From: john cooper <john.cooper@third-harmonic.com>
To: dwalker@mvista.com
Cc: Kevin Hilman <khilman@mvista.com>, Ingo Molnar <mingo@elte.hu>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
john cooper <john.cooper@third-harmonic.com>
Subject: Re: 2.6.18-rt7: rollover with 32-bit cycles_t
Date: Wed, 08 Nov 2006 08:29:04 -0500 [thread overview]
Message-ID: <4551DBA0.6080305@third-harmonic.com> (raw)
In-Reply-To: <1162956221.20694.13.camel@dwalker1.mvista.com>
Daniel Walker wrote:
> Seems like the check should really be using something like time_before()
> time_after() which takes the rollover into account .. What I don't
> understand is why we don't see those on x86 ..
Probably due to the fact it is a 64-bit counter. Even with
a free running rate of 10Ghz it would take nearly 60 years
to wrap.
On PPC and ARM 32-bit counters seems to be common which limits
range unless a prescaler is available. The better solution is
to detect wrap as a prescaled 32 bit measurement will eventually
run out of usable resolution as core frequency increases. That is
assuming 64-bit counters don't eventually show up in these
architectures.
-john
--
john.cooper@third-harmonic.com
prev parent reply other threads:[~2006-11-08 13:28 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-11-08 1:36 2.6.18-rt7: rollover with 32-bit cycles_t Kevin Hilman
2006-11-08 2:06 ` john cooper
2006-11-08 19:22 ` Kevin Hilman
2006-11-09 9:14 ` Ingo Molnar
2006-11-08 3:23 ` Daniel Walker
2006-11-08 13:29 ` john cooper [this message]
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