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

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Ian Pratt
> Sent: 09 May 2005 14:21
> To: Neugebauer, Rolf; Kip Macy
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-devel] [PATCH] Make HZ a boot-time configurable
> > 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).
> Or done away with altogether...
> As I recall, the complication with doing this is that we don't want
> periodic timer interrupts to wake the domain up when its otherwise not
> running (whether due to having been preempted or being blocked).


> At a minimum, we need to make the ticker freq programmable on a
> per-domain basis (including '0Hz').
> Alternatively, we introduce a new event notification function that
> actually 'kicks' the domain if its already runnning.
> Which option is preferred? I'd like to see this make the 3.0-testing
> cut, though I guess option 1 could be done in a hypervisor-API
> compatible fashion.

I think option 1 (per domain HZ) is better. Most OSes seem to rely on a
periodic ticker for some stats collection, scheduling etc. Changing this
to only have a reprogrammable timer looks quite tedious. 

We could have the guest OS manage this themselves, ie emulate a periodic
ticker when running (reprogramming the alarm timer periodically) and use
the alarm more in one-shot mode when idle but that would require the
guest to know when actually running....

I'm not sure if I understand your option 2?


> Ian
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