Re: [6tsch] On the fly scheduling

Guillaume Gaillard <guillaume.gaillard.maze@gmail.com> Fri, 04 October 2013 14:57 UTC

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Date: Fri, 4 Oct 2013 16:54:41 +0200
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From: Guillaume Gaillard <guillaume.gaillard.maze@gmail.com>
To: Qin Wang <qinwang@berkeley.edu>
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Cc: 6TSCH <6tsch@ietf.org>
Subject: Re: [6tsch] On the fly scheduling
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Hi Qin and all,

I agree with your separation into two tools (pre-allocation vs
post-allocation). When one wants to meet a new QoS requirement, it is
possible:
- (1) to over-provision cells in the schedule in order to be "sure" to meet
the requirement (pre-allocation);
- (2) to adapt the cell allocation to the variations of the network
parameters (post-allocation);
- (3) to do both things.

When solution (2) is adopted, one should tolerate a time of degraded QoS,
before the requirement is restored.

I agree on your list of mechanisms a solution requires. Is the algorithm
(4) (matching number of cell with QoS requirements) implementation-specific
?

Regards,

Guillaume Gaillard

Orange Labs Meylan/CITI INRIA-INSA Lyon
PhD student "SLA pour Réseaux de Capteurs Multi-Services"
Advisors D. Barthel, F. Valois, F. Theoleyre



