Volanomark slows by 80% under CFS

[Tim Chen <tim.c.chen@linux.intel.com>]

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Ingo,

Volanomark slows by 80% with CFS scheduler on 2.6.23-rc1.  
Benchmark was run on a 2 socket Core2 machine.

The change in scheduler treatment of sched_yield 
could play a part in changing Volanomark behavior.
In CFS, sched_yield is implemented
by dequeueing and requeueing a process .  The time a process 
has spent running probably reduced the the cpu time due it 
by only a bit. The process could get re-queued pretty close
to head of the queue, and may get scheduled again pretty
quickly if there is still a lot of cpu time due.  

It may make sense to queue the
yielding process a bit further behind in the queue. 
I made a slight change by zeroing out wait_runtime 
(i.e. have the process gives
up cpu time due for it to run) for experimentation. 
Let's put aside gripes that Volanomark should have used a 
better mechanism to coordinate threads instead sched_yield for 
a second.   Volanomark runs better
and is only 40% (instead of 80%) down from old scheduler 
without CFS.  

Of course we should not tune for Volanomark and this is
reference data. 
What are your view on how CFS's sched_yield should behave?

Regards,
Tim




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--- linux-2.6.23-rc1/kernel/sched_fair.c.orig	2007-07-27 09:39:11.000000000 -0700
+++ linux-2.6.23-rc1/kernel/sched_fair.c	2007-07-27 09:40:41.000000000 -0700
@@ -841,6 +841,7 @@
 	 * position within the tree:
 	 */
 	dequeue_entity(cfs_rq, &p->se, 0, now);
+    	p->se.wait_runtime = 0; 
 	enqueue_entity(cfs_rq, &p->se, 0, now);
 }