Sarah Dean PRO
asst prof in CS at Cornell
CS 4/5789: Introduction to Reinforcement Learning
Lecture 26
Prof. Sarah Dean
MW 2:45-4pm
110 Hollister Hall
Agenda
0. Announcements & Recap
1. Real World RL
2. Specification and Risks
3. Does RL Work?
Announcements
5789 Paper Review Assignment (weekly pace suggested)
HW 4 due 5/9 -- don't plan on extentions
Final exam Monday 5/16 at 7pm
Review session in lecture 5/9
Course evaluations open tomorrow
Recap: IL vs. IRL
Supervised Learning
Policy
Dataset of expert trajectory
...
\(\pi\)( ) =
\((x=s, y=a^*)\)
imitation
inverse RL
Goal: understand/predict behaviors
Recap: Max-Ent IRL
- For \(k=0,\dots,K-1\):
- \(\pi^k = \mathsf{SoftVI}(w_k^\top \varphi)\)
- \(w_{k+1} = w_k + \eta (\mathbb E_{d^{\pi^*}_\mu}[\varphi (s,a)] - \mathbb E_{d^{\pi^k}_\mu}[\varphi (s,a)])\)
- Return \(\bar \pi = \mathsf{Unif}(\pi^0,\dots \pi^{K-1})\)
- Input: reward function \(r\). Initialize \(V_H^*(s) = 0\)
- For \(h=H-1,\dots 0\):
- \(Q_h^*(s,a) = r(s,a) + \mathbb E_{s'\sim P}[V_{h+1}(s')]\)
- \(\pi_h^*(a|s) \propto \exp(Q^*_h(s,a))\)
- \(V_h^*(s) = \log\left(\sum_{a\in\mathcal A} \exp(Q^*_h(s,a) \right)\)
Soft-VI
Agenda
0. Announcements & Recap
1. Real World RL
2. Specification and Risks
3. Does RL Work?
Real World RL
AlphaGo vs. Lee Sedol, 2016
Real World RL
Real World RL
Real World RL
RL for Amazon Ads, 2022
Real World RL
Agenda
0. Announcements & Recap
1. Real World RL
2. Specification and Risks
3. Does RL Work?
RL Specification
Markov decision process \(\mathcal M = \{\mathcal S, ~\mathcal A, ~P, ~r, ~\gamma\}\)
\(s_t\)
\(r_t\)
\(a_t\)
\(\pi\)
\(\gamma\)
\(P\)
- action space and discount known
- states and reward signals observed
- transition probabilities unknown
actions & states determine environment
discount & reward determine objective
Specifying Horizon/Discount
Large discount factor leads to short-sighted agent
- \(0\) cost for \(a_0\)
- \(2\epsilon\) cost for \(a_1\)
- \(\epsilon\) reward in \(s_0\)
- \(1\) reward in \(s_1\)
\(V^{a_0}(s_0) = \frac{\epsilon}{1-\gamma}\) and \(V^{a_1}(s_0) = \frac{1}{1-\gamma} - \frac{2\epsilon}{\gamma}\)
Specifying Reward
The promise of RL:
translate specified objective into desired behavior
The reality:
Risk: Reward Hacking
Risk: Reward Hacking
Risk: Reward Hacking
While everyone seemed focused on how many views a video got, we thought the amount of time someone spent watching a video was a better way to understand whether a viewer really enjoyed it."
Youtube in 2014 vs. 2018
Risk: Reward Hacking
Facebook's "Meaningful Social Interaction" metric
Misinformation, toxicity, and violent content are inordinately prevalent among reshares"
Reward Design
Inverse Reward Design (NeuRIPS, 2017)
Idea: treat specified reward as imperfect proxy
Then attempt to learn true reward from other feedback
Specifying States & Actions
The interface through which the agent sees and impacts the world
Also delimits reasoning about the world
\(s_t\)
\(a_t\)
Risk: Too Much Information
Evolving an oscillator on hardware (Bird & Layzell, 2002)
Result: a "network of transistors sensing and utilising the radio waves emanating from nearby PCs"
Risk: Too Little Information
The first Tesla autopilot fatality in 2016
Safety systems failed to detect white truck against bright sky
"vehicles [...] will no longer be equipped with radar. Instead, these will [...] rely on camera vision and neural net processing." (Tesla, 2021)
Risk: Inappropriate Actuation
Learning to influence other drivers
Excessive caution around other drivers
Excessive aggression
Example adapted from Anca Dragan
Agenda
0. Announcements & Recap
1. Real World RL
2. Specification and Risks
3. Does RL Work?
Does RL Work?
1. Model-based design and optimization works better
Three strikes against RL:
ex - Model Predictive Control at Boston Dynamics
Does RL Work?
1. Model-based design and optimization works better
Three strikes against RL:
data-driven optimization suffers from local minima, large sample complexity (Deep RL doesn't work yet, 2018)
Does RL Work?
2. Simulation essentially necessary, but huge sim2real gap
Three strikes against RL:
RL exploits bugs in simulator code (Nathan Lambert, 2021)
Does RL Work?
3. Questionable evaluation practices
Three strikes against RL:
State-of-the-art algorithms outperformed by simple baselines: Simple random search provides a competitive approach
to reinforcement learning, 2017
Generality?
This perspective ignores the instance-specific tuning that often goes into making RL algorithms work
"Machine learning has become alchemy" Ali Rahimi & Ben Recht, 2017
King Midas cursed by Dionysus
When Silicon Valley tries to imagine superintelligence, what it comes up with is no-holds-barred capitalism.
Ted Chiang, 2018.
I think many AV teams could handle a pogo stick user in pedestrian crosswalk. Having said that, bouncing on a pogo stick in the middle of a highway would be really dangerous. Rather than building AI to solve the pogo stick problem, we should partner with the government to ask people to be lawful and considerate. Safety isn’t just about the quality of the AI technology.
- Andrew Ng, 2018
All ML is RL once deployed
ex - credit-score designed within supervised learning framework, but used to make lending decisions
\(\{x_i, y_i\}\)
\(x\)
\(\widehat y\)
\((x, y)\)
When a measure becomes a target, it ceases to be a good measure"
Goodhardt's law
ML and social dynamics
When a measure becomes a target, it ceases to be a good measure"
Goodhardt's law
ML and social dynamics
Buzzfeed noticed the success of content that exploited racial divisions, fad/junky science, extremely disturbing news and gross images.
ML and social dynamics
Some political parties in Europe told Facebook the algorithm had made them shift their policy positions so they resonated more on the platform, according to the documents."
Technologies are developed and used within a particular social, economic, and political context. They arise out of a social structure, they are grafted on to it, and they may reinforce it or destroy it, often in ways that are neither foreseen nor foreseeable.”
Ursula Franklin, 1989
Exo-Feedback
control feedback
data feedback
external feedback
"...social, economic, and political context..."
"...neither foreseen nor forseeable..."
Recap
1. Real World RL
2. Specification and Risks
3. Does RL Work?
Upcoming
1. AlphaGo case study
2. Review for final
CS 4/5789: Lecture 26
By Sarah Dean
Private
