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Feedback Control for System
Tuning
Cody Rioux - @codyrioux
Real-Time Analytics - Insight Engineering
Overview
● Feedback Control
○ Definition
○ Conceptual Model
● Concepts
○ Math
○ Rules of Thumb
● Case Study
○ Controlling Chris’ Effective Hours
● Recap
Controlling dynamic systems through continuous feedback.
Feedback Control
What is Feedback Control?
A form of Process Control for
controlling the behavior of black
box systems.
Cody EngineAccelerator Speed
Whats a
setpoint?
If you drive like me...
No regard for the speed limit, you’re just a feed-forward automaton.
You (Driver) Engine (V8)Accelerator Speed
Speedometer
Speed Limit
Driving A Car
Cruise Control Engine (V8)Accelerator Speed
Speed Reading
Speed Limit
What if we got a machine to do it?
SystemInput Output
A Conceptual Model
Controller SystemInput OutputSetpoint
A Conceptual Model
This happens to be a feed-forward system.
Controller SystemInput Output
Feedback / Error
Setpoint
A Conceptual Model
Controller SystemInput Output
Feedback / Error
Setpoint
A Conceptual Model
Controller SystemInput Output
Feedback / Error
Setpoint
A Conceptual Model
Controller SystemInput Output
Feedback / Error
Setpoint
A Conceptual Model
Controller SystemInput Output
Feedback / Error
Setpoint
A Conceptual Model
Controller SystemInput Output
Feedback / Error
Setpoint
A Conceptual Model
Feedback Principle
Continuously compare the actual
output to its desired reference value;
then apply a change to the system
inputs that counteracts any deviation of
the actual output from the reference.
You’ll need an advanced math degree for these...
Concepts
Controller SystemInput Output
Feedback / Error
Setpoint
A Conceptual Model
error = setpoint - output
input = gain * error
Things to keep in mind...
Gain controls the
magnitude of our
adjustments. This is
important.
Univariate feedback is
inherently simple,
multivariate feedback
inherently complex.
Prefer small and
frequent adjustments to
large infrequent
adjustments.
Oscillation creates
instability. We don’t like
that.
End-to-end control system for maximum analytics productivity.
Case Study: Controlling Chris’
Caffeine Intake
What if this were...
● … a timeout value?
● … a queue size?
● … a server cluster size?
We can keep Chris rolling at 8 hours a day 365 if we wanted to.
Recap
Benefits of Feedback Control
● Automatic - No Need for Analytical Model
● Dynamic - System responds to change
● Real Time - Change Happens Fast
● Self-Correcting - Mistakes are Quickly Corrected
Feedback control is a viable
solution for configuring
systems under dynamic
conditions.
Only Scratching the Surface
● Integral Controllers
● Derivative Controllers
● PID Controllers
● Statistical Process Control
Literature
Feedback Control for Computer
Systems (Janert, 2013)
Questions and Discussion
crioux@netflix.com
@codyrioux
linkedin.com/in/codyrioux
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