Last week, Google DeepMind published a paper in the journal Nature in which they claimed to have found a more efficient sorting algorithm by using Deep Reinforcement Learning (DLR). DeepMind is known for pushing the boundaries in Reinforcement Learning (RL). A few years ago, they beat the best player in the game of Go with their agent AlphaGo, after using a similar program to crush existing chess engines. In 2022 DeepMind introduced AlphaTensor, a program that found a more efficient matrix multiplication algorithm with DLR. The techniques used are similar to DeepMind’s newest achievement: improving a standard sorting algorithm. In this article, I will discuss how they achieved this by introducing Reinforcement Learning, Monte Carlo Tree Search, and the details of DeepMind’s approach and agent AlphaDev.
Writing a sorting algorithm is simple: compare all values one by one to find the lowest (or highest) value, save this value at the first element of your return list, and continue to the next value until no more value is left. Although this algorithm will work, it is far from efficient. Because sorting is very common in computer programs, efficient sorting algorithms are subject to intensive research. DeepMind focused on two variants of sorting: Fixed sorting and variable sorting. Fixed sorting involves sorting a sequence of values with a fixed, predefined length. In variable sorting, the size of the list of values is not defined in advance. Only the maximum size of the list is provided. Both variants of sorting are used extensively in state-of-the-art sorting algorithms. Often, a large list is sorted by repeatedly sorting small sections of the list.
Current state-of-the-art algorithms use so-called sorting networks in both fixed and variable sorting. In this article, I will not discuss the details of sorting networks. You can find more information here.
In this section, I will introduce the main concepts of Reinforcement Learning. Reinforcement Learning is a branch of Machine Learning, in which an agent is tasked with finding the best actions in an environment, given the current state. The state of an environment contains all relevant aspects of an environment that can or should influence the action to be taken. The best action is defined as the action that maximizes the (discounted) cumulative reward. Actions are taken sequentially, and after each action, the obtained reward and the new state are recorded. Usually, an environment has some termination criteria after which the next episode starts. Early versions of RL used tables to keep track of the value of certain states and actions with the idea of always taking the action with the highest value. Deep neural networks often replace these tables because they can generalize better and enumerating all states is often impossible. When deep neural networks are used for Reinforcement Learning, the term Deep Reinforcement Learning is used.
Monte Carlo Tree Search
AlphaDev is trained using Deep Reinforcement Learning and Monte Carlo Tree Search, which is a search algorithm to find the best action given some initial state. It is often combined with DRL, for instance in AlphaZero and AlphaGo. It deserves its own article, but a summary is provided here.
Monte Carlo Tree Search (MCTS) builds a tree of possible outcome states where the current state is the root node. For a given number of simulations, this tree will be explored to find the consequences of taking certain actions. In each simulation, a rollout (sometimes called playout) is used to expand one node into child nodes if the game is not terminated at that point. Which action to take is based on selection criteria. In our case, the probability of selecting an action is given by the policy network which will be discussed below. The value of a node is a measure of how good the state of that node is. It is determined by the value network, which is also discussed in a future section. If a terminal state is reached, the value is replaced by the true reward value of the environment.
The values are propagated up the search tree. In this way, the value of the current state depends on the value of the states that can be reached from the current state.
The DRL agent of DeepMind that is trained to improve the sorting algorithm implementation is called AlphaDev. The task of AlphaDev is to write a more efficient sorting algorithm in Assembly. Assembly is the bridge between high-level languages such as C++, Java, and Python and machine code. If you use sort in C++, the compiler compiles your program into assembly code. The assembler then converts this code to machine code. AlphaDev is tasked with selecting a series of assembly instructions to create a sorting algorithm that is both correct and fast. This is a difficult task because adding one instruction can make the program completely wrong, even if it was correct before.
AlphaDev finds an efficient sorting algorithm by creating and solving the AssemblyGame. In each turn, it has to select one action corresponding to a CPU instruction. By formulating this problem as a game, it can easily fit the standard Reinforcement Learning framework.
Reinforcement Learning Formulation
A standard RL formulation contains states, actions, rewards, and a termination criterium.
