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.


                                            Gilles Chanteperdrix.

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