Hello, I have been trying to understand how group commit implementation works the way it does after 9.2 group commit enhancement patch (9b38d46d9f5517dab67dda1dd0459683fc9cda9f on REL9_2_STABLE). I admit it's a pretty old commit though I seek some clarification as to how it provides the performance gain as it does. Also, I have observed some behavior in this regard that I could not understand.
Profiling results show that XLogFlush is called about twice as much after this patch while for XLogWrite count remains about same as before. Since, XLogFlush is the place where this patch offers the said performance gain by alleviating the lock contention on WALWriteLock using the new LWLockAcquireOrWait(). So far so good. I do not understand why XLogFlush is invoked twice (as see from the profiling results) as an effect of this patch. I used "pgbench -c 32 -t 1000 pgbench" in both cases with TPS result (after applying the patch) not being significantly different (as in not twice as much on my system). Using: pgbench scale=10 shared_buffers=256MB max_connections=1000 checkpoint_segments=15 synchronous_commit=on Comments? -- Amit Langote -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
