On 2013-01-12 16:36:39 -0500, Tom Lane wrote:
> Andres Freund <andres@2ndquadrant.com> writes:
> >>> It does *not* combine elog_start and elog_finish into one function if
> >>> varargs are available although that brings a rather measurable
> >>> size/performance benefit.
>
> >> Since you've apparently already done the measurement: how much?
> >> It would be a bit tedious to support two different infrastructures for
> >> elog(), but if it's a big enough win maybe we should.
>
> > Imo its pretty definitely a big enough win. So big I have a hard time
> > believing it can be true without negative effects somewhere else.
>
> Well, actually there's a pretty serious negative effect here, which is
> that when it's implemented this way it's impossible to save errno for %m
> before the elog argument list is evaluated. So I think this is a no-go.
> We've always had the contract that functions in the argument list could
> stomp on errno without care.
>
> If we switch to a do-while macro expansion it'd be possible to do
> something like
>
> do {
> int save_errno = errno;
> int elevel = whatever;
>
> elog_internal( save_errno, elevel, fmt, __VA__ARGS__ );
> } while (0);
>
> but this would almost certainly result in more code bloat not less,
> since call sites would now be responsible for fetching errno.
the numbers are:
old definition: 10393.658ms, 5497912 bytes
old definition + unreachable: 10011.102ms, 5469144 bytes
stmt, two calls, unreachable: 10036.132ms, 5468792 bytes
stmt, one call, unreachable: 9443.612ms, 5462232 bytes
stmt, one call, unreachable, save errno: 9615.863ms, 5489688 bytes
So while not saving errno is unsurprisingly better its still a win.
Greetings,
Andres Freund
-- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
