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SubjectRe: [RFC PATCH 1/3] Unified trace buffer

On Fri, 26 Sep 2008, Ingo Molnar wrote:
>
> Firstly they need a low-frequency (10khz-100khz) shared clock line
> across all CPUs. A single line - and since it's low frequency it could
> be overlaid on some existing data line and filtered out. That works
> across NUMA nodes as well and physics allows it to be nanosec accurate
> up to dozens of meters or so.

Can this possibly be true? I mean, light travels only one foot every
nanosecond. Can it really keep nanosecond accuracy up to dozens of meters
away? If you send the same signal to CPU1 that is 1 foot away, as well as
send it to CPU2 that is 2 feet away. CPU2 will get that signal at least 1
nanosec after CPU1 receives it.

Of course if the hardware is smart enough to know this topology, then it
could account for these delays in traffic.

-- Steve



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