dtrace users opinion solicited (timestamps)

[Andrew Brampton]

2009/7/9 Andriy Gapon <avg at freebsd.org>:
>
>
> 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.
>

According to wikipedia newer Intel processors have a constant rate TSC
whos freq does not change. If this features is available on most
processors today, then I am happy to stick with option 1.

Another problem with this is that on a multicore machine each core may
have different TSC values. Has anyone thought how to address this
issue? Could we calculate the offset of each core from core0, and then
ensure the offset is applied to the tsc value when it is needed?

thanks
Andrew