Jan Kiszka wrote:
 > Gilles Chanteperdrix wrote:
 > > Jan Kiszka wrote:
 > >  > The root of all this: When Nick Piggin posted his first suggestion for
 > >  > ticket spinlocks on LKML, I immediately liked the idea. For details
 > >  > check LWN [1], in a nutshell: This algorithm enforces strict FIFO order
 > >  > for the admission to contended spinlocks, thus it improves the
 > >  > determinism on SMP systems with more than 2 CPUs.
 > >  > 
 > >  > Meanwhile, ticket spinlocks are mainline (2.6.25). But that version has
 > >  > to drawbacks for us: it doesn't support nesting like xnlock does, and it
 > >  > is x86-only so far.
 > >  > 
 > >  > So I designed a version for Xenomai which is both nestable and
 > >  > arch-independent. It is certainly not as optimal as mainline's version,
 > >  > but our code path stresses the locking code differently anyway.
 > >  > 
 > >  > This thing here /seems/ to work, but I'm lacking CPUs at home to test.
 > >  > You can't truly stress ticket locks with only a single dual-core :-/.
 > >  > QEMU runs into a live-lock with -smp 2, this patch applied and two
 > >  > moderate latency loops, but that might be an artifact of its
 > >  > single-threaded VCPU scheduling. And kvm currently locks up under SMP
 > >  > even without any change, but kvm and SMP is a story of its own. There is
 > >  > hope: 16-way is waiting at work... :)
 > > 
 > > Xenomai uses a Big Kernel Lock, an approach known to not scale very
 > > well. So, if we want to scale correctly on machines with many cpus, we
 > > should change our locking strategy first.
 > 
 > Per-cpu IPC objects, per-cpu nklock - I'm all with you! But that's stuff
 > for a massive restructuring we could schedule for Xenomai 3.

This does not look that easy. What I meant is rather that Xenomai is
probably run mostly on machines with not so many cpus.

-- 


                                            Gilles Chanteperdrix.

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