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

From: Marcelo Tosatti
Subject: Re: [Qemu-devel] QEMU and Real Time OS
Date: Mon, 2 Feb 2015 18:08:45 -0200
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, Jan 30, 2015 at 08:37:47AM +0100, Jan Kiszka wrote:
> On 2015-01-30 00:06, Paolo Bonzini wrote:
> > 
> > 
> > On 29/01/2015 20:37, Marc MarĂ­ 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.

Yes, we have achieved low latencies by using a dedicated pCPU for a
guest vCPU. 

This also avoids iothread <-> guest vCPU -RT priority issues.

> 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
> -- 
> Siemens AG, Corporate Technology, CT RTC ITP SES-DE
> Corporate Competence Center Embedded Linux

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