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Dear all:
I have found a big bug on xen concerning time
virtualization. Please let me show you the whole process:
1 Phenomenon
when I run a JVM based program in
IE browser in my Virtual Machine, I have found clearly that time at the right
bottom corner in my VM gets more slower and
slower.
I studied the bug deeply, and
found something below.
2 Xen
vmx_vmexit_handler -->
......... --> handle_rtc_io --> rtc_ioport_write -->
rtc_timer_update --> set RTC's REG_A to a high rate-->
create_periodic_time(disable the former timer, and init a new
one)
Win7 is installed in the vm. This
calling path is executed so frequent that may come down to set the RTC's REG_A
hundreds of times every second but with the same rate(976.562us(1024HZ)), it is so abnormal to me to see such
behavior.
3 OS
I have tried to find why the win7
setted RTC's regA so frequently. finally got the result, all that comes
from a function: NtSetTimerResolution -->
0x70,0x71
when I attached
windbg into the guest OS, I also found the source, they are all called from a
upper system call that comes from JVM(Java Virtual
Machine).
4
JVM
I don't know why JVM
calls NtSetTimerResolution to set the same RTC's rate
down (976.562us(1024HZ)) so frequently.
But found something
useful, in the java source code, I found many
palaces written with time.scheduleAtFixedRate(), Informations from Internet told
me this function scheduleAtFixedRate demands a higher time resolution.
so I
guess the whole process may be this:
java wants a higher
time resolution timer, so it changes the RTC's rate from 15.625ms(64HZ) to
976.562us(1024HZ), after that, it reconfirms whether the time is accurate as
expected, but it's sorry to get some notice it 's not accurate either. so it
sets the RTC's rate from 15.625ms(64HZ) to 976.562us(1024HZ) again and
again..., at last, results in a slow system timer in
vm.
there is also a
frequently called function going into my eyes:
QueryPerformanceCounter.
what my problems are:
1 why the JVM sets the same RTC's rate 976562 down to the coms again and
again? what he found is abnormal?
2 even a abnormal user program calls create_periodic_time to set the rate
again and again, how do we avoid the influence upon our time in vm? how do we
compensate the elapsed time at the switching point? especially when the switch
is so frequent.
can some big figures give me some advices on this?
thanks!
tupeng212
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