linux-kernel - Re: [patch] CFS scheduler, -v6

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Date:	Sat, 28 Apr 2007 20:53:27 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Gene Heskett <gene.heskett@...il.com>, Mark Lord <lkml@....ca>,
	Zach Carter <linux@...hcarter.com>,
	buddabrod <buddabrod@...il.com>
Subject: Re: [patch] CFS scheduler, -v6

On Sat, Apr 28, 2007 at 03:53:38PM +0200, Ingo Molnar wrote:
> > Won't it help if you update rq->rb_leftmost above from the value 
> > returned by rb_first(), so that subsequent calls to first_fair will be 
> > sped up?
> 
> yeah, indeed. Would you like to do a patch for that?

My pleasure :)

With the patch below applied, I ran a "time -p make -s -j10 bzImage"
test.

2.6.20 + cfs-v6			-> 186.45 (real)
2.6.20 + cfs-v6 + this_patch	-> 184.55 (real)

or about ~1% improvement in real wall-clock time. This was with the default 
sched_granularity_ns of 6000000. I suspect larger the value of
sched_granularity_ns and the number of (SCHED_NORMAL) tasks in system, better 
the benefit from this caching.


Cache value returned by rb_first(), for faster subsequent lookups.

Signed-off-by : Srivatsa Vaddagiri <vatsa@...ibm.com>


---


diff -puN kernel/sched_fair.c~speedup kernel/sched_fair.c
--- linux-2.6.21/kernel/sched_fair.c~speedup	2007-04-28 19:28:08.000000000 +0530
+++ linux-2.6.21-vatsa/kernel/sched_fair.c	2007-04-28 19:34:55.000000000 +0530
@@ -86,7 +86,9 @@ static inline struct rb_node * first_fai
 {
 	if (rq->rb_leftmost)
 		return rq->rb_leftmost;
-	return rb_first(&rq->tasks_timeline);
+	/* Cache the value returned by rb_first() */
+	rq->rb_leftmost = rb_first(&rq->tasks_timeline);
+	return rq->rb_leftmost;
 }
 
 static struct task_struct * __pick_next_task_fair(struct rq *rq)
_




	

-- 
Regards,
vatsa
-
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