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SubjectRe: [patch 0/9] x86_64: reliable TSC-based gettimeofday
On Thu, Feb 01, 2007 at 12:20:59PM +0100, Andi Kleen wrote:
> On Thursday 01 February 2007 10:59, jbohac@suse.cz wrote:
>
> >
> > Inter-CPU monotonicity can not, however, be guaranteed in a vsyscall, so
> > vsyscall is not used by default.
>
> Only for unsynchronized machines I hope

yes, sorry, only on unsynchronized machines

> The big strategic problem is how to marry your patchkit to John Stultz's
> clocksources work which is also competing for merge. Any thoughts on that?

I'll look into that next week. Sorry, I wanted to do that a long time
ago, but I spent weeks (over a month) fighting a nasty livelock
in the code. (Morale: think twice before using a spinlock inside
a {do .. while (read_seqretry(..))} loop)
> >When strict inter-CPU monotonicity is not needed, the vsyscall version of
> >gettimeofday may be forced using the "nomonotonic" command line parameter.
> >gettimeofday()'s monotonicity is guaranteed on a single CPU even with the very
> >fast vsyscall version. Across CPUs, the vsyscall version of gettimeofday is
> >not guaranteed to be monotonic, but it should be pretty close. Currently, we
> >get errors of tens/hundreds of microseconds.
>
> I think a better way to do this would be to define a new CLOCK_THREAD_MONOTONOUS
> (or better name) timer for clock_gettime().

I absolutely agree. Will do that. This should give userspace a
decently accurate and very fast time source.

--
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, SUSE CZ

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