[Qemu-devel] Re: [kvm-devel] [PATCH 0/4] Rework alarm timer infrastrucure - take2

[Luca <kronos.it@gmail.com>]

On 8/22/07, Avi Kivity <avi@qumranet.com> wrote:
> Luca wrote:
> >>> This is QEMU, with dynticks and HPET:
> >>>
> >>> % time     seconds  usecs/call     calls    errors syscall
> >>> ------ ----------- ----------- --------- --------- ----------------
> >>>  52.10    0.002966           0     96840           clock_gettime
> >>>  19.50    0.001110           0     37050           timer_gettime
> >>>  10.66    0.000607           0     20086           timer_settime
> >>>  10.40    0.000592           0      8985      2539 sigreturn
> >>>   4.94    0.000281           0      8361      2485 select
> >>>   2.41    0.000137           0      8362           gettimeofday
> >>> ------ ----------- ----------- --------- --------- ----------------
> >>> 100.00    0.005693                179684      5024 total
> >>>
> >>>
> >> This looks like 250 Hz?
> >>
> >
> > Nope:
> >
> > # CONFIG_NO_HZ is not set
> > # CONFIG_HZ_100 is not set
> > # CONFIG_HZ_250 is not set
> > # CONFIG_HZ_300 is not set
> > CONFIG_HZ_1000=y
> > CONFIG_HZ=1000
> >
> > and I'm reading it from /proc/config.gz on the guest.
> >
>
> Yeah, thought so -- so dyntick is broken at present.

I see a lot of sub ms timer_settime(). Many of them are the result of
->expire_time being less than the current qemu_get_clock(). This
results into 250us timer due to MIN_TIMER_REARM_US; this happens only
for the REALTIME timer. Other sub-ms timers are generated by the
VIRTUAL timer.

This first issue is easily fixed; if expire_time < current time then
the timer has expired and hasn't been reprogrammed (and thus can be
ignored).
VIRTUAL just becomes more accurate with dyntics, before multiple
timers were batched together.

> Or maybe your host kernel can't support such a high rate.

I don't know... a simple printf tells me that the signal handler is
called about 1050 times per second, which sounds about right.

Luca