On 11.09.2010 07:01, Stéphane Charette wrote:
I have 2 virtualbox guest instances where the /proc/cpuinfo reports the
CPU as 0.0 MHz. This is causing problems for an app that is parsing the
/proc/cpuinfo and getting hung up on that value. (It actually is
causing a divide-by-zero since the app is using that value to do some
calculations.)
The host is Ubuntu 10.04 64-bit, and both of the guests are also Ubuntu
10.04 64-bit. I'm using VB v3.2.8r64453.
Is this normal, or should there be a value in that field? I understand
how when virtualized the cpu speed is a loose number, and if a system is
idle it does't get any physical CPU cycles. But I'm wondering if there
should still be something in that field, or whether this app needs to
get the cpu speed from some other source to make in work in VirtualBox.
I assume Linux reads this information from some CPU MSR, and VirtualBox
deliberately sets only a selected number of MSRs - essentially the bare
minimum. All others return 0.
From my point of view the fact that an app misbehaves if this
information isn't present is a bug in the application. I really wonder
what legitimate use it can make of the CPU clock frequency (especially
when taking it from /proc/cpuinfo, which cannot possibly be 100%
accurate given how fast today's CPUs change clock frequencies for power
management reasons).
See here for an example output from the first guest. The 2nd guest is
identical with the exception of a slightly higher bogomips:
steph...@vm-1:~$ uname -a
Linux as1k-vm-33 2.6.32-24-server #39-Ubuntu SMP Wed Jul 28 06:21:40 UTC
2010 x86_64 GNU/Linux
steph...@vm-1:~$ cat /proc/cpuinfo
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 15
model name : Intel(R) Core(TM)2 Duo CPU T7500 @ 2.20GHz
stepping : 10
cpu MHz : 0.000
[...]
Thanks for any help you can offer.
I've tried this with a "Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz"
and a Linux 2.6.28.4 kernel (VT-x enabled for the VM), and it apparently
measures the CPU frequency somehow. For a single-core VM the value was
very accurate (2400.653), and for a quad-core VM the value it was about
50% too high: 3231.627. However in all cases I had a non-zero "cpu MHz"
value.
Not sure what that says about my MSR theory above. Linux kernels change
all the time, sometimes in surprising ways...
Klaus
Stéphane Charette
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