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.

Example:

Driving a Car

Photo Credit @dinkyhim

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