2013/10/4 Qin Wang <qinwang@berkeley.edu>

> Hi Pascal and all,
>
> It is a interesting approach. I would like to put different approaches
> above together.
>
> Objective: meet some QoS requirement.
> Tool 1: pre-allocation, like what Pascal suggested, assign number of cells
> to a bundle, and on/off them on demand.
> Tool 2: post-allocation, like discussed in other emails in this thread,
> adjust the number of cells in a bundle according to statistics information.
>
> In reality, I think a mote may use the combination of tool 1 and tool 2,
> e.g. at the beginning, mote allocates some cells for a bundle, and use them
> in the mode of on/off on demand; when statistics information show 2 more
> cells needed, then mote is triggered to allocates 3 more cells for the
> bundle, and then also use them in the mode of on/off on demand.
>
> If it is the case, a solution should include the the following parts:
> (1) mechanism to create/delete softcells
> (2) mechanism to provide statistics information
> (3) mechanism to on/off cell on demand
> (4) algorithm to determine how many cells needed for a QoS requirement
> like (dataflow/burst-feature, delay-tolerance, {statistics info},  ...)
>
> What do you think?
>
> Qin
>
>
>
>
> On Fri, Oct 4, 2013 at 5:30 PM, Pascal Thubert (pthubert) <
> pthubert@cisco.com> wrote:
>
>>  Dear all:****
>>
>> ** **
>>
>> Statistics and hysteresis can be costly in terms of memory and CPU, may
>> not be the best strategy to address bursts of traffic in a stochastic IP
>> network, and the strategy to derive the allocation from the past
>> observation may be hard to determine in a particular case and even harder
>> to generalize.****
>>
>> ** **
>>
>> The needs of bandwidth may be very dynamic for instance in the case of
>> alarms/alerts, or in the case of a brutal variation of monitored data.
>> Changes in the network topology impacting the rank of a node within the DAG
>> may also impact brutally its needs of bandwidth. So we cannot only rely on
>> past observations and but also need to maintain some capabilities that rely
>> on current needs that operate efficiently in terms of control and yet very
>> dynamically to accept both brutal bursts of traffic and longer term
>> variations.****
>>
>> ** **
>>
>> Considering that we have ample room in a SlotFrame, a simple strategy,
>> derived from traditional memory management, could be to augment the number
>> of cells in a bundle each time it is saturated. As an image, GNU’s obstack
>> seem to grow linearly one chunk at a time, but I know of chunk
>> implementations that double the size of a dynamic chunk each time it is
>> saturated and a geometrical growth may a better approach for us. ****
>>
>> ** **
>>
>> In a bundle, a portion of the cells is always on. The rest can stay idle
>> (no wake to send or listen) unless a previous cell indicates that there is
>> more to come. If we agree on a strategy like this then we can start working
>> out the details, in particular how handle lazy release or one-shot
>> override, how we observe unallocated slots to make them candidate for
>> allocation, etc...****
>>
>> ** **
>>
>> As you see, even implementing such a simple strategy can already be quite
>> some work in a constrained device. ****
>>
>> ** **
>>
>> Cheers,****
>>
>> ** **
>>
>> Pascal****
>>
>> ** **
>>
>> *From:* 6tsch-bounces@ietf.org [mailto:6tsch-bounces@ietf.org] *On
>> Behalf Of *Maria Rita PALATTELLA
>> *Sent:* vendredi 4 octobre 2013 09:19
>> *To:* Thomas Watteyne; 6TSCH
>>
>> *Subject:* Re: [6tsch] On the fly scheduling****
>>
>>  ** **
>>
>> +1 for me too.****
>>
>> ** **
>>
>> For sure, according to the specific requirements of the applications,
>> different algorithms may be used for building the schedule. But the
>> functionalities offers by 6top (i.e., collect statist. Info, send commands
>> for scheduling cells) will be still the same (no matter which algorithm is
>> used). ****
>>
>> ** **
>>
>> The statistics needed could change because different algorithms may use
>> different input as parameters.  So, we will have to include a set of
>> statistic information, or anyway, make sure it is possible to extend them.
>> ****
>>
>> ** **
>>
>> Maria Rita****
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> *From:* 6tsch-bounces@ietf.org [mailto:6tsch-bounces@ietf.org<6tsch-bounces@ietf.org>]
>> *On Behalf Of *Thomas Watteyne
>> *Sent:* Thursday, October 03, 2013 10:11 PM
>> *To:* 6TSCH
>> *Subject:* Re: [6tsch] On the fly scheduling****
>>
>> ** **
>>
>> That would indeed be clean.****
>>
>> ** **
>>
>> On Thu, Oct 3, 2013 at 12:33 PM, Qin Wang <qinwang@berkeley.edu> wrote:**
>> **
>>
>> Hi all,****
>>
>> ** **
>>
>> +1 for the proposal. ****
>>
>> ** **
>>
>> Regarding to how to approach, I agree with Thomas. There is an entity
>> running on top of 6top, which reads queue information and other statistics
>> information from 6top, sends instruction like create/delete softcells to
>> 6top. I think we can use the design methodology of Objective Function in
>> RPL, i.e. define the statistics information and the interface to/from 6top,
>> and leave the specific algorithm open.****
>>
>> ** **
>>
>> What do you think?****
>>
>> ** **
>>
>> Qin ****
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> On Fri, Oct 4, 2013 at 12:45 AM, Thomas Watteyne <
>> watteyne@eecs.berkeley.edu> wrote:****
>>
>> +1 for the proposal.****
>>
>> ** **
>>
>> I believe it could be a very simple and powerful approach. Diego, would
>> you agree that this can be considered a distributed mechanism sitting on
>> top of 6top?****
>>
>> ** **
>>
>> That is, 6top provides:****
>>
>>    - commands to modify the number of soft cells in a bundle****
