[Planetlab-users] No guarantees regarding monotonically increasing time?

[Elvita Lobo]

In addition, I found this when I was exploring the same problem some time
ago:

http://lists.planet-lab.org/pipermail/users/2004-March/000050.html

Elvita.


On 8/31/07, Rob Sherwood <capveg at cs.umd.edu> wrote:
>
> On Fri, Aug 31, 2007 at 03:05:09AM -0500, Patel, Jay A wrote:
> > Hi,
> >
> > A few hours ago -- one of the nodes I was running an experiment on
> > (system18.ncl-ext.net) experienced a negative movement in time. I
> > assumed that PL nodes adjusted their clocks using NTP. Doesn't NTP
> > guarantee monotonically increasing time? Or, did I just observe a manual
> > time adjustment?
> >
> > Has anyone experienced this?
>
> Yes, this is unfortunately a known fact about planetlab.  It's documented
> in:
>
>
>        A Platform for Unobtrusive Measurements on PlanetLab,
>        Rob Sherwood and Neil Spring
>        USENIX Workshop on Real, Large Distributed Sytems, November 2006.
>        http://www.cs.umd.edu/~capveg/worlds06.paper.pdf
>
> and
>
>        Using PlanetLab for Network Research: Myths, Realities, and Best
> Practices,
>        Neil Spring, Larry Peterson, Andy Bavier, and Vivek Pai
>        ACM SIGOPS Operating Systems Review, 40:1, pages 17--24, January
> 2006.
>        http://www.cs.umd.edu/~nspring/papers/osr.pdf
>
> among other places.  The best I can recommend is that you make sure
> that your program doesn't logically depend on monotonic time
> increases.  For accurate timing measurements, I am exploring use of the
> rdtsc assembly instruction that returns the number of CPU ticks.  It is
> guaranteed to increases monotonically under certain conditions, and is
> relatively straight forward to convert to wallclock time if you know
> the machine's CPU speed.
>
> Hope this helps,
>
> - Rob
> .
>
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