Re: idle linux guest takes up to 8.5% of host CPU

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Adam Monsen wrote:
> Hi Avi,
>
> On 7/22/07, Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>   
>> Adam Monsen wrote:
>>     
> [...]
>   
>>> Any idea why an idle Fedora 7 guest would use more host CPU than an
>>> idle Windows XP guest?
>>>       
>> Several reasons:
>>
>> - Linux userspace sucks up more cpu than Windows.  That's especially
>> true if you have X running.
>>     
>
> This is a very minimal F7 install... no X, basically just an Apache
> HTTP server with mod_mono.
>
>   
>> - If running a 64-bit kernel, Linux is running at 250Hz or 300Hz whereas
>> Windows runs at 100Hz unless you're doing multimedia. F7 32-bit kernels
>> adjust the timer dynamically.
>>     
>
> I'm pretty sure I'm running a 32-bit kernel on the host;
> "CONFIG_X86_32" is defined in the kernel options, and "getconf
> LONG_BIT" returns 32. Same for the Linux guest.
>
> According to the Intel Web site (
> http://www.intel.com/products/processor_number/chart/pentium_d.htm )
> my processor supports "Intel(r) 64", but I don't think it is truly a
> "64-bit processor".
>
>   
>> - If your machine is idle, it scales down the clock frequency, making
>> guest idle usage seem higher (that doesn't account for the difference
>> between Linux and Windows, though).
>>
>> That said, I usually see much lower numbers.  Maybe you have an older
>> cpu.
>>     
>
> Could be, depending on how old is old.
>
> Dell tells me I bought a "Dual Core Pentium D, 930 Processor, 2X2MB
> Cache, 3.0GHz 800MHz Front Side Bus". It came as part of a Dell
> PowerEdge 830 package; the cheapest I could find that supported Intel
> VT.
>   

In terms of the VT lifetime, that's old.  I think these are the first
processors to have featured VT.

You can try downloading the latest kvm package, and do

  ./configure --with-patched-kernel
  make -C user
  user/kvmctl user/test/bootstrap user/test/vmexit.flat

That will give you the cost (in cycles) of a guest<->host switch.


(btw, you're running the distro kernel, right?  I don't know how current
the kvm version there is)

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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