On Fri, Oct 24, 2014 at 4:05 PM, Andres Freund <and...@2ndquadrant.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. EnterpriseDB: http://www.enterprisedb.com
