Re: Real-time projects that could use userspace RCU

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* John Kacur (jkacur@redhat.com) wrote:
> On Tue, May 11, 2010 at 2:21 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
> > Hi Thomas,
> >
> > Paul told me you were quite interested in the userspace RCU library when he told
> > you about it (http://lttng.org/urcu). Do you have some userspace applications or
> > libraries with real-time needs in mind that could use it ? We could help moving
> > them to liburcu. The wait-free read-side is, as you certainly know, a
> > characteristic of RCU that can be very useful to RT applications.
> >
> > [CCing linux-rt-users, as it seems appropriate to ask them too.]
> >
> > Thanks,
> 
> Do you have any kind of benchmarks? If you had something appropriate
> we could add it to the rt-tests suite (which includes cyclictest). Not
> only would this provide an objective measure, but it could also act as
> a reference implementation for userspace programmers.

Yes, the library already has its set of benchmark test programs. The results can
be found in http://lttng.org/pub/thesis/desnoyers-dissertation-2009-12.pdf
section 6.5. It shows that RCU read-side is a few orders of magnitude faster
than lock-based approaches and scales linearly with the number of cores.

The same PDF, sections 7.6.2 and 7.6.3, presents the architecture-level modeling
of the RCU mb algorithm in Promela, along with the formal proof by model
checking for both correctness and progress (the read-side is proven wait-free).

> 
> See here.
> git clone git://git.kernel.org/pub/scm/linux/kernel/git/clrkwllms/rt-tests.git
> 
> cyclictest is the original program written by Thomas, maintained by
> Clark Williams now. Most - but not all, of the additional tests are
> modelled after this program, so you might want to have a look at that
> if you're not already familiar with it.

Thanks for the pointer, I did know about cyclictest, but not the others. Since
the read-side does not involve the OS nor blocking, I wonder which of these
tests would be even a near-match though.

Thanks,

Mathieu

> 
> Thanks
> 
> John

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com