Investigate slowness in Biome

Biome is intended to be fast. Very fast. And yet, sometimes it happens that we fail to live up to that promise. Most often, the cause is a specific file or a dependency that is causing woes. Or sometimes you accidentally forget to ignore a dist/ or a build/ folder. Whichever the cause, it can be rough to find out what is causing this slowness, so this guide is intended to help you figure it out.

First things first

Before going too deep, we can perform a few steps to see if our issue is caused by one of the following:

• If you have a dist/ or build/ folder with output files, or other such folders with minified files, please verify if ignoring those using both files.experimentalScannerIgnores as well as files.includes makes a difference.
• Do you have project rules enabled? They are known to cause performance overhead, in return for more advanced analysis. Also see our FAQ entry on this topic. See if disabling them makes a difference.
  • If disabling them does help, but you want to find out what specifically made it so slow, a first step can be to add node_modules to files.experimentalScannerIgnores. Did that help? Great! But you probably don’t want to keep it like this, because type information from dependencies won’t be available anymore. You can tweak files.experimentalScannerIgnores further to ignore a specific dependency instead of the entire node_modules folder, but you will probably want to read on to figure out which dependency to ignore…

If none of the above helped, or if they did help, but you want to dig deeper, we can use tracing to see if there is a specific file that might be the culprit…

Tracing

Since version 2.0, Biome has improved tracing capabilities to help with this investigation. Specifically, we’ll combine the following command-line arguments:

• Every Biome CLI command can be passed a --log-file=<path> argument, which will write all log messages for that invocation to the given path instead of stdout.
• The --log-level=<level> parameter accepts a tracing value. When --log-level=tracing is used, Biome also prints timing information from tracing spans to the log.
• Using --log-kind=json we can request Biome to write its logs in JSON format.

If we combine these three arguments, we can create a log file with JSON messages containing all relevant timing information for Biome. For instance:

biome lint --log-level=tracing --log-kind=json --log-file=tracing.json

This writes a tracing.json file, but it can contain a lot of data. So we’ll use jq to filter through this information.

For example, if you want to figure out which paths take the longest when building the module graph, you can use the following command:

cat tracing.json | jq 'select(.span.name == "update_module_graph_internal") | { path: .span.path, time_busy: .["time.busy"], time_idle: .["time.idle"] }' > filtered.json

Now you will have a file called filtered.json with all the relevant timings, together with the paths used during the invocations.

Similarly, if you want to figure out which files were slowest to analyse, you can use the following command:

cat tracing.json | jq '. | select(.span.name == "pull_diagnostics") | { path: .span.path, time_busy: .["time.busy"], time_idle: .["time.idle"] }' > filtered.json

Other span names that might be interesting to pull info from:

• format_file tells you how long files take to format.
• open_file_internal tells you how long files take to open, including parsing. It has a reason field that also can tell you why a file is opened: either by the scanner, updated by the watcher, or a client request, which is usually for linting or formatting. (since Biome 2.1.2)