* Christoph Lameter ([email protected]) wrote:
> On Fri, 17 Aug 2007, Mathieu Desnoyers wrote:
>
> > Why do you printk inside the timing period ? Filling the printk buffers
> > or outputting on things such as serial console could really hurt your
> > results.
>
> It was easier to code?
>
> > I hope you run your system with idle=poll and without frequency scaling
> > at all, because otherwise your cycle count would be completely off on
> > many AMD and Intel CPUs. You can have a look at this (very rough)
> > document on the topic:
>
> The cpu will definitely not be idle during these measurements and no
> frequency scaling is active.
The problem is that if the cpu is idle _before_ the measurements, the
frequency will change differently from one cpu to another. Therefore,
the cycle counters may have large offsets when you start your tests. So,
if get_cycles() is executed on different CPUs (thread being migrated)
between the beginning and the end of the test, the results would be
skewed.
Mathieu
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
