Using rdtsc to timestamp RTT of packets

Stephen/Gabor/Harry: Gents thanks for the guidance; this mailing list is
great. The message is rdtsc is just fine. Got it.

The command line (from Harry):
>lscpu | egrep "constant_tsc|nonstop_tsc"
is operationally ideal: specific, easy, and answers the issue of
invariance/constancy clearly.

On Mon, Mar 6, 2023 at 11:56 AM Stephen Hemminger <
stephen@networkplumber.org> wrote:

> On Sun, 5 Mar 2023 20:01:15 -0500
> fwefew 4t4tg <7532yahoo@gmail.com> wrote:
>
> > I think rdtsc does all this. But then I read [1]:
> >
> >    - The TSC is not always invariant
> >    - And of course context switches (if a thread is not pinned to a core)
> >    will invalidate any time difference
> >    - The TSC is not incremented when the processor enters a deep sleep. I
> >    don't care about this because I'll turn off the power saving modes
> anyway
>
> Stack Overflow is only one step better than ChatGPT in giving
> partially correct answers.
>
> TSC is almost always works well on modern processors.
> The Linux kernel aligns all the TSC values for each core at boot up.
> It is invariant (derived from a single clock source) unless you have some
> poorly designed NUMA system.  In the past, there were some CPU's that
> did bad things during suspend, but that is fixed in current generations.
>
> Bottom line: that advice is no longer true.
>