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

[Arne Vajhøj]

On 5/15/2023 8:51 AM, Johnny Billquist wrote:
> On 2023-05-15 14:03, Arne Vajhøj wrote:
>> On 5/15/2023 6:17 AM, Johnny Billquist wrote:
>>> On 2023-05-13 02:16, Arne Vajhøj 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?
>>>
>>> For sure. You have no guarantee that you will get the CPU cycles when 
>>> you need them. No matter what kind of hypervisor we're talking about, 
>>> there is overhead in the host that can affect things way more than 
>>> what might happen on bare metal.
>>
>> What host? A type 1 hypervisor does not run a host OS.
> 
> Um? What is the hypervisor then?

The hypervisor software like ESXi.

It is not running on top of a host OS like a type 2 (VirtualBox etc.) does.

>> And why would the CPU not be available, when there is no
>> over allocation of resources? It seems pretty silly of
>> the hypervisor to not want to give the CPU if no other
>> VM can get it.
> 
> Any kind of hypervisor means we're talking about virtualization. That 
> means you have real hardware which exists outside of this 
> virtualization. And that hardware can generate interrupts and demand 
> service, which then forces the hypervisor to wait before it can actually 
> get any cycles. Outside the control of the hypervisor...

Maybe. But what would that be?

Arne