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SubjectRe: [PATCH] x86: Export tsc related information in sysfs

> > Yes, understood. But the kernel doesn't expose a "gettimeofday
> > performance sucks" flag either. If it did (or in the case of
> > the patch, if tsc_reliable is zero) the application could at least
> > choose to turn off the 10000-100000 timestamps/second and log
> > a message saying "you are running on old hardware so you get
> > fewer features".
> I don't think anyone would object to exporting such a flag if
> it's cleanly designed.
> Getting the semantics right for that might be somewhat tricky
> though. How is "slow" defined?

Well... if you want to know how fast gettimeofday is, perhaps doing

gettimeofday(); gettimeofday();

is good enough?

If not, perhaps you can export 'how many clocks is gettimeofday
expected to take' variable somewhere, but...


> > A CPU-hotplugable system is a good example of a case where
> > the kernel should expose that tsc_reliable is 0. (I've heard
> That would mean that a large class of systems which
> are always hotplug capable (even if it's not used)
> would never get fast TSC time.
> Wasn't the goal here to be faster?
> > anecdotally that CPU hotplug into a QPI or Hypertransport system
> > will have some other interesting challenges, so may require some
> > special kernel parameters anyway.) Even if tsc_reliable were
> > only enabled if a "no-cpu_hotplug" kernel parameter is set,
> > that is still useful. And with cores-per-socket (and even
> > nodes-per-socket) going up seemingly every day, multi-socket
> > systems will likely be an ever smaller percentage of new
> > systems.
> Still the people running them will expect as good performance
> as possible.
> -Andi

(cesky, pictures)

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