On Tue, 30 Jun 2020 at 08:43, Fujii Masao <masao.fu...@oss.nttdata.com> wrote:
> > The problem looks to be that spinlocks are terrible with overloaded > CPU and a contended spinlock. A process holding the spinlock might easily > get scheduled out leading to excessive spinning by everybody. I think a > simple thing to try would be to replace the spinlock with LWLock. > > Yes. Attached is the POC patch that replaces per-counter spinlock with > LWLock. > Great. I think this is the one that should get considered for testing. > > I did a prototype patch that replaces spinlocks with futexes, but was > not able to find a workload where it mattered. > > I'm not familiar with futex, but could you tell me why you used futex > instead > of LWLock that we already have? Is futex portable? > Futex is a Linux kernel call that allows to build a lock that has uncontended cases work fully in user space almost exactly like a spinlock, while falling back to syscalls that wait for wakeup in case of contention. It's not portable, but probably something similar could be implemented for other operating systems. I did not pursue this further because it became apparent that every performance critical spinlock had already been removed. To be clear, I am not advocating for this patch to get included. I just had the patch immediately available and it could have confirmed that using a better lock fixes things. -- Ants Aasma Senior Database Engineerwww.cybertec-postgresql.com
