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

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Message-ID: <20141014115412.GC4019@localhost.localdomain>
Date:	Tue, 14 Oct 2014 13:54:12 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Thomas Shao <huishao@...rosoft.com>
Cc:	tglx@...utronix.de, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	olaf@...fle.de, apw@...onical.com, jasowang@...hat.com,
	kys@...rosoft.com
Subject: Re: [PATCH 2/2] hyperv: Implement Time Synchronization using host
 time sample

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?

Thanks,
Richard

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