Re: [kvm-devel] kvm & dyntick

Ingo Molnar
2007-01-14T08:51:16Z

* Dor Laor <dor.laor@qumranet.com> wrote:

> Afterwards we'll need to compensate the lost alarm signals to the
> guests by using one of
> - hrtimers to inject the lost interrupts for specific guests. The
> problem this will increase the overall load.
> - Injecting several virtual irq to the guests one after another
> (using interrupt window exit). The question is how the guest will be
> effected from this unfair behavior.

well, the most important thing would be to fix qemu to:

- not use a 1024 /dev/rtc stream of signals as its clock emulation
source

i hacked that out of qemu, only to find out that qemu then uses periodic
itimers. Instead of that it should use one-shot itimers, driven by the
expiry time of the next clock. I.e. this code in vl.c, in
host_alarm_handler():

if (qemu_timer_expired(active_timers[QEMU_TIMER_VIRTUAL],
qemu_get_clock(vm_clock)) ||
qemu_timer_expired(active_timers[QEMU_TIMER_REALTIME],
qemu_get_clock(rt_clock))) {

should start an itimer with an expiry time of:

active_timers[QEMU_TIMER_VIRTUAL]->expire_time - qemu_get_clock(vm_clock)

or:

active_timers[QEMU_TIMER_REALTIME]->expire_time - qemu_get_clock(rt_clock)

whichever is smaller. Furthermore, whenever timer->expire_time is
changed in qemu_mod_timer(), this set-the-next-itimer-expiry-time code
needs to be called. Would anyone like to try that?

this will reduce the host Qemu wakeup rate from 1000-1100/sec to the
guest's 4-5/sec wakeup rate - resulting in 0.01% CPU overhead from a
single idle guest. Current unmodified Qemu causes 10-20% CPU overhead
from a single idle guest.

Ingo