Date: Thu, 07 Jul 2011 09:53:12 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: David Xu <davidxu@FreeBSD.org> Cc: freebsd-arch@FreeBSD.org Subject: Re: SCHED_ULE preempt_thresh: PRI_MIN_KERN -> PRI_MIN_IDLE Message-ID: <4E1557D8.7040604@FreeBSD.org> In-Reply-To: <4E150DE5.8000801@freebsd.org> References: <4E147611.6060100@FreeBSD.org> <4E150DE5.8000801@freebsd.org>
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on 07/07/2011 04:37 David Xu said the following: > 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. Quite possible, but some data/reports wouldn't hurt. Please also note that preempt_thresh=PRI_MIN_IDLE is not exactly FULL_PREEMPTION, which sets preempt_thresh=PRI_MAX_IDLE. -- Andriy Gapon
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