[ntp:questions] How bad is USB?
Todd Glassey CISM CIFI
tglassey at earthlink.net
Fri May 1 15:22:03 UTC 2009
Richard B. Gilbert wrote:
> David J Taylor wrote:
>> Garrett Wollman wrote:
>>> In article <K9WdnS4y5pIuymfUnZ2dnUVZ_t2dnZ2d at giganews.com>,
>>> Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
>>>> USB is nearly useless for NTP! USB has latencies sufficiently large
>>>> and variable to render it unsuitable for use with NTP.
>> Garrett, Richard,
>> You've both commented that USB has drawbacks, but in reality what
>> performance might be obtained? Not everyone needs microsecond
>> precision, and USB might allow millisecond precision - i.e. possibly
>> better than what might be obtained by Internet access alone, or by using
>> a radio source.
>> Has anyone made any actual tests or measurements? Are there any results
>> available from an actual installation of USB on either Windows or UNIX?
So let me answer the question about precision obtainable under USB 1PPS
interrupt processing. The performance of the ISR (interrupt service
routine) in the driver depends on who's drivers for the USB hub
functionality you use. Also whether OS interrupt coalescing is turned
off or not.
Check also of course to verify what version of USB you are using
> You can try it and see what happens! The results may be sufficiently
> good for your purposes. You will almost certainly not get microsecond
> accuracy. If you are willing to settle for +/- 10 milliseconds you can
> almost certainly get that.
> There are (at least) two groups of people who hang out here:
> a. The "chimeheads" in pursuit of the "one true time", and
> b. The "synchs" who don't really care what time it is as long as all
> their machines have the SAME time.
Actually there is a third - a combination of the two who wants the one
true time everywhere.
> Both groups use the same tool, NTPD. It turns out that having a stable
> and accurate source of time makes it much easier to get a whole herd of
> machines to agree on what time it is.
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