* Dor Laor <[email protected]> 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
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