The state of the AssemblyGame consists of two parts. First, a representation of the current program. AlphaDev uses the output of a Transformer, a neural network architecture, to represent the current algorithm. Transformers have had much success with large language models recently and are a suitable choice for encoding the current algorithm because it is text-based. The second part of the state is a representation of the state of the memory and registers after running the current algorithm. This is done by a conventional deep neural network.
The actions of the AssemblyGame are appending a legal assembly instruction to the program. AlphaDev can choose to append any instruction as long as it is legal. DeepMind created the following six rules for legal actions:
- Memory locations are always read in incremental order
- Registers are allocated in incremental order
- Read and write to each memory location once
- No consecutive compare instructions
- No comparison or conditional moves to memory
- Non-initialized registers cannot be used
The last two actions are illegal from a programming standpoint, the others are enforced because they will not change the program. By removing these actions the search space is limited without affecting the algorithm.
The reward of the AssemblyGame is a combination of two parts. The first element of the reward is a correctness score. Based on a series of test sequences, the provided algorithm is evaluated. The closer the algorithm gets to properly sorting the test sequence, the higher the reward for correctness. The second element of the reward is the speed or latency of the program. This is measured by the number of lines of the program, if there are no conditional branches. In practice, this means that the length of the program is used for fixed sorting because no conditions are required for that program. For variable sorting, the algorithm has to condition on the actual length of the sequence. Because not all parts of the algorithm are used when sorting, the actual latency of the program is measured instead of the number of lines.
Termination and Goal
According to the paper released by DeepMind, they terminate the AssemblyGame after “a limited number of steps”. What this exactly means is unclear, but I assume they limit the number of steps based on the current human benchmark. The goal of the game is to find a correct and fast algorithm. If either an incorrect or a slow algorithm is provided by AlphaDev, the game is lost.
Policy and Value Networks
To use Monte Carlo Tree Search a policy and a value network are required. The policy network sets the probability for each action and the value network is trained to evaluate a state. The policy network is updated based on the number of visits of a certain state. This creates an iterative procedure, where the policy is used to perform MCTS and the policy network is updated with the number of visits of a state in MCTS.
The value network outputs two values, one for the correctness of the algorithm and one for the latency of the algorithm. According to DeepMind, this yields better results than combining these values in one score by the network. The value network is updated based on the obtained rewards.
After training AlphaDev, it found shorter algorithms for fixed sorting with sequence lengths 3 and 5. For fixed sorting with length 4, it found the current implementation, so no improvement was made there. AlphaDev achieved these results for fixed sorting by applying two new ideas. These new ideas, called the swap and the copy move, result in fewer comparisons between values and therefore a faster algorithm. For fixed sorting with length 3, DeepMind proved that no shorter program exists by brute forcing through all options with a shorter program length. For longer programs, this approach is infeasible, as the search space grows exponentially.
For variable sorting, AlphaDev came up with a new design of the algorithm. For instance for variable sorting a sequence with a length at most 4, AlphaDev suggested to first sort 3 values and then perform a simplified version of sort for the last remaining element. The figure below shows the algorithm design for Varsort(4) given by AlphaDev.
Overall, faster algorithms were found for fixed and variable sorting, proving that DRL can be used to implement efficient algorithms. The sorting implementation in C++ has been updated to use these new algorithms. Details on the performance gains achieved by AlphaDev can be found in the table below.
To test if this approach generalizes to other programming implementations, DeepMind also tested AlphaDev on a competitive coding challenge and a protocol buffer used by Google. In both cases, AlphaDev was able to come up with more efficient implementations, proving that DLR with Monte Carlo Tree Search is a valid approach to finding efficient implementations for common algorithms.
Deep Reinforcement Learning has shown to be successful in many different environments, from games like Go and Chess to algorithm implementation as seen here. Although the domain is challenging, often dependent on low-level implementation details, and hyperparameter choices, these successes show the results that can be achieved by this powerful concept.
I hope this article gave you an insight into the latest achievement in RL. If you want a more detailed explanation of Monte Carlo Tree Search or other algorithms that I discussed, please let me know!
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