Re: What time is it kvm-clock?

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Feb 25, 2016 at 4:20 AM, Radim Krčmář <rkrcmar@redhat.com> wrote:
> 2016-02-24 19:50-0800, Owen Hofmann:
>> On Wed, Feb 24, 2016 at 5:19 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> Of course the guest can run its own NTP daemon or similar adjtimex
>>> caller and cause the guest to stop tracking the host.  But if the host
>>> passed CLOCK_MONOTONIC through, then the guest would, by default,
>>> treat kvm-clock as an exactly 1GHz source and would then expose a
>>> disciplined NTP-tracking CLOCK_MONOTONIC through to its user apps even
>>> without an NTP client on the guest.
>>>
>>> If integration with the POSIX clock core were provided, the guest
>>> would learn to consume the host's CLOCK_REALTIME as well, as long as
>>> the host uses the tsc as its clocksource.
>>
>> Your proposal, which I'd describe as a direct passthrough (to the
>> extent possible) of the host gettimeofday vdso to a kvm guest, sounds
>> like a much better way to get clock frequency adjustments from the
>> host to the guest. But I don't know if I can think of a reason to do
>> this besides "hey you don't have to run ntp". Is there a situation you
>> have in mind that this helps out?
>
> Running NTP only on the host is a good reason.
> (And probably the only reason I'd call good, because any software that
>  passes TSC or CLOCK_MONOTONIC timestamps between hosts needs to handle
>  their differences.)

There are handful of distributed algorithms that benefit from clocks
with a bounded worst-case synchronization error.  I think that Google
uses some.  If some cloud provider were to provide, say, 10ms max
CLOCK_REALTIME error and pass CLOCK_REALTIME through using kvm-clock,
it could be quite useful.

--Andy