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    πŸ”₯ single-command flamegraph profiling πŸ”₯

    Discover the bottlenecks and hot paths in your code, with flamegraphs.

    Visualize Stack Traces

    0x can profile and generate an interactive flamegraph for a Node process with a single command, on any platform which Node runs on (macOs, Linux, Windows, Android...).


    • Node v12.x and above
    • Default usage supports any Operating System that Node runs on!
    • Chrome
      • Other browsers may open flamegraphs in a degraded, but functional form


    An example interactive flamegraph can be viewed at http://davidmarkclements.github.io/0x-demo/


    npm install -g 0x


    Use 0x to run a script:

    0x my-app.js

    Immediately open the flamegraph in the browser:

    0x -o my-app.js

    Automatically execute profiling command against the first port opened by profiled process:

    0x -P 'autocannon localhost:$PORT' my-app.js

    Use a custom node executable:

    0x -- /path/to/node my-app.js

    Pass custom arguments to node:

    0x -- node --zero-fill-buffers my-app.js


    When ready to generate a flamegraph, send a SIGINT or a SIGTERM.

    The simplest way to do this is pressing CTRL+C.

    When 0x catches the SIGINT or the SIGTERM, it process the stacks and generates a profile folder (<pid>.0x), containing flamegraph.html.

    The UI

    The flamegraph.html file contains the 0x UI, which is explained in docs/ui.md.

    Production Servers

    A lightweight, production server friendly, approach to generating a flamegraph is described in docs/production-servers.md.

    The Profile Folder

    By default, a Profile Folder will be created and named after the PID, e.g. 3866.0x (we can set this name manually using the --output-dir flag).

    The Profile Folder is explained in more detail in docs/profile-folder.md


    Clone this repo, run npm i -g and from the repo root run

    0x examples/rest-api

    In another tab run

    npm run stress-rest-example

    To put some load on the rest server, once that's done use ctrl + c to kill the server.

    Command Line API

    --help | -h

    Print usage info.

    --open | -o

    Open the flamegraph in the browser using open or xdg-open (see https://www.npmjs.com/package/open for details).

    --on-port | -P

    Run a given command and then generate the flamegraph. The command as specified has access to a $PORT variable. The $PORT variable is set according to the first port that profiled process opens.

    For instance, here's an example of using autocannon to load-test the process:

    0x -P 'autocannon localhost:$PORT' app.js

    When the load-test completes, the profiled processed will be sent a SIGINT and the flamegraph will be automatically generated.

    Remember to use single quotes to avoid bash interpolation, or else escape variable (e.g. 0x -P "autocannon localhost:$PORT" app.js won't work wheras 0x -P "autocannon localhost:\$PORT" app.js will).

    Note: On Windows interpolation usually occurs with %PORT%, however in this case the dollar-prefix $PORT is the correct syntax (because the interpolation is not shell based).

    Default: ''


    The name of the HTML file, without the .html extension Can be set to - to write HTML to STDOUT (note due to the nature of CLI argument parsing, this must be set using =, e.g. --name=-).

    If either this flag or --output-html-file is set to - then the HTML will go to STDOUT.

    Default: flamegraph


    Set the title to display in the flamegraph UI.

    Default: the command that 0x ran to start the process

    --output-dir | -D

    Specify artifact output directory. This can be specified in template form with possible variables being {pid}, {timestamp}, {name} (based on the --name flag) and {outputDir}(variables must be specified without whitespace, e.g. { pid } is not supported).

    Default: {pid}.0x

    --output-html | -F

    Specify destination of the generated flamegraph HTML file. This can be specified in template form with possible variables being {pid}, {timestamp}, {name} (based on the --name flag) and {outputDir} (variables must be specified without whitespace, e.g. { pid } is not supported). It can also be set to - to send the HTML output to STDOUT (note due to the nature of CLI argument parsing, this must be set using =, e.g. --output-html=-).

    If either this flag or --name is set to - then the HTML will go to STDOUT.

    Default: {outputDir}/{name}.html


    Use an OS kernel tracing tool (perf on Linux). This will capture native stack frames (C++ modules and Libuv I/O), but may result in missing stacks from Node.js due to the optimizing compiler.

    See docs/kernel-tracing.md for more information.

    Default: false

    --quiet | -q

    Limit output, the only output will be fatal errors or the path to the flamegraph.html upon successful generation.

    Default: false

    --silent | -s

    Suppress all output, except fatal errors.

    Default: false


    Don't generate the flamegraph, only create the Profile Folder, with relevant outputs.

    Default: false


    Delay the collection of stacks by a specified time(ms) relative to the first entry.

    Default: 0


    Supply a path to a profile folder to build or rebuild visualization from original stacks.

    Default: undefined


    Supply a path to a CPU profile (.cpuprofile). See examples/cpu-profile for examples.

    CPU Profile output does not have as much information but it can be exported from Chrome Devtools in the browser. There's also an automated headless tool for doing so: automated-chrome-profiling. For creating Node.js Cpu Profiles in Node see v8-profiler or v8-profiler-next. They can also be generated from Node.js 12 and above using the command-line flag --cpu-prof.

    Default: undefined


    Show output from perf(1) tools.

    Default: false


    Save the intermediate tree representation of captured trace output to a JSON file.

    Default: false

    Programmatic API

    0x can also be required as a Node module and scripted:

    const zeroEks = require('0x')
    const path = require('path')
    async function capture () {
      const opts = {
        argv: [path.join(__dirname, 'my-app.js'), '--my-flag', '"value for my flag"'],
        workingDir: __dirname
      try {
        const file = await zeroEks(opts)
        console.log(`flamegraph in ${file}`)
      } catch (e) {

    The Programmatic API is detailed in docs/api.md.


    Memory Issues

    Very complex applications with lots of stacks may hit memory issues.

    The --stack-size flag can be used to set the memory to the maximum 8GB in order to work around this when profiling:

    node --stack-size=8024 $(which 0x) my-app.js

    There may still be a problem opening the flamegraph in Chrome. The same work around can be used by opening Chrome from the command line (platform dependent) and nesting the --stack-size flag within the --js-flags flag: --js-flags="--stack-size 8024".


    DEBUG=0x* 0x my-app.js



    Sponsored by nearForm

    This tool is inspired from various info and code sources and would have taken much longer without the following people and their Open Source/Info Sharing efforts:






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    • matteo.collina
    • mafintosh
    • davidmarkclements