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SubjectRe: [RFC 0/3] Improve fallback LPJ calculation

Adding Tony and Linux-OMAP cc:, as this patchset was written specifically
with the OMAP in mind (thread rooted at http://lkml.org/lkml/2010/9/10/104 ).
Any comments would be appreciated.

Phil

On 10/09/10 11:54 +0200, Carmody Phil.2 (EXT-Ixonos/Helsinki) wrote:
>
> The motivation for patch 2/3 is that currently our OMAP calibrates
> itself using the trial-and-error binary chop fallback that some other
> architectures no longer need to perform. This is a lengthy process,
> taking 0.2s in an environment where boot time is of great interest.
>
> The patch has two optimisations. Firstly, it replaces the initial
> repeated-doubling to find the relevant power of 2 with a tight loop
> that just does as much as it can in a jiffy. Secondly, it doesn't
> binary chop over an entire power of 2 range, it choses a much smaller
> range based on how much it squeezed in, and failed to squeeze in,
> during the first stage. Both are significant optimisations, and
> bring our calibration down from 23 jiffies to 6, and, in the process,
> arrive at a more accurate lpj value.
>
> The 'bands' and 'sub-logarithmic' growth may look over-engineered,
> but they cost a small level of in inaccuracy of the initial guess
> (for all architectures) in order to avoid the very large inaccuracies
> that appeared during testing (on x86_64 architectures, and presumably
> others with less metronomic operation). Note that due to the existence
> of the TSC and other timers, the x86_64 doesn't use this fallback
> routine, but I wanted to code defensively, able to cope with all kinds
> of processor behaviours.
>
> 1/3 is simply cosmetic to prepare for 2/3.
> 3/3 is simply to assist testing and not intended for integration.
>
> I would appreciate it if people with exotic or unusual architectures
> could help test this.
>
> Cheers,
> Phil
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