[Dprglist] Stability of CPU oscillator

[Murray Altheim]

Hi John,

In a former life I had to cause to deal with something akin to this and
noted that given consumer PCs have had no reason to have stable clocks,
they generally don't. This kind of specification isn't published because
it doesn't matter to most people (apart from kernel developers) and it's
not competitive to do so.

You may have run upon this before, but to quote:

   "The time stamp counter (TSC) provided by x86 processors is a
    high-resolution counter that can be read with a single instruction
    (RDTSC), which makes it a tempting target for applications that
    need fine-grained timestamps. Unfortunately, it is also rather
    unreliable, so the kernel jumps through hoops to decide whether
    to use it and to try to detect when it goes awry. An effort to
    export the kernel's knowledge about the reliability of the TSC
    has met strong resistance for a number of reasons, but the biggest
    is that the kernel developers don't think that applications should
    be accessing the counter directly."

    The trouble with the TSC (2010)
    https://lwn.net/Articles/388188/

also:

    Internal Clock Drift Estimation in Computer Clusters (2008)
    https://www.hindawi.com/journals/jcnc/2008/583162/

I don't know if things have improved since then, my guess would be
probably not. A few years ago I bought a top of the line Dell 13"
XPS laptop and found I could hear the coil whine -- it turns out
mine was a 7th generation laptop and the whine had been there since
the beginning. Dell just never bothered to fix it since few of their
customers could hear the noise and even fewer apparently complained.
Commercial PC manufacturers don't fix things unless they need to.

The closest thing I can think of as a really stable clock source for
about US$30 would be to use the PPS signal off of a GPS sensor, though
this isn't strictly a high speed clock these typically have an accuracy
in the 10ns range. I would assume since the application is GPS the
stability must be very good, somewhere in the same range, dunno, that's
probably more your department. It's possible that some of the GPS units
have other pin outputs that might be helpful. I know this isn't what
you're looking for (since it's not a typical commercial product like a
PC) but GPS clocks would seem to be the cheapest and most reliable I
can think of.

Cheers,

Murray

On 5/06/20 2:55 pm, John Swindle via DPRGlist wrote:
> Folks:
> 
> What is the typical stability of the oscillator in a consumer x86 platform?
> 
> I don't find that specified for systems, or I don't know where 
> to look for the spec. The part numbers on the crystal often don't
> seem to be sufficient to find the stability in the XTAL datasheet.
> 
> I don't have access to test equipment to measure it.
> 
> I appreciate any help, but just to be clear: I'm asking about the
> stability, not the accuracy, so it would be ppm of drift and/or 
> jitter over time, not ppm of accuracy.
> 
> I intend to use the invariant TSC in x86 processors, but if the oscillator
> is not stable, then the invariant TSC numbers will not be as useful to me.

...........................................................................
Murray Altheim <murray18 at altheim dot com>                       = =  ===
http://www.altheim.com/murray/                                     ===  ===
                                                                    = =  ===
     In the evening
     The rice leaves in the garden
     Rustle in the autumn wind
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            -- Minamoto no Tsunenobu