Re: Wait free LW_SHARED acquisition - v0.2

On 2014-10-24 15:59:30 +0530, Amit Kapila wrote:
> > > and w.r.t performance it can lead extra
> > > function call, few checks and I think in some cases even can
> > > acquire/release spinlock.
> >
> > I fail to see how that could be the case.
>
> Won't it happen incase first backend sets releaseOK to true and another
> backend which tries to wakeup waiters on lock will acquire spinlock
> and tries to release the waiters.

Sure, that can happen.

> > And again, this is code that's
> > only executed around a couple syscalls. And the cacheline will be
> > touched around there *anyway*.
>
> Sure, but I think syscalls are required in case we need to wake any
> waiter.

It won't wake up a waiter if there's none on the list.
> > > > And it'd be a pretty pointless
> > > > behaviour, leading to useless increased contention. The only time it'd
> > > > make sense for X to be woken up is when it gets run faster than the S
> > > > processes.
> > >
> > > Do we get any major benefit by changing the logic of waking up waiters?
> >
> > Yes.
>
> I think one downside I could see of new strategy is that the chance of
> Exclusive waiter to take more time before getting woked up is increased
> as now it will by pass Exclusive waiters in queue.

Note that that *already* happens for any *new* shared locker that comes
in. It doesn't really make sense to have share lockers queued behind the
exclusive locker if others just go in front of it anyway.

> > > Code is more readable, but I don't understand why you
> > > want to do refactoring as part of this patch which ideally
> > > doesn't get any benefit from the same.
> >
> > I did it first without. But there's required stuff like
> > LWLockDequeueSelf(). And I had several bugs because of the list stuff.
> >
> > And I did separate the conversion into a separate patch?
>
> Yeah, but the main patch for wait free LW_SHARED also uses
> it.

Well, the only thing that it could have done given that the other patch
is a preqrequisite is reverting the behaviour?

Greetings,

Andres Freund

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