Re: [Qemu-devel] Re: request : qemu-smp as target

[Joe Batt <Joe@soliddesign.net>]

On Tue, 2005-05-17 at 21:21 +0100, Paul Brook wrote:
> > What inter processor synchronization issues are there?  Could you take
> > this a step further and use processes on different machines for each
> > processor? (There are many shared memory implementations to choose
> > from.)  Are there so many resources shared  
> > between the CPUs to make this a ridiculous proposition?
> 
> Baically most SMP/shared memory systems assume very low latency communication 
> between CPUs and memory. For example on opteron systems remote memory latency 
> is of the order of 200 cpu cycles. Typical ethernet latency is several 
> million cycles.

But how often will the virtual CPUs need the same page and is there any
other shared resource other than memory?  I don't know how independent
each CPU is.  Though in side discussions, everyone agrees with you, I
haven't seen numbers to convince my gut.  If page only needs to be
faulted back and forth every couple million cycles, then it might work.

> The only solution I can imagine being even vaguely worthwhile is a running 
> user-mode qemu on top of a native openmozix system.
OpenMosix is very interesting, but is a pain to setup.  How about this:

  ssh -f host1 qemu -cpu-server $KEY
  ssh -f host2 qemu -cpu-server $KEY
  qemu -cpu-client host1:$KEY \
       -cpu-client host2:$KEY \
       -hda server.image

> > I have ignorantly implemented an SH2 emulator, 
> 
> Cool. Any chance you're going to make these changes publicly available?
It was a Java implementation for a customer.  Not my property and not
integrated with any free software.

> Paul
-- 
Joe Batt <Joe@soliddesign.net>