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SubjectRe: Scheduler benchmarks - a follow-up

* Rob Hussey <robjhussey@gmail.com> 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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