Date: Wed, 06 Jul 2011 17:49:53 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: freebsd-arch@FreeBSD.org Subject: SCHED_ULE preempt_thresh: PRI_MIN_KERN -> PRI_MIN_IDLE Message-ID: <4E147611.6060100@FreeBSD.org>
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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. -- Andriy Gapon
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