linux-kernel - Re: Scheduler benchmarks

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Message-ID: <20070917130524.GA10707@elte.hu>
Date:	Mon, 17 Sep 2007 15:05:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Rob Hussey <robjhussey@...il.com>
Cc:	linux-kernel@...r.kernel.org, ck@....kolivas.org
Subject: Re: Scheduler benchmarks - a follow-up


* Rob Hussey <robjhussey@...il.com> wrote:

> http://www.healthcarelinen.com/misc/benchmarks/BOUND_hackbench_benchmark2.png

heh - am i the only one impressed by the consistency of the blue line in 
this graph? :-) [ and the green line looks a bit like a .. staircase? ]

i've meanwhile tested hackbench 90 and the performance difference 
between -ck and -cfs-devel seems to be mostly down to the more precise 
(but slower) sched_clock() introduced in v2.6.23 and to the startup 
penalty of freshly created tasks.

Putting back the 2.6.22 version and tweaking the startup penalty gives 
this:

                             [hackbench 90, smaller is better]

            sched-devel.git      sched-devel.git+lowres-sched-clock+dsp
            ---------------      --------------------------------------
                      5.555                  5.149
                      5.641                  5.149
                      5.572                  5.171
                      5.583                  5.155
                      5.532                  5.111
                      5.540                  5.138
                      5.617                  5.176
                      5.542                  5.119
                      5.587                  5.159
                      5.553                  5.177
            --------------------------------------
                 avg: 5.572             avg: 5.150 (-8.1%)

('lowres-sched-clock' is the patch i sent in the previous mail. 'dsp' is 
a disable-startup-penalty patch that is in the latest sched-devel.git)

i have used your .config to conduct this test.

can you reproduce this with the (very-) latest sched-devel git tree:

  git-pull git://git.kernel.org/pub/scm/linux/kernel/git/mingo/linux-2.6-sched-devel.git

plus with the low-res-sched-clock patch (re-) attached below?

	Ingo
---
 arch/i386/kernel/tsc.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux/arch/i386/kernel/tsc.c
===================================================================
--- linux.orig/arch/i386/kernel/tsc.c
+++ linux/arch/i386/kernel/tsc.c
@@ -110,9 +110,9 @@ unsigned long long native_sched_clock(vo
 	 *   very important for it to be as fast as the platform
 	 *   can achive it. )
 	 */
-	if (unlikely(!tsc_enabled && !tsc_unstable))
+	if (1 || unlikely(!tsc_enabled && !tsc_unstable))
 		/* No locking but a rare wrong value is not a big deal: */
-		return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
+		return jiffies_64 * (1000000000 / HZ);
 
 	/* read the Time Stamp Counter: */
 	rdtscll(this_offset);
-
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