Messages in this thread | | | Date | Fri, 4 Jun 2010 16:24:39 +0200 | From | Pavel Machek <> | Subject | Re: [PATCH] x86: Export tsc related information in sysfs |
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Hi!
> > 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...
Pavel
> > 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 >
-- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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