This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


Re: [Xen-devel] RE: [PATCH] rendezvous-based local time calibration WOW!

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] RE: [PATCH] rendezvous-based local time calibration WOW!
From: John Levon <levon@xxxxxxxxxxxxxxxxx>
Date: Wed, 6 Aug 2008 16:21:47 +0100
Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Delivery-date: Wed, 06 Aug 2008 08:22:29 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <20080806090906796.00000008444@djm-pc>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <20080806133829.GA28204@xxxxxxxxxxxxxxxxxxxxxxx> <20080806090906796.00000008444@djm-pc>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.9i
On Wed, Aug 06, 2008 at 09:09:06AM -0600, Dan Magenheimer wrote:

> Again no guarantees but I think we are now under the magic
> threshold where the skew is smaller than the time required
> for scheduling a VCPU onto a different CPU.  If so,
> consecutive gethrtime's by the same thread in a domain
> should always be monotonic.

Right! That sounds positive.

> The overhead of measuring the inter-CPU stime skew is
> too large to do at every cross-PCPU-schedule so doing
> any kind of adjustment would be difficult.
> But it might make sense for the Xen scheduler to do a
> get_s_time() before and after a cross-PCPU-schedule
> to detect the problem and printk if it occurs
> (possibly rate-limited in case it happens a lot on
> some badly-behaved machine).

If we're doing a get_s_time() before the schedule, don't we merely* have
to ensure that the new s_time is after the last recorded one on the
previous CPU? (Yes, I'm handwaving terribly)


Xen-devel mailing list

<Prev in Thread] Current Thread [Next in Thread>