20-24 September 2021
US/Pacific timezone

Per-task I/O boost tracking

20 Sep 2021, 09:45
30m
Microconference1/Virtual-Room (LPC Virtual)

Microconference1/Virtual-Room

LPC Virtual

150
Scheduler MC Scheduler MC

Speakers

Ms Beata Michalska (Arm) Dietmar Eggemann

Description

Sugov implements a rather simplistic concept of boosting I/O-bound
tasks, through tracking I/O wakeups reported on each CPU and adjusting a
synthetic boost value to potentially influence upcoming frequency changes.
The actual boost value depends on a number of different conditions, like
timings of the task wake-ups and CPU frequency update requests or the
CPUfreq policy.
This makes things rather fuzzy and exposes the following drawbacks which
might result in unexpected lost of I/O boost build-ups or undesired CPU
frequency spikes:

1) Sugov does not differentiate between I/O boost request sources so it
can't detect multiple unrelated tasks that do have sporadic I/O wake-
ups.

2) As the boost value is being maintained per CPU, boost accumulated on
one CPU might be lost upon task migration.

3) There is no guarantee that the task(s) that did trigger the I/O boost
is/are still runnable on that CPU.

4) Relevant task uclamp restrictions are not being taken into account.

5) No notion of dependency on the actual device's performance and
throughput. I.e. boosting CPU frequency might turn out to be
pointless in case the device cannot cope with the increasing IO
request rate.

This presentation shows how these shortcomings could be solved or at
least mitigated by moving from per-core to per-task I/O boost tracking
implementation.

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Primary authors

Ms Beata Michalska (Arm) Dietmar Eggemann

Presentation Materials