linux-kernel - Re: [PATCH] Drivers: hv: util: Implement Time Synchronization using host time sample

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Message-ID: <20140925060745.GA9450@sucs.org>
Date:	Thu, 25 Sep 2014 07:07:45 +0100
From:	Sitsofe Wheeler <sitsofe@...il.com>
To:	Olaf Hering <olaf@...fle.de>
Cc:	Thomas Shao <huishao@...rosoft.com>, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org,
	driverdev-devel@...uxdriverproject.org, apw@...onical.com,
	jasowang@...hat.com, kys@...rosoft.com, haiyangz@...rosoft.com
Subject: Re: [PATCH] Drivers: hv: util: Implement Time Synchronization using
 host time sample

On Tue, Sep 23, 2014 at 09:56:10AM +0200, Olaf Hering wrote:
> On Tue, Sep 23, 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.

You [Microsoft?] do? Can you link to public sources where is this stated
please? I don't see any mention of doing this on
http://technet.microsoft.com/en-us/library/dn720239.aspx .. The only
official Microsoft information with respect to Hyper-V guest time
synchronisation I've seen has been for Windows guests but perhaps I've
looked in the wrong places.

The reason I ask is because regular ntpd is not enough to discipline a
Linux Hyper-V guest's clock. So much drift can occur under load that
ntpd can't bring the clock under sync. For now, I've been using Chrony
which has a higher tolerance for correcting drifting clocks. I'm not the
only one seeing this either (see http://serverfault.com/a/488528/203726
and
http://serverfault.com/questions/523389/linux-clock-loses-10-minutes-every-week
).

It would be good to something official about this issue as it is painful
when happens.

-- 
Sitsofe | http://sucs.org/~sits/
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