[lttng-dev] real time Userspace RCU

Fri Apr 15 15:46:48 UTC 2016

I really appreciate your reply!!

I have one more question.
>From real time aspect, preemption of URCU read-side critical sections
can cause priority-inversion issue.
Does current URCU implementation deal with this problem?
And if so, how does it solve this?

Thanks
Yuxin

On Thu, Mar 31, 2016 at 7:22 AM, Paul E. McKenney
<paulmck at linux.vnet.ibm.com> wrote:
> 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
>> >>> >>> _______________________________________________
>> >>> >>> lttng-dev mailing list
>> >>> >>> 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
>>
>