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@xxxxxxxxxx GnuPG FP : F327 283A E769 2E36 18D5 4DE2 1B05 6A63 AE9C 1E30
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