Re: Scheduler benchmarks - a follow-up

[Ingo Molnar]

* Rob Hussey <robjhussey@xxxxxxxxx> wrote:

> Hi all,
> 
> After posting some benchmarks involving cfs 
> (http://lkml.org/lkml/2007/9/13/385), I got some feedback, so I 
> decided to do a follow-up that'll hopefully fill in the gaps many 
> people wanted to see filled.

thanks for the update!

> I'll start with some selected numbers, which are preceded by the 
> command used for the benchmark.
> 
> for((i=2; i < 201; i++)); do lat_ctx -s 0 $i; done:
> (the left most column is the number of processes ($i))
> 
> 	2.6.21		2.6.22-ck1	2.6.23-rc6-cfs-devel
> 
> 15	5.88		4.85		5.14
> 16	5.80		4.77		4.76

the unbound results are harder to compare because CFS changed SMP 
balancing to saturate multiple cores better - but this can result in a 
micro-benchmark slowdown if the other core is idle (and one of the 
benchmark tasks runs on one core and the other runs on the first core). 
This affects lat_ctx and pipe-test. (I'll have a look at the hackbench 
behavior.)

> Bound to Single core:

these are the more comparable (apples to apples) tests. Usually the most 
stable of them is pipe-test:

> pipe-test:
> 
> 	2.6.21	2.6.22-ck1	2.6.23-rc6-cfs-devel
> 
> 1  	9.27	8.50 		8.55
> 2  	9.27	8.47 		8.55
> 3  	9.28	8.47 		8.54
> 4  	9.28	8.48 		8.54
> 5  	9.28	8.48 		8.54

so -ck1 is 0.8% faster in this particular test. (but still, there can be 
caching effects in either direction - so i usually run the test on both 
cores/CPUs to see whether there's any systematic spread in the results. 
The cache-layout related random spread can be as high as 10% on some 
systems!)

many things happened between 2.6.22-ck1 and 2.6.23-cfs-devel that could 
affect performance of this test. My initial guess would be sched_clock() 
overhead. Could you send me your system's 'dmesg' output when running a 
2.6.22 (or -ck1) kernel? Chances are that your TSC got marked unstable, 
this turns on a much less precise but also faster sched_clock() 
implementation. CFS uses the TSC even if the time-of-day code marked it 
as unstable - going for the more precise but slightly slower variant.

To test this theory, could you apply the patch below to cfs-devel (if 
you are interested in further testing this) - this changes the cfs-devel 
version of sched_clock() to have a low-resolution fallback like v2.6.22 
does. Does this result in any measurable increase in performance? 

(there's also a new sched-devel.git tree out there - if you update to it 
you'll need to re-pull it against a pristine Linus git head.)

	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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