[vbox-dev] questions about number of CPUs and host scheduling behaviour

Nikolay Igotti nikolay.igotti at oracle.com
Fri May 6 10:17:10 GMT 2011

    Hi Bayard,

 Question is how do you generate load in guest, and what's are real 
bottlenecks. Generally, guest SMP maps to multiple
threads of execution for guest code, but mind page table 
synchronization, device access locks and other factors adding
overhead in SMP case sometimes more severe than it would be on the real 

Also if your box has just 4 CPUs I wouldn't recommend assign all them to 
the guest.


Bayard Bell wrote:
> Anyone?
> On 23 Apr 2011, at 11:50, Bayard Bell wrote:
>> I've got an OpenSolaris guest that I'm using as a compile server, with Mac OS X Server as the host. I've assigned 4 CPUs to the guest, and the guest in fact sees 4 CPUs. From the host perspective, however, what I see is that the guest never ranges substantially above 200% (or 2 CPU) utilisation, even when the run queue is backed up and 4 processes appear to be on the CPU. I'm comparing the compile times to reference against other configurations, and what I'm seeing in VirtualBox leads me to believe that I'm being presented 4 CPUs but can't actually consume more than 2. I haven't made any apples-to-apples comparison yet, but this nevertheless seems to be able to keep the system running under load that can't  be sustained with only 2 CPUs assigned, which seems to indicate that the benefits of assigning more than 2 CPUs may be more about reducing context switching and CPU migration overhead on the guest than providing the full benefit of increased compute resources (or: IOW words the benefit seems equivalent to provide hyperthreaded virtual CPUs rather than cores).
>> Is this expected behaviour? I've looked through the documentation and wasn't able to find any information on this. I'm running 4.0.6 and also saw this behaviour on 4.0.4.
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