[libvirt] "Choose alarm timer" feature for kvm
Hi everyone In KVM it is possible to select the alarm timer being used (dynticks, hpet, rtc, unix). Is someone already working on the support of this for libvirt? If not I'll see that I can come up with patches for libvirt and virt-inst within the next month (if time permits). Cheers, Tiziano -- ------------------------------------------------------- Tiziano Müller Gentoo Linux Developer Areas of responsibility: Samba, PostgreSQL, CPP, Python, sysadmin E-Mail : dev-zero@gentoo.org GnuPG FP : F327 283A E769 2E36 18D5 4DE2 1B05 6A63 AE9C 1E30
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 ? 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 ?
Is someone already working on the support of this for libvirt?
Not that I'm aware of.
If not I'll see that I can come up with patches for libvirt and virt-inst within the next month (if time permits).
Daniel -- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://ovirt.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|
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. Rich. -- 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. http://et.redhat.com/~rjones/virt-top
Am Mittwoch, den 27.08.2008, 11:28 +0100 schrieb Richard W.M. Jones:
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.
Ok, agreed. I'll bring the issue up once there's a real benefit of having this configurable. Cheers, Tiziano -- ------------------------------------------------------- Tiziano Müller Gentoo Linux Developer Areas of responsibility: Samba, PostgreSQL, CPP, Python, sysadmin E-Mail : dev-zero@gentoo.org GnuPG FP : F327 283A E769 2E36 18D5 4DE2 1B05 6A63 AE9C 1E30
participants (3)
-
Daniel P. Berrange -
Richard W.M. Jones -
Tiziano Müller