Simon Riggs wrote:
Group commit is a well-documented technique for improving performance,
but the gains only show themselves on very busy systems. It is possible
in earlier testing any apparent value was actually hidden by the
BufMgrLock issues we have now resolved in 8.1. We now see XLogInsert as
being very nearly the highest routine on the oprofile. That tells me
that it could now be time for group commit to show us some value, if any
exists.
DB2 and Berkeley-DB use group commit, while other rdbms use log writer
processes which effectively provide the same thing. It would surprise me
if we were unable to make use of such a technique, and worry me too.
I would ask that we hold off on their execution, at least for the
complete 8.1 beta performance test cycle. We may yet see gains albeit,
as Tom points out, that benefit may only be possible on only some
platforms.
I don't remember the details exactly, but isn't it so that postgres has some
kind of group commits even without the commit_delay option? I.e. when
several backends are waiting for commit concurrently, the one to get to
commit will actually commit wal for all waiting transactions to disk?
I remember the term "ganged wal writes" or something similar. Tom, can you
elaborate on this? Please tell me if I am totally off track. ;-)
Best Regards,
Michael Paesold
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TIP 8: explain analyze is your friend
