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Date: Thu, 03 Oct 2002 14:15:09 -0700
From: Matthew Dobson <colpatch@us.ibm.com>
Reply-To: colpatch@us.ibm.com
Organization: IBM LTC
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To: Ingo Molnar <mingo@elte.hu>
CC: Nick Piggin <piggin@cyberone.com.au>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Michael Hohnbaum <hohnbaum@us.ibm.com>
Subject: Re: [rfc][patch] kernel/sched.c oddness?
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Ingo Molnar wrote:
> this was done intentionally, and this scenario (1+2 tasks) is the very
> worst scenario. The problem is that by trying to balance all 3 tasks we
> now have 3 tasks that trash their cache going from one CPU to another.  
> (this is what happens with your patch - even with another approach we'd
> have to trash at least one task)
> 
> By keeping 2 tasks on one CPU and 1 task on the other CPU we avoid
> cross-CPU migration of threads. Think about the 2+3 or 4+5 tasks case
> rather, do we want absolutely perfect balancing, or good SMP affinity and
> good combined performance?
OK...  But what about the (imbalance / 2) part?  Either the comment 
needs to change, or the code.  Attatched is a slightly revised patch for 
the code.  The comment patch would be even easier:

- 
/* It needs an at least ~25% imbalance to trigger balancing. */
+ 
/* It needs an at least ~50% imbalance to trigger balancing. */

Either way works for me.  I'd like to see something done, as the 
comments don't match the code right now...

Cheers!

-Matt

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diff -Nur --exclude-from=/usr/src/.dontdiff linux-2.5.40-vanilla/kernel/sched.c linux-2.5.40-sched_cleanup/kernel/sched.c
--- linux-2.5.40-vanilla/kernel/sched.c	Tue Oct  1 00:07:35 2002
+++ linux-2.5.40-sched_cleanup/kernel/sched.c	Thu Oct  3 14:09:31 2002
@@ -689,10 +689,10 @@
 	if (likely(!busiest))
 		goto out;
 
-	*imbalance = (max_load - nr_running) / 2;
+	*imbalance = max_load - nr_running;
 
 	/* It needs an at least ~25% imbalance to trigger balancing. */
-	if (!idle && (*imbalance < (max_load + 3)/4)) {
+	if (!idle && (*imbalance <= (max_load + 3)/4)) {
 		busiest = NULL;
 		goto out;
 	}
@@ -746,6 +746,11 @@
 	task_t *tmp;
 
 	busiest = find_busiest_queue(this_rq, this_cpu, idle, &imbalance);
+	/*
+	 * We only want to steal a number of tasks equal to 1/2 the imbalance,
+ 	 * otherwise, we'll just shift the imbalance to the new queue.
+	 */
+	imbalance /= 2;
 	if (!busiest)
 		goto out;
 

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