* Daniel Walker <[email protected]> wrote:
> > > > I see sched_clock() as fast first, accurate second. Whereas the
> > > > clocksource thing is accurate first, fast second.
> > >
> > > This is true .. However, if there is a speed different it's small.
> >
> > Ugh. Have you ever compared pmtimer (or even hpet) against TSC based
> > sched_clock()? What you write is so wrong that it's not even funny.
> > You keep repeating this nonsense despite having been told multiple
> > times that you are dead wrong.
>
> Yes I have, and your right there is a difference, and a big difference
> .. Above I was referring only to the TSC clocksource, since that's an
> apples to apples comparison .. I would never compare the TSC to the
> acpi_pm, that's no contest ..
You still dont get it i think: in real life we end up using the TSC in
sched_clock() _much more often_ than we end up using the TSC for
clocksource! So your flawed suggestion does not fix anything, it in fact
introduces a really bad regression: instead of using the TSC (or
jiffies) we'd end up using the pmtimer or hpet for every lock operation
when lockstat is enabled, bringing the box to a screeching halt in
essence.
so what you suggest has a far worse effect on the _majority_ of systems
that are even interested in running lockstat, than the case you
mentioned that some seldom-used arch which is lazy about sched_clock()
falls back to jiffies granularity. It's not a big deal: the stats will
have the same granularity. (the op counts in lockstat will still be
quite useful)
sched_clock() is a 'fast but occasionally inaccurate clock', while the
GTOD clocksource is an accurate clock (but very often slow).
Ingo
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