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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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