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Old 09-23-2012, 10:52 AM
Russell Coker
 
Default nacl and CPU frequency.

On Sun, 23 Sep 2012, "Thomas Preud'homme" <robotux@celest.fr> wrote:
> Le dimanche 23 septembre 2012 01:42:01, Ben Hutchings a écrit :
> > So the build process is trying to determine which method will work?
> > Then what if it settles on some method that is available on the build
> > machine's kernel, but not the target distribution's kernel? I think you
> > need to either (1) make it defer this to run-time or (2) force the
> > decision at build-time, and be conservative.
>
> If I understood correctly he is ready to patch the build system to use a
> specific method for this precise reason but is asking which method would
> work on all Debian system.

How can build-time recognition of CPU features be useful in any way for
Debian? We can usually be certain that most systems which run the code will
have different CPUs to the system used for building.

If hard-coding the number to some reasonable value isn't good enough then code
needs to be written to detect it at run-time.

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Old 09-23-2012, 10:56 PM
Craig Small
 
Default nacl and CPU frequency.

On Sat, Sep 22, 2012 at 05:29:45PM +0100, peter green wrote:
> I therefore conclude that if it's not returning elapsed times in true CPU
> cycles it probablly doesn't matter much if the supposed CPU speed and the
> real CPU speed are not exactly the same.
>
> As such unless someone objects I plan to patch the code to fallback to
> using bogomips as a "psuedo CPU speed" if true CPU speed cannot be
> determined.
procps has been struggling with this for years, though ita mainly around
the CPU ticks versus real seconds. If you are really bored, have a look
at some old procps bugs where this sort of thing has broken.

We've narrowed it down to a small handful of checks. The real value is
probably unknown for some devices so we just set it to a fixed value.

The point is, trying to guess this sort of stuff will probably give you
the wrong number so a "pseduo CPU speed" is probably as bad as anything
else you can come up with, so find something simple.

- Craig
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