Re: [RTnet-developers] Debugging

[Glen W. <gl...@je...>]

Great stuff thank you.

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Glen Wernersbach
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On Dec 7, 2010, at 12:57 PM, Jan Kiszka <jan...@si...> wrote:

> Am 07.12.2010 18:23, Glen Wernersbach wrote:
>> Jan,
>> 
>> I wanted to follow up on the line below. My understand is that you cannot do
>> hard real time in virtualization.
> 
> That's not true. Nothing conceptually prevents that hypervisor provide
> the same determinism like real hardware. You should just not expect to
> see the same latencies. In practice, not every hypervisor is designed
> for hard or even just soft RT, just like not every OS provides this.
> 
> So far, QEMU/KVM is not capable of providing hard guarantees, and it
> even has problems with soft RT unless you very carefully tune
> everything. However, last time I tried I was able to measure << 1 ms
> guest user space latencies in a PREEEMPT-RT or Xenomai on PREEMPT-RT
> with KVM setup (modern AMD host). And that's an order of magnitude where
> quite a few real applications start to be happy.
> 
>> You seem to indicate we can debug our
>> RTNET app to some extend using QEMU and KVM.
>> 
>> Just wonder what you experience was?
> 
> It depends on what aspects of your app, middleware, driver you want to
> debug and how they react if deadlines are missed. If they jump out of
> the window because some operation took surprisingly long, then you
> either need to avoid this code path or relax the check when running in a
> VM. That's fairly easy with new code, but it can be hairy if not
> impossible with existing large projects.
> 
> Transferred to RTnet: I'm able to run a full-featured network
> (RTmac/TDMA, RTcfg) in multiple VMs with moderate cycles. Of course,
> deadlines will be missed all the time whenever the host is overloaded or
> just temporarily busy with other work. But a lot of tests are
> nevertheless possible.
> 
> Jan
> 
> -- 
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