Date: Thu, 07 Jul 2011 09:37:41 +0800 From: David Xu <davidxu@freebsd.org> To: Andriy Gapon <avg@freebsd.org> Cc: freebsd-arch@freebsd.org Subject: Re: SCHED_ULE preempt_thresh: PRI_MIN_KERN -> PRI_MIN_IDLE Message-ID: <4E150DE5.8000801@freebsd.org> In-Reply-To: <4E147611.6060100@FreeBSD.org> References: <4E147611.6060100@FreeBSD.org>
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On 2011/07/06 22:49, Andriy Gapon wrote: > > I do not have sufficient knowledge of SCHED_ULE, so maybe I shouldn't even talk > about this, but I couldn't help but notice that many (many) users have reported in > the past heavy interactivity problems with SCHED_ULE under high load, especially > I/O-related load. > > The universal advice has always been to tune preempt_thresh via sysctl > kern.sched.preempt_thresh=224. I think that David Xu was the first person that I > saw recommending this. In all cases users have reported significant improvements. > I must add that I also have the experience and I do use preempt_thresh=224 to > this day. > > Now, I would like to discuss this phenomenon in two veins: > 1. Why do we see the interactivity problem with the default setting of > preempt_thresh=PRI_MIN_KERN (provided that PREEMPTION is enabled and > FULL_PREEMPTION is not)? Could this be a general ULE issue? Or could it be > because of some particular hogs (like, purely hypothetically speaking, GEOM threads)? > > 2. Why don't we change the default (for PREEMPTION and !FULL_PREEMPTION case) to > preempt_thresh=PRI_MIN_IDLE? Plus sides of this have been reported via anecdotes. > What down sides could there be? > > Unfortunately somehow I just couldn't grasp ULE priorities and preemption, so I'd > like to ask for help of those who already have understood these things. > > Thank you. I think people must have found full preemption hurts performance for some benchmarks, normally batch-like scheduler (FIFO) have best peformance for server applications. But for desktop, you want to tune preempt_thresh to higher value, this should reduce interactivity jitter. Regards,
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