[PATCH] benchtests: Restore the clock_gettime option

[H.J. Lu]

On Tue, May 19, 2020 at 3:16 PM Alexander Monakov <amonakov@ispras.ru> wrote:
>
> On Tue, 19 May 2020, H.J. Lu wrote:
>
> > On Tue, May 19, 2020 at 2:18 PM Alexander Monakov <amonakov@ispras.ru> wrote:
> > >
> > >
> > >
> > > On Tue, 19 May 2020, H.J. Lu via Libc-alpha wrote:
> > >
> > > > commit 7621e38bf3c58b2d0359545f1f2898017fd89d05
> > > > Author: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
> > > > Date:   Tue Jan 29 17:43:45 2019 +0000
> > > >
> > > >     Add generic hp-timing support
> > > >
> > > > removed the clock_gettime option.  On x86, fewer cycles doesn't
> > > > necessarily mean faster exection due to frequency drop.  We should
> > > > restore the clock_gettime option.
> > >
> > > Can you please elaborate more which x86 CPUs you have in mind here,
> > > as since Nehalem (2008) when invariant TSC was introduced, the rdtsc(p)
> > > instructions count at a fixed rate rather than at CPU clock rate.
> > > And before 2008, there were no turbo frequencies and no AVX frequency drop.
> >
> > https://stackoverflow.com/questions/56852812/simd-instructions-lowering-cpu-frequency
>
> I am well aware. Again: rdtsc does not count CPU cycles on recent Intel CPUs.
> RDTSC reads a register that increments at a fixed rate. So its increment is
> proportional to wall clock time. When a workload is causing a reduction in
> actual CPU frequency, RDTSC increment frequency is not affected and so it
> remains suitable for measuring the actual wall-clock time.
>
> Alexander

We'd like to have it as an option on x86.

-- 
H.J.