Re: [Qemu-devel] QEMU and Real Time OS

[Jan Kiszka]

On 2015-01-30 11:36, Frederic Konrad wrote:
> On 30/01/2015 11:26, Marc Marí wrote:
>> El Fri, 30 Jan 2015 08:37:47 +0100
>> Jan Kiszka <address@hidden> escribió:
>>> On 2015-01-30 00:06, Paolo Bonzini wrote:
>>>>
>>>> On 29/01/2015 20:37, Marc Marí wrote:
>>>>> Is this an expected behaviour? I can't see why.
>>>>>
>>>>> I'd like to know if there is a certain reason why it doesn't work.
>>>>> Or if it should work and the problem is too much I/O overhead. Or
>>>>> any other hint to understand it.
>>>> It is due to latencies in the host.  You need at least to use
>>>> preempt-rt kernels in the host as well.
>>> That alone won't help much. You also need to fine-tune the guest to
>>> avoid running into QEMU locks that continuously synchronizes the guest
>>> on things like VGA or disk I/O emulation.
>>>
>>> When using KVM, thus being able to run VCPUs widely independent of
>>> each other and the device models, you need to push cyclictest on an
>>> isolated second virtual CPU of the guest. Luiz and Marcelo can
>>> probably confirm this based on their ongoing experiments.
>>>
>>> With TCG, we would first of all have to make it true SMP and
>>> independent of the I/O device lock. That's what Frederic is working
>>> on [1].
>>>
>>> Jan
>>>
>>> [1] http://permalink.gmane.org/gmane.comp.emulators.qemu/314406
>>>
>> Thanks for the answers. I think I'm stuck with ARM926, which I think is
>> not prepared for SMP. I'll have to look if I can use Cortex for my
>> experiments.
>>
>> I'll continue interested with the improvements for RT on TCG, but for
>> the moment I'll go to work on real harware, even though is easier to
>> run and debug on an emulator.
>>
>> Thanks
>> Marc
> Hi Marc,
> 
> I think the important point here is "TCG thread independent of the I/O
> device lock".
> I need it for multithread TCG but that doesn't mean you need an SMP guest
> platform for that.

Provided the guest is not stressing any costly device emulation. That
can cause priority inversions when an urgent event should rather be
delivered in the meantime. That can easily happen when running real-time
Linux as guest, therefore the SMP proposal.

Jan

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