On Tue, Apr 19, 2016 at 9:57 AM, Amit Kapila <amit.kapil...@gmail.com> wrote: > On Sun, Apr 17, 2016 at 2:26 AM, Andres Freund <and...@anarazel.de> wrote: >> >> On 2016-04-16 16:44:52 -0400, Noah Misch wrote: >> > That is more controversial than the potential ~2% regression for >> > old_snapshot_threshold=-1. Alvaro[2] and Robert[3] are okay releasing >> > that way, and Andres[4] is not. >> >> FWIW, I could be kinda convinced that it's temporarily ok, if there'd be >> a clear proposal on the table how to solve the scalability issue around >> MaintainOldSnapshotTimeMapping(). > > It seems that for read-only workloads, MaintainOldSnapshotTimeMapping() > takes EXCLUSIVE LWLock which seems to be a probable reason for a performance > regression. Now, here the question is do we need to acquire that lock if > xmin is not changed since the last time value of > oldSnapshotControl->latest_xmin is updated or xmin is lesser than equal to > oldSnapshotControl->latest_xmin? > If we don't need it for above cases, I think it can address the performance > regression to a good degree for read-only workloads when the feature is > enabled.
Thanks, Amit -- I think something along those lines is the right solution to the scaling issues when the feature is enabled. For now I'm focusing on the back-patching issues and the performance regression when the feature is disabled, but I'll shift focus to this once the "killer" issues are in hand. -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
