Re: [RFC patch 0/4] TSC calibration improvements

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, 4 Sep 2008, Ingo Molnar wrote:
> > 
> > ah - perhaps a dynamic statistical approach with an estimation of 
> > worst-case calibration error (~= standard deviation) and a quality 
> > threshold to reach? That could dramatically increase the number of 
> > samples while also making it much faster in practice. Nifty!
> 
> Oh, no, I'm _much_ more nifty than that!
> 
> Instead of being very clever, I have a very _stupid_ algorithm, one 
> that has very hardcoded expectations of exactly what it will see. And 
> if it doesn't see exactly that, it just fails early.
>
> I'd post the patch, but I really need to actually _test_ it first, and 
> I haven't rebooted yet.

ok, will wait with patience :)

i've been using adaptive calibration with great success in user-space, 
to run benchmarks on multiple boxes with a variable number of lmbench 
iterations. Once the observable statistical properties of the series of 
measurements looks valid it stops the test iterations and emits a 
result. This both makes things faster (more predictable hw/kernel 
executes certain lmbench tests much faster) and it makes the results 
more reliable (less noise, better cross-hardware, cross-kernel and 
cross-test comparisons). I've got a quality threshold (and a max 
iterations threshold) hardcoded as well.

	Ingo