RE: [PATCH] [RFC] Building guests on monotonic Xen system time

["Dan Magenheimer" <dan.magenheimer@oracle.com>]

> I have a pretty clear vision of what I want now, so I will make the
> necessary adjustments to the patch. Then we can go from there 

I noticed you fixed a related bug (cset 17760), but haven't
seen anything that changes timer_mode==0 back to non-monotonic.
Is that still on your list, or do you want me to submit a
patch for that?

Related, I'm wondering if we should change the defaults for
timer_mode and hpet now.  I'll start a new thread on that
to avoid topic drift.

Dan  

> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@eu.citrix.com]
> Sent: Saturday, May 24, 2008 1:34 AM
> To: dan.magenheimer@oracle.com; Xen-Devel (E-mail)
> Cc: Dave Winchell
> Subject: Re: [Xen-devel] [PATCH] [RFC] Building guests on 
> monotonic Xen
> system time
> 
> 
> On 23/5/08 23:44, "Dan Magenheimer" 
> <dan.magenheimer@oracle.com> wrote:
> 
> > Here's another option:  I think delay_for_missed_ticks is
> > the only mode where time must be set backwards.  All of the
> > other modes track wallclock.  It looks possible that we
> > could enforce monotonicity for the other three modes and
> > not enforce it for delay_for_missed_ticks (which is,
> > unfortunately, the default).
> >
> > If this is acceptable to you (and you don't disagree with
> > my "only mode...set backwards" conjecture), I'll try to
> > code a patch for this.
> 
> I agree with your analysis. I couldn't remember if any modes 
> other than 0
> would require non-monotonic time across VCPUs, but it does 
> appear that this
> property is exclusive to mode 0.
> 
> > P.S. I think the answer is no, but is there any way to specify
> > gang-scheduling for a domain?  If so, we could say if your
> > OS is non-resilient to preemption and your application requires
> > time monotonicity, you must specify gang scheduling.
> 
> There isn't currently a way to request gang scheduling. I 
> think that is a
> second step anyway. It sounds like you could simply turn off your
> monotonicity checks for timer mode 0 and I'll be happy.
> 
> I have a pretty clear vision of what I want now, so I will make the
> necessary adjustments to the patch. Then we can go from there 
> if you think
> further modifications are required.
> 
>  -- Keir
> 
> 
>