linux-kernel - Re: [patch 0/9] x86_64: reliable TSC-based gettimeofday

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Message-ID: <20070201145236.GA23394@dwarf.suse.cz>
Date:	Thu, 1 Feb 2007 15:52:43 +0100
From:	Jiri Bohac <jbohac@...e.cz>
To:	Andi Kleen <ak@...e.de>
Cc:	jbohac@...e.cz, linux-kernel@...r.kernel.org,
	Vojtech Pavlik <vojtech@...e.cz>, ssouhlal@...ebsd.org,
	arjan@...radead.org, tglx@...utronix.de, johnstul@...ibm.com,
	zippel@...ux-m68k.org, andrea@...e.de
Subject: Re: [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@...e.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@...e.cz>
SUSE Labs, SUSE CZ

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