On 08/03/2009 12:09 PM, Antoine Martin wrote:
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It seems to be an unsettled issue, but, would any kind soul suggest
the current
best practice for setting the clock in Ubuntu Linux and Windows guests?
For Linux the best source clock is the kvm pv clock (exist from 2.6.27
and above).
# qemu-system-x86_64 -clock ?
Available alarm timers, in order of precedence:
dynticks
hpet
rtc
unix
I see no "pv clock"...
Which one should I use then?
Did I miss a ./configure or .config option?
No, we were talking about different clocks.
I was explaining the guest source clock while you wanted some info for
the host-qemu clock.
Hah, gotcha. You're talking about the guest kernel as in:
clocksource=[hpet|pit|tsc|acpi_pm|cyclone|scx200_hrt|kvm-clock]
# cat /sys/devices/system/clocksource/clocksource0/current_clocksource
kvm-clock
So all good, thanks.
You can use the default - dynticks. hpet and rtc might be good if you
need a fine grain granularity on older< 2.6.24 host kernels.
(I'm on dynticks)
I guess dynticks reduces context switches on the host, but I still get
hundreds per second on guests that are otherwise idle.
How would I go about finding what makes them tick?
You can use the kvm_stat script to check the guest activity.
As for host timers, you can use sudo strace -e trace=signal -c -p
`pgrep qemu` to get the number of SIGALRM calls.
You can reduce this number since there is a threshold in qemu for the
dyntick.
My winXp guest does the following:
sudo strace -c -p `pgrep qemu`
Process 14241 attached - interrupt to quit
^CProcess 14241 detached
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
82.46 0.089660 19 4801 select
14.51 0.015775 33 480 futex
1.70 0.001847 0 9511 4804 read
0.72 0.000787 1 824 ioctl
0.23 0.000246 0 2361 write
0.19 0.000211 0 2463 timer_gettime
0.13 0.000144 0 2361 rt_sigaction
0.06 0.000066 0 2377 timer_settime
0.00 0.000000 0 3 poll
0.00 0.000000 0 3 writev
------ ----------- ----------- --------- --------- ----------------
100.00 0.108736 25184 4804 total
Thanks
Antoine
Cheers
Antoine
For windows, standard acpi HAL uses the rtc clock by default. As long as
you use the -rtc-td-hack it won't drift.
When the tsc is not stable on the host or the host cpu might get into
deep sleep state (c2), you better use another source clock in the guest
- for windows it should be the pmtimer (using the boot.ini).
Thanks!
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