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.
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.
-- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com