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

Mon May 15 10:14:16 EDT 2023

Den 2023-05-15 kl. 16:09, skrev Arne Vajhøj:
> On 5/15/2023 9:45 AM, Arne Vajhøj wrote:
>> On 5/15/2023 8:53 AM, Johnny Billquist wrote:
>>> 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.
>>
>> What would those interrupts do?
>>
>> A type 1 hypervisor does not do that much.
>>
>> It has not very much in common with a type 2 hypervisor. I see it
>> as more of a software equivalent of system partitioning (DEC Galaxy,
>> IBM LPAR).
> 
> As I don't know much about hypervisors and real time, then
> no need to take my opinion too serious.
> 
> But one can look at the market.
> 
> VMWare officially talks about real-time:
> 
> https://docs.vmware.com/en/VMware-Edge-Compute-Stack/1.0/ecs-enterprise-edge-ref-arch/GUID-99972F62-3FAE-42B2-A850-66402274B616.html
> 
> And a whole bunch of special type 1 hypervisors for real time usage exist:
> 
> Wind River Helix - https://www.windriver.com/solutions/learning/hypervisor 
> and https://www.windriver.com/products/helix
> 
> RTS - https://www.real-time-systems.com/
> 
> Acontis - https://www.acontis.com/en/acontis-hypervisor.html
> 
> Some people besides me seems to think that VM and real time is possible.
> 
> Arne
> 
> 

And I use to say that "real time" is worthless if you do not
specify what you mean with it. What are your expections?

For a user standing with some I/O devices like a hand-scanner
and a label printer, "real time" can be "within 1-2 seconds".

For a system talkning to some machinery, "real time" can be a
few 10's of milliseconds or even less.