I used to play with a realtime Linux system back in the 90s that had a sort of virtualization architecture.  It had a realtime executive that could run realtime tasks.  One of its tasks was the Linux kernel itself so this way the tasks could "talk" with linux processes and make use of linux capabilities it lacked.

When I first worked with it, it ran the 1.3 kernel and it was really fast. A 6µsec latency.  It got progressively worse with 2x kernels.  But still much better than the 150µsec quoted earler.

I can't remember what it was called. Somebody from New Mexico developed it.  There was also an offshoot development by a group in Italy.



On 08/27/2014 12:07 PM, Michael Duvall wrote:
> Hello,
>
> While I am thoroughly interested in SL topics, I rarely comment on threads.  Today is an exception.  I work for a real-time linux vendor.  I concur with David Somerseth's summation.  Real-time cannot be achieved under virtualization.
>
> Regards,
> --
> *Michael Duvall*
> Systems Analyst, Real-Time
> [log in to unmask] <mailto:[log in to unmask]>
>
> (954) 973-5395 direct
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>
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>
> -----Original Message-----
> *From*: David Sommerseth <[log in to unmask] <mailto:[log in to unmask]>>
> *Reply-to*: "[log in to unmask]" <[log in to unmask]>
> *To*: John Lauro <[log in to unmask] <mailto:[log in to unmask]>>, Paul Robert Marino <[log in to unmask] <mailto:[log in to unmask]>>
> *Cc*: Brandon Vincent <[log in to unmask] <mailto:[log in to unmask]>>, [log in to unmask] <[log in to unmask] <mailto:[log in to unmask]@gmail.com%3e>>, [log in to unmask] <[log in to unmask] <mailto:[log in to unmask]@fnal.gov%3e>>, Nico Kadel-Garcia <[log in to unmask] <mailto:[log in to unmask]>>
> *Subject*: Re: about realtime system
> *Date*: Wed, 27 Aug 2014 12:27:50 -0400
>
> 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>
>
>