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Date: Thu, 07 Jul 2011 09:37:41 +0800
From: David Xu <davidxu@freebsd.org>
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To: Andriy Gapon <avg@freebsd.org>
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Cc: freebsd-arch@freebsd.org
Subject: Re: SCHED_ULE preempt_thresh: PRI_MIN_KERN -> PRI_MIN_IDLE
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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,