>>    - commands to retrieve usage statistics of the cells/bundles****
>>
>>  The way I see it, your proposal consists of an algorithm which feeds
>> from the usage statistics and triggers changes in the number of soft cells
>> in a bundle. Correct?****
>>
>> ** **
>>
>> The questions to answer for now is whether 6top provides the right
>> statistics.****
>>
>> ** **
>>
>> Maria Rita, one big difference with TASA is that OTF scheduling is
>> distributed.****
>>
>> ** **
>>
>> Thomas****
>>
>> ** **
>>
>> On Thu, Oct 3, 2013 at 9:11 AM, Maria Rita PALATTELLA <
>> maria-rita.palattella@uni.lu> wrote:****
>>
>> Diego,( all)
>>
>> what you are suggesting (i.e., reserve cells based on queue size, delay)
>> is actually the main idea behind TASA (Traffic Aware Scheduling Algorithm).
>>
>> TASA builds the schedule based on the local (number of pkt generated by
>> the node) and global queue level ( i.e., local + pkt to be forwarded,
>> generated by children). It gives priority to nodes with longer queues and
>> it aims to reduce the latency for delivering the pkt. At the same time
>> while building the schedule it minimizes the number of scheduled cells in
>> order to reduce the network duty cycle.
>> TASA is centralized and thus it assumes that the PCE has all the info
>> needed for setting up the schedule. in other words, it knows the traffic
>> generated by each nodes, and the paths followed by each pkt.
>> With a "on the fly solution", we will not need to know all this info a
>> priori. but we will use 6top monitoring functions and the control flows
>> message for scheduling the cells.
>>
>> Btw, I agree with all the points raised up by Xavi.  We will have to
>> address his questions.
>>
>> And I support Pascal's suggestions about how to deal with bundle.
>>
>> Maria Rita****
>>  ------------------------------
>>
>> *From:* 6tsch-bounces@ietf.org [6tsch-bounces@ietf.org] on behalf of
>> Pascal Thubert (pthubert) [pthubert@cisco.com]
>> *Sent:* Thursday, October 03, 2013 4:09 PM
>> *To:* xvilajosana@eecs.berkeley.edu; Prof. Diego Dujovne****
>>
>>
>> *Cc:* 6TSCH
>> *Subject:* Re: [6tsch] On the fly scheduling****
>>
>> ** **
>>
>> +1 too.****
>>
>>  ****
>>
>> I think that the queue size matters at enqueue but the latency is really
>> what we care for at dequeue, that is how long did this device keep this
>> message in queue (even if we are far from ****
>>
>> buffer bloat conditions in such a device). If one of the 2 conditions
>> (size at enqueue, latency at dequeue) is reached then the bundle should be
>> increased. ****
>>
>>  ****
>>
>> I agree with Xavi that we want to avoid changing the bundle size all the
>> time. We discussed that with Qin and others earlier on the ML. One way of
>> increasing the bundle dynamically at a very low cost (not even a
>> hysteresis)  is to have it large amount of cells from the start but used
>> like 10% by default (xmit/listen happens only once in 10 time slots). A bit
>> in the frame indicates whether the next (normally unused) slot will indeed
>> be used. The bit can be present in the data and acked in the ack. This can
>> also implicitly be triggered for retries.****
>>
>>  ****
>>
>> Please keep us tuned!****
>>
>>  ****
>>
>> Cheers,****
>>
>>  ****
>>
>> PS Note that Cisco has IPR on chaining time slots and flagging whether
>> the next is used or not. We already declared our IPR against the
>> architecture draft and provided terms.****
>>
>>  ****
>>
>> Pascal****
>>
>>  ****
>>
>> *From:* 6tsch-bounces@ietf.org [mailto:6tsch-bounces@ietf.org] *On
>> Behalf Of *Xavier Vilajosana Guillen
>> *Sent:* jeudi 3 octobre 2013 15:46
>> *To:* Prof. Diego Dujovne
>> *Cc:* 6TSCH
>> *Subject:* Re: [6tsch] On the fly scheduling****
>>
>>  ****
>>
>> Diego,
>>
>> +1****
>>
>> it seems to a me a very interesting idea to explore. Maybe we can start
>> putting some rules of this mechanism on the table and prepare a simulation.
>> I am completely in with that idea.****
>>
>> Some questions arise:****
>>
>> 1-how fast do you react to changes on the queue size to avoid hysteresis
>> -- i.e how do you maintain certain stability in the schedule (so you don't
>> start installing and removing links very often)****
>>
>> 2-how you map queue size (only one or if more than one queue) to actual
>> link requirements****
>>
>> 3-how you recover from link collisions in case of multiple nodes schedule
>> the same cells.****
>>
>> 4-how to decide to who (what neighbor) install more links according to
>> queue size?****
>>
>>  ****
>>
>> cheers!
>> Xavi****
>>
>>  ****
>>
>>  ****
>>
>> On Thu, Oct 3, 2013 at 3:23 AM, Prof. Diego Dujovne <
>> diego.dujovne@mail.udp.cl> wrote:****
>>
>> Dear all,
>>             I've been looking into the idea of "on the fly scheduling",
>> presented on the Sept 27th webex call as "on-the-fly decentralized
>> reservation".
>> The basic mechanism would be based on analysing the queue size
>> on a node and dynamically adapt the number of reserved
>> cells to satisfy queue size, delay and/or power
>> consumption thresholds.
>>             This mechanism would work inside 6top, between pairs of nodes.
>> As a first approach, it would be based on the minimal draft.
>> What do you think on this starting point?
>> I (gladly) receive comments to add or modify this proposal.
>>
>>                                      Diego
>>
>>
>>
>>
>> --
>> DIEGO DUJOVNE
>> Académico Escuela de Ingeniería en Informática y Telecomunicaciones
>> Facultad de Ingeniería UDP
>> www.ingenieria.udp.cl
>> (56 2) 676 8125
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>>
>>  ****
>>
>>
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