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

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Date:	Thu, 25 Sep 2014 09:40:42 +0000
From:	Thomas Shao <huishao@...rosoft.com>
To:	Sitsofe Wheeler <sitsofe@...il.com>, Olaf Hering <olaf@...fle.de>
CC:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"KY Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH] Drivers: hv: util: Implement Time Synchronization using
 host time sample



> -----Original Message-----
> From: Sitsofe Wheeler [mailto:sitsofe@...il.com]
> Sent: Thursday, September 25, 2014 2:08 PM
> To: Olaf Hering
> Cc: Thomas Shao; gregkh@...uxfoundation.org; linux-
> kernel@...r.kernel.org; driverdev-devel@...uxdriverproject.org;
> apw@...onical.com; jasowang@...hat.com; KY Srinivasan; Haiyang Zhang
> 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.
> 

Hi Sitsofe,

As far as I know, currently the document about time sync in Hyper-V is only for Windows. Below is a doc from MSDN:
http://blogs.msdn.com/b/virtual_pc_guy/archive/2010/11/19/time-synchronization-in-hyper-v.aspx 
We are working on the document about Time Sync on Hyper-V to cover both Linux and Windows.

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