Gilles Chanteperdrix wrote:
On Fri, Apr 4, 2008 at 3:25 PM, Jan Kiszka <[EMAIL PROTECTED]> wrote:Gilles Chanteperdrix wrote:On Fri, Apr 4, 2008 at 12:45 PM, Jan Kiszka <[EMAIL PROTECTED]> wrote:Sebastian Smolorz wrote:Jan Kiszka wrote:Sebastian Smolorz wrote:Jan Kiszka wrote:This patch may do the trick: it uses the inverted tsc-to-nsfunctioninstead of the frequency-based one. Be warned, it is totally untestedinsideXenomai, I just ran it in a user space test program. But it may give an idea.Your patch needed two minor corrections (ns instead of ts infunctionsxnarch_ns_to_tsc()) in order to compile. A short run (30 minutes) oflatency-t1 seems to prove your bug-fix: There seems to be no drift.That's good to hear.If I got your patch correctly, it doesn't make xnarch_tsc_to_nsmoreprecise but introduces a new function xnarch_ns_to_tsc() which is alsolessprecise than the generic xnarch_ns_to_tsc(), right?Yes. It is now precisely the inverse imprecision, so to say. :)So isn't there still the danger of getting wrong values whencallingxnarch_tsc_to_ns() not in combination with xnarch_ns_to_tsc()?Only if the user decides to implement his own conversion. Xenomaiwithall its skins and both in kernel and user space should always runthroughthe xnarch_* path.OK, would you commit the patch?Will do unless someone else has concerns. Gilles, Philippe? ARM and Blackfin then need to be fixed similarly, full patch attached.Well, I am sorry, but I do not like this solution; - the aim of scaled math is to avoid divisions, and with this patch we end up using divisions;Please check again, no new division due to my patch, just different parameters for the existing one.I just checked your patch rapidly, but saw that xnarch_ns_to_tsc was using llimd, so does use division. My fast_llimd could be used to replace both the llimd calls in xnarch_tsc_to_ns and xnarch_ns_to_tsc.- with scaled math we do wrong calculations, and making a wrong xnarch_ns_to_tsc only works for values which should be passed to xnarch_tsc_to_ns.IMHO, the error is within the range of the clock's precision, if not even below. So struggling for mathematically precise conversion of imprecise physical values makes no sense to me. Therefore I once proposed the scaled-math optimization.Now that I have understood what really happens, I disagree with this approach. Take the implementation of clock_gettime (or rtdm_clock_read, for that matter). They basically do xnarch_tsc_to_ns(ipipe_read_tsc()). The relative error may be small, but in the very frequent use case of substracting two results of consecutive reads of ipipe_read_tsc, the result of the substraction is essentially garbage, because the result of the substraction may be of the same order as the absolute error of the conversion. And I insist, this use case of clock_gettime or rtdm_clock_read is a very realistic use case.
This use case is valid, but I don't see the error scenario you sketch: The error of the conversion is only relevant for large deltas, tsc_to_ns(B)-tsc_to_ns(A)=B-A for any small B-A. Cornelius' test nicely showed constantly increasing deviation, not something that jumped around. Essentially, we are just replacing
xnarch_llimd(ts, 1000000000, RTHAL_CPU_FREQ); with xnarch_llimd(ts, xnarch_tsc_scale, 1<<xnarch_tsc_shift);which introduces a linearly increasing error of the _absolute_ results, not of relative ones. But if you can prove me wrong, I would take everything back and agree on kicking out the scaled math immediately!
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