[ntp:questions] Sure GPS - Very High Jitter and Offset

Ken Link klink at numberzero.org
Tue Aug 16 03:58:45 UTC 2011

Thanks! That helped me realize I had broken the connection to the CD
pin on the port, which would explain why it wasn't seeing a PPS

Now, I get timestamps in the assert and clear files: 1313465775.004708342#4545

However, the source is still jittery! 15ms sometimes, like it's not
using the PPS signal at all.

The important configuration lines are:
server mode 18 minpoll 4 prefer
fudge flag1 1 flag2 0 flag3 1 flag4 0

I am also now booting with "noapic" in the kernel options, verified it
wasn't using APIC interrupts with 'cat /proc/interrupts', but that
didn't seem to help.

At this point I'm not sure what I'm forgetting - I've disabled FIFO,
added low_latency, verified the kernel configuration is 100HZ, I'm
seeing the PPS assert and clear signals at the kernel level, NTP is
configured to use PPS in the nmea driver, verified my clocksource is
acpi_pm (the only available clocksource on my system), disabled APIC,
disabled any power-saving/frequency scaling modes in BIOS...

What am I missing?


On Mon, Aug 15, 2011 at 1:59 PM, Miroslav Lichvar <mlichvar at redhat.com> wrote:
> On Mon, Aug 15, 2011 at 12:46:04PM -0500, Ken Link wrote:
>> Does anyone know of any Linux CLI tools that display (in some way or
>> another) the serial control signals received on a serial port?
> You can cat the /proc/tty/driver/serial file in a loop and see if the
> CD bit is flipping.
> 0: uart:16550A port:000003F8 irq:4 tx:1422 rx:184849390 fe:4891 pe:31 RTS|DTR|CD
> --
> Miroslav Lichvar
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