Re: WalSndWakeup() and synchronous_commit=off

On Monday, May 14, 2012 07:55:32 PM Fujii Masao wrote:
> On Mon, May 14, 2012 at 6:32 PM, Andres Freund <andres(at)2ndquadrant(dot)com>
wrote:
> > 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.
>
> So what about moving the existing calls of WalSndWakeup() out of a critical
> section and adding new call of WalSndWakeup() into XLogBackgroundFlush()?
> Then all WalSndWakeup()s are called outside a critical section and after
> releasing WALWriteLock. I attached the patch.
Imo its simply not sensible to call WalSndWakeup at *any* of the current
locations. They simply don't have the necessary information. They will wakeup
too often (because with concurrency commits often won't require additional wal
writes) and too seldom (because a flush caused by XLogInsert wont cause a
wakeup).

Andres

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