[ntp:questions] is USB inherently evil, or only if something else is on the bus?

Chuck Swiger cswiger at mac.com
Wed Aug 8 21:57:46 UTC 2012

On Aug 8, 2012, at 2:34 PM, Rick Jones wrote:
> Is that unsuitability inherent in USB, so it matters not whether there
> is anything else on the USB, or is it more a case of being "bad"
> generally only when other things are on the same USB?  I'm still
> looking to go serial, but was wondering.

The unsuitability is inherent to USB, although some of the issues are
better under USB 3.  USB 1/2 involves host-based polling rather than
devices being able to handle device-initiated communications which
create an interrupt which gets handled much more immediately.

You're getting ~10ms of latency for a typical USB connection, versus
something around a microsecond for an uncontested INT line (ie, serial
or parallel).

> Also, speaking of things considered "bad" and drifting - fudging the
> LOCAL(0) is definitely frowned upon right?  If I happen to have say
> four servers in a location which might loose its connectivity to the
> outside world I probably don't want those servers to fall-back on
> LOCAL(0) right?  Would configuring each to have the other three as
> "peer" entities be the way to go?

Folks probably never should use the LOCAL clock with a recent ntpd.

By definition, if you're using it your offset is always zero: if you
know it is off, updating it to the right time rather than fudging makes
far more sense.  Use orphan mode instead:



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