On Thu, Jun 24, 2010 at 08:25:10AM +0200, Andrej Podzimek wrote:
> Yes, the amount of idle and system time is *incredible*. Something
> must be wrong. More than 20 running processes are reported by 'top'
> most of the time, but at most 4 to 6 are on CPU at any given moment.
What kind of workload are you running? What are these 20 running
processes doing?
> This happens on a 64-bit VirtualBox (3.2.4 r62467) guest running on a
> Linux host (4-core Core i7 (8 threads)). Setting a lower number of
> CPUs (such as 4) does not help. Uniprocessor guests seem to work
> normally (with IO APIC disabled).
>
> I've read about bugs affecting the IO APIC performance under
> VirtualBox, but all the reports say they are only relevant for 32-bit
> guests. This is a 64-bit one.
>
> I migrated the machine into a QEMU-KVM environment and performance got
> much better, but still far from ideal. Instead of 15:35:50
> (usr:sys:idl), I got something like 70:30:0 (usr:sys:idl). (So there
> was no inexplicable idle time. But still quite a lot of kernel
> overhead.)
Hard to say. Some virtualized environments have bugs that confuse the
operating system.
> Is this a known issue? Is there a solution? Could I diagnose it with
> DTrace somehow? A piece of advice would be very helpful.
You can use DTrace to profile the kernel.
# dtrace -n 'profile-1997hz { @a[stack(50)] = count();} END { trunc(@a, 30); }'
The above will show you the 30 most common kernel stacks. That might be
a start.
-j
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