Hi Jan, On Tue, Jun 22, 2010 at 08:29:03PM +0200, Jan Kiszka wrote: > luis.henrix@xxxxxxxxx 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)? We're talking about 1 ms deadline -- there is a task that needs to "do some work" every millisecond. And we're talking about a hard RT system (although I suspect the old software actually.... well, never mind :) ) > > 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. It looks like it is going to hurt me. But still, this is something I still need to measure. There is always the option to re-design the old app and not to use virtualisation at all. > > > > 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. Ouch! Here's something I was not planning to do :) > > 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. Agree, it looks like the easiest way. But still need to take a look at XtratuM (suggested by Nicholas). Thanks! -- Luis Henriques -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
