[ntp:questions] How to determin hardware latency for PPS offset given simple tools.

unruh unruh at invalid.ca
Fri Jun 7 15:17:54 UTC 2013

On 2013-06-07, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> On 06/06/2013 20:17, Doug Calvert wrote:
>> unruh has a nice page about interrupt coalescing:
>> http://axion.physics.ubc.ca/scatter/rt.html
> This is a quite interesting note, unfortunately rendered rather less 
> useful by the graphs, which have no axis names or units marked, and 
> which appear to have inconsistent scales (although this may simply be 
> lack of legibility).  For example:
>    http://axion.physics.ubc.ca/scatter/rtdt1.gif
> The units on the vertical axis appear to be labelled:
>    -0.0001  -5x10-6  0  5x10-6  0.0001

Yes, supermongo, the plotting program, does tend to make 5 and 6 similar
at the small sizes used in superscripts.  But as you say, it is a linear
scale, and thus that must be a 5 rather than a 6. 
> which I interpret as -100us, -5 us, 0,  +5us  +100us, but it appears to 

Since that is, as you point out, an interpretation that produces silly
results, it is probably the interpretation that is at fault. 
> be a linear axis.  Is the 5x10-6 supposed to be 5x10-5?  The character 
> needs serious improvement as it looks like 5x10-6 to me.  But in any 
> case, as microseconds are referred to in the text, would it not be a lot 
> clearer if the graphs were also in microseconds, rather than some small 
> fraction of seconds?
> Similar problems in:
>    http://axion.physics.ubc.ca/scatter/gps.gif
> where the -5 and -6 suffices are very difficult to distinguish.  I do 
> hope that Bill can re-do these graphs some time as it would enhance the 
> value of this work.

I am not at all sure that I can find the data anymore to redo the
About all I could do is to use gimp to replace the axes labels and make
the 5 look a bit more like 5 rather than 6. The problem is at the small
scale of the superscripts, the little culicue up at the bottom of the 5
in the font used becomes a single pixel closing off the the loop, making
it look a lot like a 6. So I used gimp to open it up again, and also to
labels on the axes. 

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