On 19 February 2012 05:24, Robert Haas <robertmhaas@gmail.com> wrote:
> I have attached tps scatterplots. The obvious conclusion appears to
> be that, with only 16MB of wal_buffers, the buffer "wraps around" with
> some regularity: we can't insert more WAL because the buffer we need
> to use still contains WAL that hasn't yet been fsync'd, leading to
> long stalls. More buffer space ameliorates the problem.
Incidentally, I wondered if we could further improve group commit
performance by implementing commit_delay with a WaitLatch call, and
setting the latch in the event of WAL buffers wraparound (or rather, a
queued wraparound request - a segment switch needs WALWriteLock, which
the group commit leader holds for a relatively long time during the
delay). I'm not really sure how significant a win this might be,
though. There could be other types of contention, which could be
considerably more significant. I'll try and take a look at it next
week.
--
Peter Geoghegan http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services