On Fri, Dec 11, 2015 at 6:34 PM, Andres Freund <andres@anarazel.de> wrote: > > On 2015-12-11 15:56:46 +0300, Alexander Korotkov wrote: > > > > Yes, there is a cycle with retries in LWLockAcquire function. The case of > > retry is when waiter is waked up, but someone other steal the lock before > > him. Lock waiter is waked up by lock releaser only when lock becomes free. > > But in the case of high concurrency for shared lock, it almost never > > becomes free. So, exclusive locker would be never waked up. I'm pretty sure > > this happens on big Intel machine while we do the benchmark. So, relying on > > number of retries wouldn't work in this case. > > I'll do the tests to verify if retries happens in our case.
makes sense and if retries never happen, then I think changing LWLockRelease()
such that it should wake the waiters if there are waiters on a lock and it has not
waked them for some threshold number of times or something like that might
work.
> > I seriously doubt that making lwlocks fairer is the right way to go > here. In my testing the "unfairness" is essential to performance - the > number of context switches otherwise increases massively. >
Agreed, if the change being discussed hurts in any kind of scenario, then
we should better not do it, OTOH the case described by Alexander seems
to be genuine and I have seen similar complaint by customer in the past
for another database I worked with and the reason for the problem is same.