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Re: [libvirt] "Choose alarm timer" feature for kvm

On Sun, Aug 24, 2008 at 08:20:05PM +0100, Daniel P. Berrange wrote:
> On Sun, Aug 24, 2008 at 08:40:46PM +0200, Tiziano M?ller wrote:
> > Hi everyone
> > 
> > In KVM it is possible to select the alarm timer being used (dynticks,
> > hpet, rtc, unix).
> What exactly do these options do ? Is there anything describing the
> pros/cons of the different options & is there a way to determine 
> what the best option is for a particular VM ? 

I've had a long look at the source code in KVM-73 associated with
these timers and have some comments:

- QEMU tests the available timer sources in order and chooses the
first one which works (allegedly the "best" one).  The order, on
Linux, is:

   Timer          Implementation method
   (1) dynticks   POSIX timer_create
   (2) hpet       Linux-specific hardware (/dev/hpet) - see below
   (3) rtc        old /dev/rtc, millisecond accuracy
   (4) "unix"     POSIX setitimer

- Dynticks is at the top of the list, and has an extra 'rearm'
operation (the others only support starting and stopping timers).  But
I don't understand how this rearm operation is an improvement.

- HPET is second in the list and most accurate.  It's backed by
hardware and has sub-microsecond accuracy.

- It's not clear if dynticks (== timer_create) uses HPET internally in
the kernel?

- I can't see how any of this would affect guests -- except that
choosing a more accurate timer would be in some way 'better' because
the guest would then get more accurate emulation events.

> Basically I'm wondering where/if this should be exposed in the XML
> format, or whether libvirt could/should just internally pick the 
> best one ?

I can't see how libvirt can do any better than qemu/kvm is doing
already.  So unless someone comes along with an argument that using
(eg) hpet is better for Windows guests or something, then I suggest
that libvirt doesn't need to do anything.


Richard Jones, Emerging Technologies, Red Hat  http://et.redhat.com/~rjones
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.

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