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Subject[RESEND][RFC] BFQ I/O Scheduler
[sorry for reposting, wrong subject]

Hi,
we are working to a new I/O scheduler based on CFQ, aiming at
improved predictability and fairness of the service, while maintaining
the high throughput it already provides.

The patchset, too big for lkml posting, is available here:
http://feanor.sssup.it/~fabio/linux/bfq/patches/

The Budget Fair Queueing (BFQ) scheduler turns the CFQ Round-Robin
scheduling policy of time slices into a fair queueing scheduling
of sector budgets. More precisely, each task is assigned a budget
measured in number of sectors instead of amount of time, and budgets
are scheduled using a slightly modified version of WF2Q+. The
budget assigned to each task varies over time as a function of its
behaviour. However, one can set the maximum value of the budget
that BFQ can assign to any task.

The time-based allocation of the disk service in CFQ, while having
the desirable effect of implicitly charging each application for
the seek time it incurs, suffers from unfairness problems also
towards processes making the best possible use of the disk bandwidth.
In fact, even if the same time slice is assigned to two processes,
they may get a different throughput each, as a function of the
positions on the disk of their requests. On the contrary, BFQ can
provide strong guarantees on bandwidth distribution because the
assigned budgets are measured in number of sectors. Moreover, due
to its Round Robin policy, CFQ is characterized by an O(N) worst-case
delay (jitter) in request completion time, where N is the number
of tasks competing for the disk. On the contrary, given the accurate
service distribution of the internal WF2Q+ scheduler, BFQ exhibits
O(1) delay.

We made several tests to measure the aggregate throughput, long-term
bandwidth distribution and single-request completion time guaranteed
by CFQ and BFQ; what we present here was obtained with an outdated
version of the code, we are in the process of collecting data for
the current one (see [1]).

In the first type of tests, to achieve a higher throughput than CFQ
(with the default 100 ms time slice), the maximum budget for BFQ
had to be set to at least 4k sectors. Using the same value for the
maximum budget, in the second type of tests, BFQ guaranteed a maximum
deviation from the desired bandwidth distribution in the order of
3% over all the experiments. On the contrary CFQ exhibited a maximum
deviation of 28% in consequence of the different positions of the
files on the disk.

Slowest task's bw (MB/s) Fastest task's bw (MB/s)
-------------------------------------------------------------------
BFQ (2 files) 9.81 +/- 0.47 9.95 +/- 0.43
CFQ (2 files) 8.61 +/- 0.67 11.92 +/- 0.44
-------------------------------------------------------------------
BFQ (5 files) 4.29 +/- 0.10 4.31 +/- 0.09
CFQ (5 files) 4.01 +/- 0.17 5.24 +/- 0.14
Finally, we set up a VLC video streaming server to stream an
increasing number of movies in presence of disturbing ON/OFF random
file readers. Each test ended when a 1% packet loss was reached
(a packet was deemed as lost if transmitted with a delayed of more
than 1 second). With BFQ it was possible to transmit at most 24
movies in parallel (again with a 4k sectors maximum budget), against
15 movies with CFQ (with a time slice of 20 ms). This is likely
to be a consequence of the higher jitter of CFQ.

Nr. of movies Aggr. bw (MB/s)
---------------------------------------------------------------
BFQ (max_budget=4096) 24.00 +/- 0.00 7.56 +/- 0.87
BFQ (max_budget=16384) 18.70 +/- 9.45 12.78 +/- 5.64
CFQ (slice_sync=20) 14.35 +/- 1.40 12.59 +/- 2.12
More stuff related to BFQ (extended results, the test programs used
and the setup for the tests, a document describing the algorithm in
detail and so on) can be found at:

[1] http://algo.ing.unimo.it/people/paolo/disk_sched/

We would greatly appreciate any sort of feedback from you, comments,
suggestions, corrections and so on. Thank you for your attention.



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