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

["Neugebauer, Rolf" <rolf.neugebauer@xxxxxxxxx>]

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
> 
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