Re: [patch 0/4] [RFC] Another proportional weight IO controller

[Paolo Valente <paolo.valente@unimore.it>]

Fabio and I are a little bit worried about the fact that the problem
of working in the time domain instead of the service domain is not
being properly dealt with.  Probably we did not express ourselves very
clearly, so we will try to put in more practical terms.  Using B-WF2Q+
in the time domain instead of using CFQ (Round-Robin) means introducing
higher complexity than CFQ to get almost the same service properties
of CFQ.  With regard to fairness (long term) B-WF2Q+ in the time domain
has exactly the same (un)fairness problems of CFQ.  As far as bandwidth
differentiation is concerned, it can be obtained with CFQ by just
increasing the time slice (e.g., double weight => double slice).  This
has no impact on long term guarantees and certainly does not decrease
the throughput.

With regard to short term guarantees (request completion time), one of
the properties of the reference ideal system of Wf2Q+ is that, assuming
for simplicity that all the queues have the same weight, as the ideal
system serves each queue at the same speed, shorter budgets are completed
in a shorter time intervals than longer budgets.  B-WF2Q+ guarantees
O(1) deviation from this ideal service.  Hence, the tight delay/jitter
measured in our experiments with BFQ is a consequence of the simple (and
probably still improvable) budget assignment mechanism of (the overall)
BFQ.  In contrast, if all the budgets are equal, as it happens if we use
time slices, the resulting scheduler is exactly a Round-Robin, again
as in CFQ (see [1]).

Finally, with regard to completion time delay differentiation through
weight differentiation, this is probably the only case in which B-WF2Q+
would perform better than CFQ, because, in case of CFQ, reducing the
time slices may reduce the throughput, whereas increasing the time slice
would increase the worst-case delay/jitter.

In the end, BFQ succeeds in guaranteeing fairness (or in general the
desired bandwidth distribution) because it works in the service domain
(and this is probably the only way to achieve this goal), not because
it uses WF2Q+ instead of Round-Robin.  Similarly, it provides tight
delay/jitter only because B-WF2Q+ is used in combination with a simple
budget assignment (differentiation) mechanism (again in the service
domain).

[1] http://feanor.sssup.it/~fabio/linux/bfq/results.php

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