"Syncing cpus" on a multi-cpu, dual core system

[M. Warner Losh]

In message: <200612141437.28724.mldodson at houston.rr.com>
            "M. L. Dodson" <mldodson at houston.rr.com> writes:
: On a computational chemistry list I subscribe to there is a
: current thread about multi-cpu systems needing to have the cpu
: frequencies synced (this is in a Linux context).  This is
: evidently not just having the cpus running at nominally the same
: frequency but something else in addition.  A posting in the thread
: said variations less than 0.1% were not problematic.  However, the
: poster said it was an issue in a dual cpu, dual core system he had
: set up.
: 
: My questions are:
: 1. Is this real or an urban legend?
: 2. If real, is this a Linuxism or is FreeBSD affected as well?
: 3. How do you "sync" the cpus, if it is needed?
: 4. anything else some one wants to expound on along this line.

Linux keeps the cpu's frequencies 'synchronized' so that it can use
the fast time keeping hardware (TSC).  FreeBSD uses different
mechanisms for its timekeeping, so doesn't need to keep them in sync
at all, and doesn't even try at this point.  Maybe this is what they
are talking about...

Warner