Using Linux Perf

Linux Perf provides low level CPU profiling with JavaScript, native and OS level frames.

Important: this tutorial is only available on Linux.

How To

Linux Perf is usually available through the linux-tools-common package. Through either --perf-basic-prof or --perf-basic-prof-only-functions we are able to start a Node.js application supporting perf_events.

--perf-basic-prof will always write to a file (/tmp/, which can lead to infinite disk growth. If that’s a concern either use the module: linux-perf or --perf-basic-prof-only-functions.

The main difference between both is that --perf-basic-prof-only-functions produces less output, it is a viable option for production profiling.

# Launch the application an get the PID
$ node --perf-basic-prof-only-functions index.js &
[1] 3870

Then record events based in the desired frequency:

$ sudo perf record -F 99 -p 3870 -g

In this phase, you may want to use a load test in the application in order to generate more records for a reliable analysis. When the job is done, close the perf process by sending a SIGINT (Ctrl-C) to the command.

The perf will generate a file inside the /tmp folder, usually called /tmp/ (in above example: /tmp/ containing the traces for each function called.

To aggregate those results in a specific file execute:

$ sudo perf script > perfs.out

The raw output can be a bit hard to understand so typically the raw file is used to generate flamegraphs for a better visualization.

Example nodejs flamegraph

To generate a flamegraph from this result, follow this tutorial from step 6.

Because perf output is not a Node.js specific tool, it might have issues with how JavaScript code is optimized in Node.js. See perf output issues for a further reference.

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