A Meta Lesson

March 21, 2019

Two giants of AI, Rich Sutton and Rodney Brookes, have recently written essays ostensibly presenting two opposing viewpoints for the future of AI research. I highly recommend reading A Bitter Lesson and the follow up A Better Lesson. Both authors pride themselves on elegant brevity, which is admirable. But both viewpoints are so rich they really need further elaboration. So this attempt at synthesis will be much more verbose but hopefully edifying. Rich Sutton the sage has delivered his teachings: it’s traditional for us junior monks researchers to provide the accompanying exposition and analysis.

First a bit of background: Rich Sutton, as far as I can tell from studying his work and hearing him speak, believes in a kind of reward-minimalism. This is the idea that everything can and should be learned from a reward signal alone. Give me a world, tell me what is better and what is worse and I will give you an agent. He seems to have a critical view of any system that tries to include any innate skill or ability. For example, why build in SLAM (Simultaneous Location and Mapping)? If the agent needs to locate itself to get a high reward, it will learn to locate. If it’s unnecessary — it won’t. If there’s a better way to get where it needs to go, it will eventually learn that instead of making a map. Why should we corrupt our agents with our own limited ideas about how to solve a problem?

Many reward-minimalists believe that some kind of meta-learning will take place if a sufficiently intelligent agent interacts with a complex environment. I think Rich has a rich and nuanced view on this, which I don’t fully comprehend. I tried to ask him and David Silver at a NeurIPS panel last year but the question came out all garbled because I was nervous and jet-lagged. Either way, it’s intuitive that an agent shouldn’t just learn how to increase its reward. In a sufficiently rich environment it should improve its methods of learning, e.g. develop curiosity, reasoning etc. This is also related to recursive self-improvement, the idea that an agent could learn to modify itself and that the subsequent better version could do that again, improving each time.

Rodney Brookes, on the other hand, has long had a chip on his shoulder about the amount of compute needed to do AI. He pioneered building robots with ingenious techniques that use many individually simple rules that could interact and override each other, called a subsumption architecture. The emergent behaviour of these interacting simple rules could robustly solve surprisingly complex problems with very low computational power and primitive sensors. He expressed his contrarian approach in the title of his book “Fast, Cheap and Out of Control”. He created the iRobot company and the Roomba vacuum cleaner along these principles — and it worked.

It is therefore not surprising that Sutton and Brooks have butted heads on the role of increased computation power for the development of AI. Although I was surprised at Brooks for his polemic tone and some mischaracterisations of Sutton’s arguments. I think that Sutton and Brooks both take clear positions with strong examples, my main contention would be that Brooks discounts using learning and search to improve learning itself and so misses the deeper implications of Sutton’s position.

I’m going to try and summarise both their positions in a few sentences, but you should definitely read both essays, especially as they are so short.

Rich Sutton (approx.): learning and search always outperform hand-crafted solutions given enough compute.

Rodney Brooks (approx.): No, human ingenuity is actually responsible for progress in AI. We can’t just solve problems by throwing more compute at them.

I think both positions are interesting, important and well supported by evidence. But if you read both essays, you’ll see that these positions are also not mutually exclusive, in fact they can be synthesised. But to accept this interpretation you need to take your view one level ‘up’, so to speak. I’ll illustrate this with a quote from Rodney Brooks:

One of the most celebrated successes of Deep Learning is image labelling, using CNNs, Convolutional Neural Networks, but the very essence of CNNs is that the front end of the network is deigned by humans to manage translational invariance, the idea that objects can appear anywhere in the frame. To have a Deep Learning network also have to learn that seems pedantic to the extreme, and will drive up the computational costs of the learning by many orders of magnitude.

