1. Feedback Control for System
Tuning
Cody Rioux - @codyrioux
Real-Time Analytics - Insight Engineering

2. Overview
● Feedback Control
○ Definition
○ Conceptual Model
● Concepts
○ Math
○ Rules of Thumb
● Case Study
○ Controlling Chris’ Effective Hours
● Recap

3. Controlling dynamic systems through continuous feedback.
Feedback Control

4. What is Feedback Control?
A form of Process Control for
controlling the behavior of black
box systems.

5. Example:
Driving a Car
Photo Credit @dinkyhim

6. Cody Engine
Accelerator Speed
Whats a
setpoint?
If you drive like me...
No regard for the speed limit, you’re just a feed-forward automaton.

7. You (Driver) Engine (V8)
Accelerator Speed
Speedometer
Speed Limit
Driving A Car

8. Cruise Control Engine (V8)
Accelerator Speed
Speed Reading
Speed Limit
What if we got a machine to do it?

9. System
Input Output
A Conceptual Model

10. Controller System
Input OutputSetpoint
A Conceptual Model
This happens to be a feed-forward system.

11. Controller System
Input Output
Feedback / Error
Setpoint
A Conceptual Model

12. Controller System
Input Output
Feedback / Error
Setpoint
A Conceptual Model

13. Controller System
Input Output
Feedback / Error
Setpoint
A Conceptual Model

14. Controller System
Input Output
Feedback / Error
Setpoint
A Conceptual Model

15. Controller System
Input Output
Feedback / Error
Setpoint
A Conceptual Model

16. Controller System
Input Output
Feedback / Error
Setpoint
A Conceptual Model

17. 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.

18. You’ll need an advanced math degree for these...
Concepts

19. Controller System
Input Output
Feedback / Error
Setpoint
A Conceptual Model

20. error = setpoint - output

21. input = gain * error

22. Things to keep in mind...

23. Gain controls the
magnitude of our
adjustments. This is
important.

25. Univariate feedback is
inherently simple,
multivariate feedback
inherently complex.

26. Prefer small and
frequent adjustments to
large infrequent
adjustments.

28. Oscillation creates
instability. We don’t like
that.

30. End-to-end control system for maximum analytics productivity.
Case Study: Controlling Chris’
Caffeine Intake

35. What if this were...
● … a timeout value?
● … a queue size?
● … a server cluster size?

36. We can keep Chris rolling at 8 hours a day 365 if we wanted to.
Recap

37. 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

38. Feedback control is a viable
solution for configuring
systems under dynamic
conditions.

39. Only Scratching the Surface
● Integral Controllers
● Derivative Controllers
● PID Controllers
● Statistical Process Control

40. Literature
Feedback Control for Computer
Systems (Janert, 2013)

41. Questions and Discussion
crioux@netflix.com
@codyrioux
linkedin.com/in/codyrioux