Monday, 6 November, 2017 UTC


JavaScript performance has always been important to the V8 team, and in this post we would like to discuss a new JavaScript Web Tooling Benchmark that we have been using recently to identify and fix some performance bottlenecks in V8. You may already be aware of V8’s strong commitment to Node.js and this benchmark extends that commitment by specifically running performance tests based on common developer tools built upon Node.js. The tools in the Web Tooling Benchmark are the same ones used by developers and designers today to build modern web sites and cloud-based applications. In continuation of our ongoing efforts to focus on real-world performance rather than artificial benchmarks, we created the benchmark using actual code that developers run every day.
The Web Tooling Benchmark suite was designed from the beginning to cover important developer tooling use cases for Node.js. Because the V8 team focuses on core JavaScript performance, we built the benchmark in a way that focuses on the JavaScript workloads and excludes measurement of Node.js-specific I/O or external interactions. This makes it possible to run the benchmark in Node.js, in all browsers, and in all major JavaScript engine shells, including ch (ChakraCore), d8 (V8), jsc (JavaScriptCore) and jsshell (SpiderMonkey). Even though the benchmark is not limited to Node.js, we are excited that the Node.js benchmarking working group is considering using the tooling benchmark as a standard for Node performance as well (nodejs/benchmarking#138).
The individual tests in the tooling benchmark cover a variety of tools that developers commonly use to build JavaScript-based applications, for example:
  • The Babel transpiler using the es2015 preset.
  • The parser used by Babel — named Babylon — running on several popular inputs (including the lodash and Preact bundles).
  • The acorn parser used by webpack.
  • The TypeScript compiler running on the typescript-angular example project from the TodoMVC project.
See the in-depth analysis for details on all the included tests.
Based on past experience with other benchmarks like Speedometer, where tests quickly become outdated as new versions of frameworks become available, we made sure it is straight-forward to update each of the tools in the benchmarks to more recent versions as they are released. By basing the benchmark suite on npm infrastructure, we can easily update it to ensure that it is always testing the state of the art in JavaScript development tools. Updating a test case is just a matter of bumping the version in the package.json manifest.
We created a tracking bug and a spreadsheet to contain all the relevant information that we have collected about V8’s performance on the new benchmark up to this point. Our investigations have already yielded some interesting results. For example, we discovered that V8 was often hitting the slow path for instanceof (v8:6971), incurring a 3–4× slowdown. We also found and fixed performance bottlenecks in certain cases of property assignments of the form of obj[name] = val where obj was created via Object.create(null). In these cases, V8 would fall off the fast-path despite being able to utilize the fact that obj has a null prototype (v8:6985). These and other discoveries made with the help of this benchmark improve V8, not only in Node.js, but also in Chrome.
We not only looked into making V8 faster, but also fixed and upstreamed performance bugs in the benchmark’s tools and libraries whenever we found them. For example, we discovered a number of performance bugs in Babel where code patterns like
value = items[items.length - 1];
lead to accesses of the property "-1", because the code didn’t check whether items is empty beforehand. This code pattern causes V8 to go through a slow-path due to the "-1" lookup, even though a slightly modified, equivalent version of the JavaScript is much faster. We helped to fix these issues in Babel (babel/babel#6582, babel/babel#6581 and babel/babel#6580). We also discovered and fixed a bug where Babel would access beyond the length of a string (babel/babel#6589), which triggered another slow-path in V8. Additionally we optimized out-of-bounds reads of arrays and strings in V8. We’re looking forward to continue working with the community on improving the performance of this important use case, not only when run on top of V8, but also when run on other JavaScript engines like ChakraCore.
Our strong focus on real-world performance and especially on improving popular Node.js workloads is shown by the constant improvements in V8’s score on the benchmark over the last couple of releases:

Since V8 5.8, which is the last V8 release before switching to the Ignition+TurboFan architecture, V8’s score on the tooling benchmark has improved by around 60%.
Over the last several years, the V8 team has come to recognize that no one JavaScript benchmark — even a well-intentioned, carefully crafted one — should be used as a single proxy for a JavaScript engine’s overall performance. However, we do believe that the new Web Tooling Benchmark highlights areas of JavaScript performance that are worth focusing on. Despite the name and the initial motivation, we have found that the Web Tooling Benchmark suite is not only representative of tooling workloads, but is representative of a large range of more sophisticated JavaScript applications that are not tested well by front end-focused benchmarks like Speedometer. It is by no means a replacement for Speedometer, but rather a complementary set of tests.
The best news of all is that given how the Web Tooling Benchmark is constructed around real workloads, we expect that our recent improvements in benchmark scores will translate directly into improved developer productivity through less time waiting for things to build. Many of these improvements are already available in Node.js: at the time of writing, Node 8 LTS is at V8 6.1 and Node 9 is at V8 6.2.
The latest version of the benchmark is hosted at
Benedikt Meurer, @bmeurer, JavaScript Performance Juggler