linux-kernel - Re: [PATCH] x86: Export tsc related information in sysfs

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Message-ID: <20100604142439.GC1760@ucw.cz>
Date:	Fri, 4 Jun 2010 16:24:39 +0200
From:	Pavel Machek <pavel@....cz>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Export tsc related information in sysfs

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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