> Date: Sat, 19 Mar 2016 13:53:07 +0100 > From: Martin Pieuchot <m...@openbsd.org> > > Applications using multiple threads often call sched_yield(2) to > indicate that one of the threads cannot make any progress because > it is waiting for a resource held by another one. > > One example of this scenario is the _spinlock() implementation of > our librthread. But if you look on https://codesearch.debian.net > you can find much more use cases, notably MySQL, PostgreSQL, JDK, > libreoffice, etc. > > Now the problem with our current scheduler is that the priority of > a thread decreases when it is the "curproc" of a CPU. So the threads > that don't run and sched_yield(2) end up having a higher priority than > the thread holding the resource. Which means that it's really hard for > such multi-threaded applications to make progress, resulting in a lot of > IPIs numbers. > That'd also explain why if you have a more CPUs, let's say 4 instead > of 2, your application will more likely make some progress and you'll > see less sluttering/freezing. > > So what the diff below does is that it penalizes the threads from > multi-threaded applications such that progress can be made. It is > inspired from the recent scheduler work done by Michal Mazurek on > tech@. > > I experimented with various values for "p_priority" and this one is > the one that generates fewer # IPIs when watching a HD video on firefox. > Because yes, with this diff, now I can. > > I'd like to know if dereferencing ``p_p'' is safe without holding the > KERNEL_LOCK. > > I'm also interested in hearing from more people using multi-threaded > applications.
This doesn't only change the sched_yield() behaviour, but also modifies how in-kernel yield() calls behave for threaded processes. That is probably not right. > Index: kern/sched_bsd.c > =================================================================== > RCS file: /cvs/src/sys/kern/sched_bsd.c,v > retrieving revision 1.43 > diff -u -p -r1.43 sched_bsd.c > --- kern/sched_bsd.c 9 Mar 2016 13:38:50 -0000 1.43 > +++ kern/sched_bsd.c 19 Mar 2016 12:21:36 -0000 > @@ -298,7 +298,16 @@ yield(void) > int s; > > SCHED_LOCK(s); > - p->p_priority = p->p_usrpri; > + /* > + * If one of the threads of a multi-threaded process called > + * sched_yield(2), drop its priority to ensure its siblings > + * can make some progress. > + */ > + if (TAILQ_FIRST(&p->p_p->ps_threads) == p && > + TAILQ_NEXT(p, p_thr_link) == NULL) > + p->p_priority = p->p_usrpri; > + else > + p->p_priority = min(MAXPRI, p->p_usrpri * 2); > p->p_stat = SRUN; > setrunqueue(p); > p->p_ru.ru_nvcsw++; > >
