Re: [PATCH 2/2] hyperv: Implement Time Synchronization using host time sample

From: Richard Cochran
Date: Tue Oct 14 2014 - 07:54:22 EST

On Tue, Oct 14, 2014 at 04:11:18AM -0700, Thomas Shao wrote:
> In current hyper-v time sync service,it only gets the initial clock time
> from the host. It didn't process the following time samples. This change
> introduced a module parameter called host_time_sync. If it is set to true,
> the guest will periodically sychronize it's time with the host clock using
> host time sample. By default it is disabled, because we still recommend
> user to configure NTP for time synchronization.

I really don't see the need for this. We have NTP. If the guests want
to, they may use it. Otherwise, they have a free running clock, just
like real machines.

> + /*
> + * Use the Hyper-V time sample to adjust the guest time. The
> + * algorithm is: If the sample offsets exceeds 1 second, we
> + * directly set the clock to the server time. If the offset is

So the guests will experience random time jumps in the kernel, without
any rhyme or reason?

> + * less than 1ms, we ignore the time sample. Otherwise we adjust
> + * the clock.
> + */

So when using this kernel module, the sychronization is never expected
to be better than one millisecond. That is not too good. I expect NTP
can do better. So what was the point of this change again?


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