[ntp:questions] Loopstats jitter field mostly zero?

Charles Swiger cswiger at mac.com
Tue Jan 8 14:01:51 UTC 2013


On Jan 8, 2013, at 2:16 AM, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> Thanks, Harlan.  I see that a function SQRT() is used, and that this function is defined as sqrt() in ntp.h:
>  #define SQRT(x) (sqrt(x))
> I recall seeing /something/ about hardware and software floating point support in the Raspberry Pi, that some hardware/firmware/software had it and some not.

AFAIK, all of the Raspberry Pis are using an ARM1176JZF-S CPU.  The J means Jazelle (aka ThumbEE); dunno about the Z; the F means H/W floating point (aka VFP); S means the TrustZone Security Extensions:


> I wonder whether that might be the cause?  I can't imagine sqrt() being broken, but maybe for very small numbers it's wrongly returning zero?  Single precision versus double precision?  As you know, I'm well out of my depth on this, but could there be a configuration flag to use software FP rather than hardware?  I wonder how best to go about solving this problem?

Well, there are software floating point and H/W testing software available which one might use to check:

http://www.netlib.org/fp/ (see UCBTEST)


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