Re: dtrace users opinion solicited (timestamps)

On Thu, Jul 9, 2009 at 19:31, Andriy Gapon<avg_at_freebsd.org> wrote:
>
> As you might be aware DTrace timestamps right now are derived from TSC value.
> http://en.wikipedia.org/wiki/Time_Stamp_Counter
>
> DTrace timestamps are measured in nano-seconds and the formula similar to the
> following is used for calculations:
> rdtsc() * 1000000000 / tsc_freq
> where rdtsc is a function that returns current TSC value and tsc_freq is a
> frequency of TSC.
>
> This formula is supposed to produce proper results if tsc_freq stays constant.
> But there are environment where this might not be the case.
> If a CPU has a non-invariant TSC and processor's clock frequency changes (e.g.
> because of powerd), then tsc_freq changes too.
> As a result, the formula would produce wildly different values and, most
> importantly, was values would non be monotonic. Timestamp values that jump back
> and forth would not only be useless for a user, they would also confuse DTrace
> internal logic.
>
> There are at least the following two alternatives:
>
> 1. Keep things as they are and warn users not to change CPU clock frequency when
> they use DTrace and the CPU doesn't have invariant TSC. I think that this should
> cause only minor inconveniences to a portion of DTrace users.
>
> 2. Use raw TSC value as a DTrace timestamp and document this difference from the
> original DTrace. Advantage: timestamp value is always monotonic. Disadvantage:
> manual conversion is needed to get "real" time (using the same formula).
> Please note that in this case timestamps would be in non-linear time dimension if
> TSC frequency changes, so to get meaningful timestamps (when needed/important) one
> would still have to make sure that TSC frequency stay constant.
>
> Please share your opinion on these approaches.
> Or suggest yest another alternative.

What about atomically changing tsc_freq every time the frequency is changed?

>
> Just in case, related sysctls:
> machdep.tsc_freq
> kern.timecounter.invariant_tsc