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 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.
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
