* 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/