-- Michael Duvall Systems Analyst, Real-Time [log in to unmask] (954) 973-5395 direct (954) 531-4538 mobile CONCURRENT | 2881 Gateway Drive | Pompano Beach, FL 33069 | www.real-time.ccur.com

On 24/08/14 18:57, John Lauro wrote:
> Why spread FUD about Vmware.  Anyways, to hear what they say on the subject:
> http://www.vmware.com/files/pdf/techpaper/latency-sensitive-perf-vsphere55.pdf
> 
> Anyways, KVM will not handle latency any better than Vmware.

You can currently not achieve true realtime characteristics when adding
virtualization, no matter the technology.  The reason is that realtime
tasks must be able to preempt running tasks to be able to keep its
deadlines.

Consider running a virtualized realtime kernel running a realtime task,
on a host VM with a realtime kernel.  When the task gets CPU time, it
preempts all other running tasks on the provided CPU core.  But if the
VM host is not aware of this happening, it may just as well not give
enough runtime in the right time-window to the realtime guest OS.  Thus
increasing the latency quite noticeably.  So for this to work, the guest
OS kernel must be able to communicate to the host OS kernel that it has
a task which needs attention right now.  And AFAIK, this mechanism is
not implemented anywhere.

I know there has been done some research on this topic some years ago,
and an interesting paper on it.  But I don't know if this has come any
further.

<http://lwn.net/images/conf/rtlws11/papers/proc/p18.pdf>