[lttng-dev] real time Userspace RCU

[Paul E. McKenney]

On Thu, Mar 10, 2016 at 08:53:05PM +0000, Mathieu Desnoyers wrote:
> ----- On Mar 10, 2016, at 3:33 PM, Yuxin Ren ryx at gwmail.gwu.edu wrote:
> 
> > Thank you for your reply.
> > 
> > I want to generally understand how to apply urcu to real time systems.
> > I know real time system focus on predictability on both timing and
> > memory consumption.
> > So how does real time urcu support predictability?
> > Could you provide me some papers, documents or any materials about any
> > aspect of real time urcu?
> 
> Adding Paul E. McKenney in CC, who may have some thoughts on this
> topic.

URCU does timing quite well, given that the read-side primitives each
execute a fixed sequence of instructions.  Updates using call_rcu()
can be used to minimize update-side latency, but if you need to bound
memory overhead, the best way to do that is to make sure that updates
are not on the critical path, and then use synchronize_rcu() instead
of call_rcu().  In that case, the total amount of memory waiting for
reclamation is bounded by the maximum size of an RCU-protected memory
block times the number of threads.

So can you design your application so that updates are off the critical
path?  If so, you can get both bounded read-side accesses and bounded
memory footprint.

This of course assumes that your data structures are simple enough
that readers don't need to use retry techniques.

The following info might be helpful:

http://www2.rdrop.com/users/paulmck/realtime/paper/DetSyncRCU.2009.08.18a.pdf
http://www2.rdrop.com/users/paulmck/realtime/paper/DetSyncRCU.2009.09.29a.pdf

http://www2.rdrop.com/users/paulmck/realtime/paper/RTLWS2012occcRT.2012.10.19e.pdf

It also depends on your timeframe.  Microseconds?  Life is hard.
Milliseconds?  Care is required, but you have a fair amount of freedom.
Seconds?  Life is not so hard.  Unless you need to do two seconds of
computation in one second or some such.  ;-)

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > 
> > Thanks again!
> > 
> > On Thu, Mar 10, 2016 at 1:52 PM, Michel Dagenais
> > <michel.dagenais at polymtl.ca> wrote:
> >> Real-time and embedded systems is an important current focus for the LTTng
> >> toolchain research. Do you have specific needs for userspace RCU?
> >>
> >> ----- Mail original -----
> >>> Hi,
> >>>
> >>>  Is there any work or research about Userspace RCU on real time or
> >>> embedded systems?
> >>> Any information is welcome.
> >>>
> >>> Thanks a lot!
> >>> Yuxin
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> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>