Re: [Xenomai-core] ns vs. tsc as internal timer base

[Philippe Gerum <rpm@xenomai.org>]

Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
>  > Gilles Chanteperdrix wrote:
>  > > Philippe Gerum wrote:
>  > >  > static inline unsigned long long ns_2_cycles(unsigned long long ns)
>  > >  > {
>  > >  >      return ns * ns2cyc_scale >> NS2CYC_SCALE_FACTOR;
>  > > 
>  > > This multiplication is 64 bits * 32 bits, the intermediate result may
>  > > need more than 64 bits, so you should compute it the same way as the
>  > > beginning of ullimd. Something like:
>  > 
>  > Sure, but the point is that if we were to use such code, we should bound 
>  > the 64bit operand and would not use it beyond the tolerable loss of 
>  > accuracy on output (e.g. 2ms).  This would require to break longer shots 
>  > in several smaller ones, relying on the internal timer management logic 
>  > to redo the shot until it has actually elapsed (which should be a rare 
>  > case for oneshot timing), a bit like we are currently doing in bounding 
>  > the values to 2^32-1 right now. Going for ullimd alike implementation 
>  > somehow impedes the overall effort in reducing the CPU footprint, I 
>  > guess. This said, I have still no clue if the gain in computation cycles 
>  > is worth the additional overhead of dealing with possibly early shots - 
>  > I tend to think it would be better on average though.
> 
> Ok, we could then write:
> 
> static inline unsigned long long ns_2_cycles(unsigned ns)
> {
>     return (unsigned long long) ns * ns2cyc_scale >> NS2CYC_SCALE_FACTOR;
> }
> 

Yep.

-- 

Philippe.