Re: commit_delay, siblings

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