[Info-vax] clock problems with OpenVMS x86 on VirtualBox

[Johnny Billquist]

On 2023-05-15 14:36, Arne Vajhøj wrote:
> On 5/15/2023 8:20 AM, Simon Clubley wrote:
>> On 2023-05-12, Arne Vajhøj <arne at vajhoej.dk> wrote:
>>> On 5/12/2023 1:30 PM, Simon Clubley wrote:
>>>> On 2023-05-12, Dave Froble <davef at tsoft-inc.com> wrote:
>>>>> On 5/12/2023 8:14 AM, Simon Clubley wrote:
>>>>>> That's going to make for some "interesting" real-time program 
>>>>>> behaviour... :-)
>>>>>
>>>>> Do you think any serious real time programmer will run a real time 
>>>>> task inside a
>>>>> VM?  I'm not a real time programmer, and I'd still not do that.
>>>>
>>>> As well as traditional real-time stuff (which I agree with you about 
>>>> BTW),
>>>
>>> I would not want to do it on a type 2 hypervisor - there must be
>>> cases where what is happening on the host OS impact the performance
>>> of the guest OS.
>>>
>>> But with a type 1 hypervisor and no over allocation of resources -
>>> would it be worse than running on bare metal?
>>
>> A type 1 hypervisor is still a software layer between the hardware and
>> the RTOS. It would have to _guarantee_ that it would _never_ get in the
>> way of the timing guarantees that a program running under a RTOS needs.
> 
> It is software that is active between the VMS and the HW.
> 
> But it is not obvious to me where any delay would come from.
> 
> If we talk type 2 then it is easy to understand. You have a host
> OS running 200 processes - 1 VM and 199 other stuff, and 4 CPU
> so those 200 processes get scheduled on those 4 CPU's, and certain
> interrupts may happen in the host OS. That VM will have
> a problem providing real time capabilities.
> 
> But in the type 1 with no over allocation of resources then
> what will cause any delays? The VM got its own CPU or CPU's
> that no other VM want, so I would expect the hypervisor to
> keep them permanently allocated to the VM. And I don't
> see the need for any interrupts in the hypervisor either.

Can you guarantee that no interrupts happens on the CPUs the hypervisor 
is using? I would suspect "no", which means you do not have full control.

It's kindof weird to me that people can even believe that it would be 
possible to add a software layer in between that have no actual impact.

   Johnny