Re: Clarifications about PPS SHM content

[Gary E. Miller]

Yo Martin!

On Tue, 24 Mar 2020 02:14:03 -0400
Martin Boissonneault <address@hidden> wrote:

> >> I don't have much experience with Debian/Raspbian/Linux
> >> optimization and process tracking. I struggled to find the cause
> >> of the time error spike that was caused by systemd's timesyncd
> >> service.  
> > Well, systemd(umb) says it all.  "Just Say No" (tm)  
> 
> Well, to the defence of whoever put timesyncd there, the logic is
> sound: Get the time right before the system starts too much to
> prevent the time from jumping backwards because of no RTC or one
> that's too far off.

Fine for coarse adjustment on eyond that, the systemd(ibble) people
are not time-nuts.

> But otherwise, it's in direct conflict with any
> other type of time synchronization. So, m'kay...

One could say that os all of systemd(umber).

> I have a Nooelec NESDR SMArtee for comparison, and the LNA does help
> me pull in more planes.

I see 300 miles do I really better?

> Both are fed by small antennas with a good
> ground-plane a few feet away.

At a GHz you want the ground plane directly the antenna.

> The receiver has been working well for over a year now!
> Rock-solid, not a single disconnection.

Yup, the RasPi's are rock-solid.

> I tested that about a year ago, so I don't have the numbers. But I
> ran /CONFIG_NO_HZ_COMMON=y/ (stock), then CONFIG_HZ_PERIODIC=y with
> 100Hz, 250Hz and 500Hz and 250Hz had the less jitter of the four 
> configurations.

Good to know.  I have not tested.  Some people recomment the NO_HZ,
without the data to prove it.

> Considering the compile  time on my Pi,

Who cares how long it takes.  As long as it is less than my nightly
eight hours.  My problem is so many things to test...

> I think the reason to set it in the kernel .config has to do with the 
> timing calibration of the kernel, as the governor that is active on
> boot uses a different frequency before the OS configuration changes
> it (600MHz vs 1400MHz).

I'd like to see real data to prove that.

> I think there is a mechanism to fix this (a
> delay before lowering the frequency), but hard-setting it in the
> kernel ensures that the calibration will run with the frequency that
> will be used normally.

The RasPi overheats at the drop of a hat.  Very rare to keep one running
at full speed.

> > Something swung your frequency hard at 23 Mar 12:00Z  
> 
> Ha! Yes, I tested an offline boot, only to find out my RTC was off
> half an hour for some reason. Being an DS3231 RTC with battery
> backup, it shouldn't be off more than a few minute a year.

Just now on #ntpsec someone was proclaiming how good their RTC is.  I
never trust them.

> > Nice.  Your disk I/O is a bit high for best timing.  Maybe buffer
> > that more?  
> 
> Good idea! How about I do it right now? I changed 
> /vm.dirty_expire_centisecs/ to 2.5 minutes,/vm.dirty_ratio = 40/ and 
> /vm.dirty_background_ratio = 30/. Do you think that's allright?

You tell me.  Did it help or hurt?

> Well, my u-Blox MAX-M8Q is configured for a static position (it is 
> static). It's height is also configured from the 1-week long position 
> averaging.
> 
>   * UBX-CFG-PRT set baudrate to 115200,
> [...]
>   * and saved the config to bbRAM.

And good to verify those setting occasionally.

> Should I turn the Fix mode to 3D and leave TMODE2 Time mode to
> 2-Fixed?

Time mode is suposed to be best for timing.  Something else to test
in your spare time.  :-)

> > Your temp change is so small that cpuheat may be enough to get it
> > solid.  No cost to that.  
> 
> On my TODO list, there's cpuheat. My problem is summer with the big 
> south-facing window during heat waves. In winter, there is no
> problem, it can generate it own heat, but in the summer with high
> ambient temps, it becomes HOT. I've put the plastic cover on the case
> for the winter (room temperature set to 18__C, door kept almost
> closed) but in the summer I only leave a piece of paper in place of
> the cover.

Optimal OXCO temp is pretty high, I seem to remember near 55C?  If your
house gets that hot in the summer you have other problems.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can't measure it, you can't improve it." - Lord Kelvin

pgpaNB_zclKoB.pgp
Description: OpenPGP digital signature