On 2020/06/30 20:30, Ants Aasma wrote:
On Tue, 30 Jun 2020 at 08:43, Fujii Masao <masao.fu...@oss.nttdata.com <mailto: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.
Understood. Thanks for the explanation! Regards, -- Fujii Masao Advanced Computing Technology Center Research and Development Headquarters NTT DATA CORPORATION
