Re: Any leap year issues caused by Ada yesterday?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Tue, 6 Mar 2012 11:08:37 -0800 (PST), Shark8 wrote:

> On Tuesday, March 6, 2012 11:37:35 AM UTC-6, Dmitry A. Kazakov wrote:
>> 
>> If the sources of the TSC and of the system clock are different, then there
>> is a systematic error. I suppose this is the major reason why it did not
>> work under VxWorks. The system time must be derived from the same quartz.
>> Otherwise you have to compensate the deviation, which could well be as big
>> as 5 microseconds per second.
> 
> Really? I thought the whole point of using Quartz crystals was because
> they vibrate at a precise rate; or is the problem analogous to the "beats"
> that you hear when two musical instruments* are slightly out of tune?

I am not a physicist, but we actually measured TSC of two identical
computers (same motherboard, processor, manufacturer etc). The deviation
was microseconds per second.
 
> (*Guitar comes immediately to mind; especially as it uses that as part of the tuning process.)

In that analogy it is like two strings having different tension or
diameter.

But what I meant was not oscillators of almost same period, but rather ones
having absolutely unrelated periods, e.g. the BIOS clock and the TSC. x86
has many clocks spread in abundance all over the motherboard. They are
physically different, unsynchronized etc.

OS designers carefully select the least accurate and most unreliable of
them for the time source... (:-))

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de