Not So Fast: Analyzing the Performance of WebAssembly vs. Native Code

TL;DR

This paper introduces Browsix-Wasm to run unmodified WebAssembly Unix applications in browsers and provides a large-scale performance comparison showing WebAssembly is significantly slower than native code, with 45-55% average slowdown.

Contribution

The paper presents Browsix-Wasm, enabling large-scale evaluation of WebAssembly performance with unmodified Unix applications in browsers, revealing substantial performance gaps.

Findings

WebAssembly runs 45-55% slower than native code on SPEC CPU benchmarks.

Peak slowdowns reach up to 2.5x in Chrome and Firefox.

Performance issues stem from missing optimizations and inherent platform limitations.

Abstract

All major web browsers now support WebAssembly, a low-level bytecode intended to serve as a compilation target for code written in languages like C and C++. A key goal of WebAssembly is performance parity with native code; previous work reports near parity, with many applications compiled to WebAssembly running on average 10% slower than native code. However, this evaluation was limited to a suite of scientific kernels, each consisting of roughly 100 lines of code. Running more substantial applications was not possible because compiling code to WebAssembly is only part of the puzzle: standard Unix APIs are not available in the web browser environment. To address this challenge, we build Browsix-Wasm, a significant extension to Browsix that, for the first time, makes it possible to run unmodified WebAssembly-compiled Unix applications directly inside the browser. We then use Browsix-Wasm to conduct the first large-scale evaluation of the performance of WebAssembly vs. native. Across the SPEC CPU suite of benchmarks, we find a substantial performance gap: applications compiled to WebAssembly run slower by an average of 45% (Firefox) to 55% (Chrome), with peak slowdowns of 2.08x (Firefox) and 2.5x (Chrome). We identify the causes of this performance degradation, some of which are due to missing optimizations and code generation issues, while others are inherent to the WebAssembly platform.