[lttng-dev] real time Userspace RCU

[Paul E. McKenney]

On Thu, Mar 31, 2016 at 09:20:07AM +0800, Yuxin Ren wrote:
> Thank you all!!
> 
> I agree URCU does timing quite well.
> But are there any formal response time analysis for URCU/RCU (both
> read  and update path)?

Not that I know of.  You could be the first!

> Or could anyone guide me how to do RTA for URCU/RCU?

Compile something with a simple RCU read-side critical section, and then
count the instructions.  QSBR will of course work best, but MB will also
have good bounds.  Signal-based will be a bit more complicated.

Much depends on what type of RTA you want to do.  Brandenburg's
dissertation has quite a bit of info and many citations:

	http://www.cs.unc.edu/~bbb/diss/

You can also take an experimental approach, though a great many
runs are required.  OSADL (https://www.osadl.org/) does quite a
bit of this work on -rt Linux.

							Thanx, Paul

> Thanks again.
> 
> On Fri, Mar 11, 2016 at 10:00 PM, Mathieu Desnoyers
> <mathieu.desnoyers at efficios.com> wrote:
> > ----- On Mar 11, 2016, at 6:45 AM, Paul E. McKenney paulmck at linux.vnet.ibm.com wrote:
> >
> >> 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.
> >
> > An intermediate solution if both update throughput and bounded-memory
> > are required (but the application would not have real-time constraints
> > on updates) would be to use the defer_rcu() API in liburcu. It amortizes
> > the cost of synchronize_rcu() over many defer_rcu() calls with a worker
> > thread, but only up to an upper bound. When the upper bound is reached,
> > the defer queue call empties the defer queue itself.
> >
> > Thanks,
> >
> > Mathieu
> >
> >>
> >> 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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> >>> >>> lttng-dev at lists.lttng.org
> >>> >>> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> >>> >>>
> >>> > _______________________________________________
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> >>>
> >>> --
> >>> Mathieu Desnoyers
> >>> EfficiOS Inc.
> >>> http://www.efficios.com
> >
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > http://www.efficios.com
>