Re: Wait free LW_SHARED acquisition - v0.2

On Fri, Oct 24, 2014 at 4:05 PM, Andres Freund <andres(at)2ndquadrant(dot)com>
wrote:
>
> 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.

Yeap, but still it will acquire/release spinlock.

> > > > > 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.

Yeah, but I think it is difficult to avoid that behaviour as even when it
wakes
Exclusive locker, some new shared locker can comes in and acquire the
lock before Exclusive locker.

I think it is difficult to say what is the best waking strategy, as
priority for
Exclusive lockers is not clearly defined incase of LWLocks.

With Regards,
Amit Kapila.
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