All true. But there is a critical omission: translational invariance isn’t necessarily the only good way to solve vision problems. We don’t want an AI algorithm that will exactly relearn to use convolutions. We want a computer vision system that can learn something even better than convolutions. Maybe some other more efficient sparse connectivity pattern or even an entirely different way to do scene parsing. This is critical, moving past simple feature engineering opened up a whole new world for computer vision. The choice of convolutions is a hand-crafted inductive bias — a choice that will eventually need to yield to learning as well. But you see what happened, in the past we hand-crafted features, now we hand-craft architectures (A point Brooks himself makes) this is exactly what Rich Sutton is critical of; we should be always seeking to do more with learning and search. Indeed, we are using learning and search to improve learning itself, meta-learning and architecture search are already an important and growing area of research. Architecture search is relatively unsophisticated at the moment, but it will hopefully give us something better than just throwing countless PhD students at the problem (an algorithm I call… graduate descent)

Brooks goes on to criticise the need for massive data sets in Deep Learning, this is indeed a fair criticism but totally mischaracterises Sutton; he doesn’t even use the word ‘data’ once in his essay. A robot that could learn to recognise objects from interaction with the world alone is certainly one of their shared goals. Even then, the best one-shot and few-shot learning systems that exist today either make use of meta-learning or learned vector representations. Just to reiterate, the most sample efficient systems that exist today are learned.

Brooks writes about the need for computational efficiency, citing the brain using only 20W. I think this point avoids unpacking compute used in training vs deployment. For example, accurate LSTM speech recognition models use very little compute and can run effectively on mobile phones. Training these models may take a very large amount of compute, optimising their hyperparameters even more. But the deployed model is frugal. Isn’t the total cost of this system when deployed on millions of phones still very good? Google’s recent AlphaStar model runs on a single GPU when it’s actually playing. Sure, training AlphaStar took a huge amount of power — improve that!

Rich Sutton isn’t arguing for wasteful learning and search, he’s calling on us to improve it. He is saying we’ll never be able to go back to hand-written StarCraft bots. Building a StarCraft bot that can rival AlphaStar with only a few hours of play would be a huge achievement for AI, but that would mean better learning and search, exactly what Sutton advocates.

If you train an LSTM with reinforcement learning to solve maze after maze it will learn complex navigation behaviour, backtracking to take past turns etc. Incredibly, it learns to learn the layout of each individual maze problem. Does it do SLAM? Does it ‘know’ where it is? Does it use Bayesian inference? It does do whatever it does, with a surprisingly small amount of compute after training. Learning-to-learn can yield systems that are more efficient than anything we can program. Meta-learning synthesises the need for efficient learning and moving away from ad-hoc hand-crafting.

Let’s elaborate on something from Rich Sutton now:

A similar pattern of research progress was seen in computer Go, only delayed by a further 20 years. Enormous initial efforts went into avoiding search by taking advantage of human knowledge, or of the special features of the game, but all those efforts proved irrelevant, or worse, once search was applied effectively at scale. Also important was the use of learning by self play to learn a value function (as it was in many other games and even in chess, although learning did not play a big role in the 1997 program that first beat a world champion). Learning by self play, and learning in general, is like search in that it enables massive computation to be brought to bear. Search and learning are the two most important classes of techniques for utilizing massive amounts of computation in AI research. In computer Go, as in computer chess, researchers’ initial effort was directed towards utilizing human understanding (so that less search was needed) and only much later was much greater success had by embracing search and learning.

Sutton is referencing the success of AlphaGo and AlphaZero here. I think this example is illustrative. AlphaZero has very little Go, Chess or Shogi specific built into it, aside from being quite amazing, it puts some of Brooks’ criticisms into focus. It is true that AlphaGo uses a lot of compute to train, but how many thousands of hours were poured into the Stockfish position evaluation function? How many more would be needed to beat AlphaZero? Yes, much research went into AlphaGo itself, but adding Chess and Shogi after Go took a relative small team less than a year, how long would it take to port Stockfish to Shogi? Furthermore, how long would Stockfish need to grind on a move to match the skill of AlphaZero? Once trained, AlphaZero’s play is more efficient than Stockfish for the same skill level.

I think that Sutton could have been more clear that he does not think more computation is a panacea in-and-of-itself, but that learning and search are naturally and passively improved by increasing computation. He could have emphasised that improving search and learning by increasing the efficiency is an important goal too. Brooks was too quick to attack only the compute related portions of Sutton’s essay, what he missed was that learning and search will ultimately improve computational efficiency.

The meta lesson is that the most important thing to improve with search and learning — is learning itself. Building an agent with real-world recursive self-improvement will require all the compute and ingenuity we can muster.