Re: Linux, RT and virtualisation

[Jan Kiszka <jan.kiszka@siemens.com>]

luis.henrix@gmail.com wrote:
> Hi,
> 
> I have the following scenario: a legacy application with RT constraints
> that needs to be replicated.  Basically, I need to run several instances
> of this application on a single multi-core box.  However, this is not as
> simple as it sounds because the application assumes several things such
> as exclusive access to HW, etc.
> 
> So, instead of re-designing the application to co-exist with different
> instances, I was wondering whether this could be done using a lazy
> approach: running each instance within a virtual machine.
> 
> I have enough cores available so that I can actually dedicate 1 or more
> cores to each VM, but the problem is: will the application still be able
> to meet its RT requirements?

What are those RT requirements (order of magnitude, hard/soft, ie. what
may happen if some deadline is missed)?

>  I guess that, if two VMs share the same
> core(s), meeting the deadlines will not be possible without having a
> special scheduler on the VMs manager.  But what about if all the VMs have
> their own cores?
> 
> Of course there is still the issue with the shared access to the HW,
> but since this HW (Ethernet NICs) also have support for virtualisation,
> I could create virtual NICs for each of the VM instances.

For the tests Frank cited, I tried to avoid device emulation as far as
possible because it can be a bottleneck in QEMU (i.e. also KVM),
specifically if you go below the millisecond and there is other guest
I/O running in parallel. Still, if that may hurt you, depends on your RT
requirements.

> 
> Any experiences/thoughts/links?  Would preemptrt+Xen be able to do this?

Xen uses QEMU (a variant of it) in Dom0 for device emulation. Moreover,
you would have to merge Xen's Dom0 patches with Preempt-RT patches -
well, challenging, I bet.

> preemptrt+kvm? Other options?

Preempt-RT + kvm will at least allow you to tweak a lot, benefit from
ongoing optimizations of both projects, or maybe even apply some "dirty
tricks" to the hypervisor. IMO, a good starting point unless your
requirements are way off.

Jan

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