WARNING - OLD ARCHIVES

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/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

RE: [Xen-devel] [PATCH] Make HZ a boot-time configurable

To: "Kip Macy" <kmacy@xxxxxxxxxx>, "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] Make HZ a boot-time configurable
From: "Neugebauer, Rolf" <rolf.neugebauer@xxxxxxxxx>
Date: Mon, 9 May 2005 10:58:27 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Neugebauer, Rolf" <rolf.neugebauer@xxxxxxxxx>
Delivery-date: Mon, 09 May 2005 09:58:06 +0000
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
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/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
Thread-index: AcVUX4wWaafC0HFDT4KrUiud5jCsGAAFEzhQ
Thread-topic: [Xen-devel] [PATCH] Make HZ a boot-time configurable
I think the periodic ticker in *Xen* is only used for time keeping and
is not reflected to guests. We basically only use the PIT as a stable
time source (see xen/arch/x86/time.c). HZ=100 should be sufficient for
this. All other timers should be run off the APIC timer which is used in
one shot mode.

Guests can set a one shot timer/alarm timer using the set_timer
hypercall. In Linux guests this is currently only used when the guest is
blocking to set the value to the next scheduled function.

In addition the scheduler is set up to send the *current* guest a
periodic ticker at 100HZ. This last value is hardcoded in
xen/common/schedule.c. again this timer is run off the local APIC.
Arguably, the frequency for this ticker should be settable per guest to
reflect its HZ value (or equivalent).

rolf

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Kip Macy
> Sent: 09 May 2005 06:27
> To: Ian Pratt
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-devel] [PATCH] Make HZ a boot-time configurable
> 
> That would be ideal, I wasn't aware that that was up for discussion.
> 
> 
>                               -Kip
> 
> On Mon, 9 May 2005, Ian Pratt wrote:
> 
> >
> > > I understand the trade-offs that xen is currently optimized
> > > for. However, I don't think that HZ=100 is right for all
> > > places and all times. I also think that users would
> > > appreciate it if they didn't have to go edit config.h and
> > > re-compile if they wanted to change HZ. For those using xen
> > > as a platform for development it probably makes sense if HZ
> > > is the same as the kernel they intend to ship. For example,
> > > very early on in testing with HZ=1000 I hit an interrupt race
> > > condition in XenFreeBSD that I have never seen before.
> >
> > I'd really prefer to kill the periodic ticker in Xen altogether. If
> > domains want a periodic ticker they should just set their alarm
timer
> > appropriately.
> >
> > Ian
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >
> 
> --
> "Debugging is twice as hard as writing the code in the first place.
> Therefore, if you write the code as cleverly as possible, you are,
> by definition, not smart enough to debug it." - Brian W. Kernighan
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel