On Tue, 2010-04-13 at 21:09 +0300, Heikki Linnakangas wrote: > Heikki Linnakangas wrote: > > I could reproduce this on my laptop, standby is about 20% slower. I ran > > oprofile, and what stands out as the difference between the master and > > standby is that on standby about 20% of the CPU time is spent in > > hash_seq_search(). The callpath is GetSnapshotDat() -> > > KnownAssignedXidsGetAndSetXmin() -> hash_seq_search(). That explains the > > difference in performance. > > The slowdown is proportional to the max_connections setting in the > standby. 20% slowdown might still be acceptable, but if you increase > max_connections to say 1000, things get really slow. I wouldn't > recommend max_connections=1000, of course, but I think we need to do > something about this. Changing the KnownAssignedXids data structure from > hash table into something that's quicker to scan. Preferably something > with O(N), where N is the number of entries in the data structure, not > the maximum number of entries it can hold as it is with the hash table > currently.
There's a tradeoff here to consider. KnownAssignedXids faces two workloads: one for each WAL record where we check if the xid is already known assigned, one for snapshots. The current implementation is optimised towards recovery performance, not snapshot performance. > A quick fix would be to check if there's any entries in the hash table > before scanning it. That would eliminate the overhead when there's no > in-progress transactions in the master. But as soon as there's even one, > the overhead comes back. Any fix should be fairly quick because of the way its modularised - with something like this in mind. I'll try a circular buffer implementation, with fastpath. Have something in a few hours. -- 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
