[6tisch] MSF traffic adaptation under stress

[David Hauweele <[e-mail]>]

Dear 6TiSCH,

In the last few months, we performed a small scale study of the
behavior of 6TiSCH's minimal scheduling function (MSF) under stress. We
were especially interested in the dynamics of MSF's automated
adaptation to traffic load. Some of our conclusions have been
summarized in a paper that we submitted to IEEE ISCC 2020. A copy of
the evaluation section of our submission is attached to this mail.

Among our surprising findings, we observed that

1) The time required for MSF to adapt to a traffic change depends on
the current number of cells allocated. To give an example, it takes
much longer to go from 0 to 10 cells than from 10 to 20 cells. We
attribute this behavior to the way MSF measures cell occupancy by
counting the number of used and passed cells. Adaptation only occurs
when the number of passed cells reaches a maximum. However, with light
cell allocation, it takes longer to reach that maximum than with higher
cell allocation.

2) MSF can lead to severe over-provisioning, which can be harmful in an
environment where the resources are scarce. We noticed that releasing
cells was especially hard for MSF due to the fixed hysteresis
thresholds. Indeed, the estimated cell occupancy must drop below 25%
for cells to be released.

We already have ideas to improve MSF's traffic adaptation mechanism,
that we plan to put under test in the coming weeks. We can also provide
you with more details if you wish. If you see interest in our evaluation
and proposals, we are eager to discuss this further with the WG.

Best regards,
David, Bruno, Aris and Georgios