An Evaluation of WebAssembly Pre-Initialization for Faster Startup Times

Abstract: WebAssembly (Wasm) has emerged as a new technology for the web that enables complex and interactive web applications, while utilizing a compact and platform-independent bytecode format. Due to its flexibility, portability, and built-in security, it has since evolved to be used in many other embeddings, such as internet-of-things, server applications, and even mobile applications. While a goal of Wasm is near-native performance, research has found that its performance is not as great as initially expected. Due to this, projects like The WebAssembly Pre-Initializer (Wizer) have emerged as potential solutions to this problem. Wizer is a tool developed by the coalition Bytecode Alliance with the purpose of speeding up the startup time, or Critical Path to Interactive (CPTI), of a Wasm module by pre-initializing it and saving a snapshot of the Wasm instance state into a new Wasm module. Wizer has been evaluated using two benchmark programs. However, no larger-scale investigation into the CPTI improvement brought by its pre-initialization has been conducted. Furthermore, saving a snapshot of the module is likely to result in a larger module in terms of file size, leading to increased compile time or, for use cases where it is relevant, network latency. This project investigates, mainly within the field of Wasm in non-web environments, the extent to which Wizer is able to improve CPTI for a Wasm module. The purpose of this is to allow both Wasm maintainers and developers to form an opinion whether pre-initialization could be standardized for use in Wasm compilers and toolchains, or whether pre-initialization should be applied to their Wasm module based on its CPTI before pre-initialization. Results are obtained by compiling a number of sample software down to Wasm, measuring their CPTI in terms of elapsed CPU cycles both without and with pre-initialization using Wizer, and comparing them. This is made possible through an extension to the Sightglass benchmarking framework also developed by Bytecode Alliance. The results show that pre-initialization using Wizer increases the CPTI if the Wasm module cannot be compiled to native CPU instructions in advance. However, if compilation can be done in advance, Wizer is able to reduce the CPTI of a Wasm module by a factor of between two to six times, depending on how it is initialized.