[ntp:questions] Flash 400 on all peers; can't get ntpd to be happy

Steve Kostecke kostecke at ntp.org
Wed Mar 9 01:56:23 UTC 2011

On 2011-03-08, Chuck Swiger <cswiger at mac.com> wrote:

> On Mar 8, 2011, at 1:18 PM, Steve Kostecke wrote:
>> On 2011-03-08, Chuck Swiger <cswiger at mac.com> wrote:
>>> Seriously, each physical machine only has one RTC and crystal
>>> oscillator. It's useful to run one instance of ntpd in the Dom0 (or
>>> host ESX) context where it can actually work and keep this real
>>> hardware clock in sync.
>> NTP disciplines the system (i.e. kernel) clock, not the hardware
>> clock on the mother board.
> That's right, although in reasonably common for platforms to
> periodically write the system clock time back to the hardware
> clock-- variously called the RTC/TOD/TOY clock which is in the
> BIOS/EFI/firmware and keeps time when the system is off.

The RTC is _updated_, not synced, by the kernel.

> Anyway, there isn't a separate RTC *or* timer crystal driving
> ACPI/HPET/etc for each VM.

Plus the VMs likely don't receive consistant time-slices.

>> I have a Debian 6.0 system running as a VMWare guest. ntpd on this
>> system has no problem disciplining the clock.
> OK. Does it do any better than using VMWare's "tools.syncTime = true"?

I don't have access to the host.

> Your jitter values are well over an order of magnitude worse than that
> of ntpd running on a non-virtualized machine, and your offsets are
> nearly an order of magnitude worse:

You're comparing apples and oranges.

> For all of that, your VM is doing pretty well running ntpd compared to
> others I'd seen. I'd imagine the host running the VM isn't especially
> busy; if it was, I wouldn't be surprised if ntpd can't manage to
> discipline the clock without "tinker panic 0".

The default panic threshold is 1024 seconds. 

>>> You are better off running ntpdate (or sntp) periodically via cron
>>> in the DomUs.
>> Perhaps in certain cases, but not across the board.
> I'd be happy to review counterexamples to my generalization...

There's my example.

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

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