
During that I was tinkering with the idea of reusing that split to vacuum/prune user tables in a per db fashion. In a very quick and hacky test that sped up the aggregate performance of concurrent pgbenches in different databases by about 30%. So, somewhat worthwile ;). The problem with that is that GetSnapshotData, which computes RecentGlobalXmin, only looks at the PGXACT structures and not PGPROC which contains the database oid. This is a recently added optimization which made GetSnapshotData() quite a bit faster & scalable which is important given the frequency it's called at. What about moving/copying the database oid from PGPROC to PGXACT? Currently a single PGXACT is 12 bytes which means we a) have several entries in a single cacheline b) have ugly sharing because we will have PGXACTs split over more than one cacheline. Comments? Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
