Re: WalSndWakeup() and synchronous_commit=off

On Friday, May 11, 2012 08:45:23 PM Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> > Its the only place though which knows whether its actually sensible to
> > wakeup the walsender. We could make it return whether it wrote anything
> > and do the wakeup at the callers. I count 4 different callsites which
> > would be an annoying duplication but I don't really see anything better
> > right now.
>
> Another point here is that XLogWrite is not only normally called with
> the lock held, but inside a critical section. I see no reason to take
> the risk of doing signal sending inside critical sections.

> BTW, a depressingly large fraction of the existing calls to WalSndWakeup
> are also inside critical sections, generally for no good reason that I
> can see. For example, in EndPrepare(), why was the call placed where
> it is and not down beside SyncRepWaitForLSN?
Hm. While I see no real problem moving it out of the lock I don't really see a
way to cleanly outside critical sections everywhere. The impact of doing so
seems to be rather big to me. The only externally visible place where it
actually is known whether we write out data and thus do the wakeup is
XLogInsert, XLogFlush and XLogBackgroundFlush. The first two of those are
routinely called inside a critical section.

Andres
--
Andres Freund http://www.2ndQuadrant.com/
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