Testing ntpd and/or timing from gpsd

[Gary E. Miller]

Yo Chris!

On Fri, 13 May 2016 10:46:05 +1000
Chris Johns <chrisj at ntpsec.org> wrote:

> On 13/05/2016 07:37, Gary E. Miller wrote:
> >
> > It would be insane for a switch to have EEE enabled without a way to
> > turn it off, so you likely have never seen it turned on.  
> 
> The switch should only enable EEE if the PHY says it is ok. If you 
> disable it in the PHY and switch should honour it. A PHY that knows 
> nothing about EEE can not be expected to interoperate with a port 
> operating in that mode.

Agreed.  But my point still stands.

> 
> > I can enable autonegotiation for EEE it on some of my switches,
> > nothing but trouble.  
> 
> I would make sure this is not a compliance issue with your switches 
> and/or PHYs connecting to it. I seem to remember some issue with
> early devices that ended up not matching the standard but I cannot
> find the references. Power up latency would be an issue as a send
> results in a reset timer starting in the PHY and depend on the MAC
> the data may be buffered in the PHY until the link has come up.

All I can say is I lost a ton of packets, all the time, when I enabled
automatic EEE.

> EEE and PTP together does not make sense to me.

Yeah, two flakey technologies added together, what could go wrong?

> > If your backbone is GigE then it is likely way better than you
> > think. Turn on PTP and your hosts will time link closer than you
> > have ever seen, if it works at all.  Hardware timestamping of the
> > ethernet packets works well, when it works at all.
> >
> > PTP shows that the main problem with high accuracty NTP is the
> > network stack and the OS, not the network itself.  
> 
> Yeap, this is important once you get down to this level. Xilinx 
> recommend using an RTOS when doing PTP on their Zynq processor and 
> Marvell talk about measuring the delay in clocks in their PHYs. 

I can't see how the OS matters at all.

> Interrupt latency seems to be the key factor.

Not a factor at all.  In hardware PTP all the timestamping is done
in the ethernet chip itself.  The software jitter matters not at all
since the time was already captured in the hardware.

When it works...

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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