node package manager


ūüĒ• single-command flamegraph profiling ūüĒ•


ūüĒ• single-command flamegraph profiling ūüĒ•

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 in a single command, on both Linux and OS X. Whilst this seems trivial... it's not. Well it wasn't before 0x.


An example interactive flamegraph can be viewed at

This demo shows a terminal preview, the default behaviour is not to show a preview, you can use -p to turn it on.


  • Node v4+

  • OS

    • Linux
    • OS X
      • Up-to-date XCode
    • SmartOS
    • not Windows (PR's welcome)


npm install -g 0x

Basic Usage

Prefix the usual command for starting a process with 0x:

0x node my-app.js

All node args are supported, for instance:

0x node --trace-opt my-app.js

When node flags aren't required, we can use the shorthand form:

0x my-app.js

If we want to test against a local node binary, simply reference the binary directly.

For example if the node binary is the current working directory:

0x ./node my-app.js


Once we're ready to generate a flamegraph we send a SIGINT.

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

When 0x catches the SIGINT, it process the stacks and generates a profile folder, containing flamegraph.html


Due to security reasons Docker containers tend to result in the following error:

Cannot read kernel map
perf_event_open(..., PERF_FLAG_FD_CLOEXEC) failed with unexpected error 1 (Operation not permitted)
perf_event_open(..., 0) failed unexpectedly with error 1 (Operation not permitted)
You may not have permission to collect stats.

We can work around this problem by running our container with the --privileged option or add privileged: true in your docker-compose.yml file. See the Docker's doc for more info.

Production Servers

Generating a flamegraph can be quite intense on CPU and memory, if we have restricted resources we should generate the flamegraph in two pieces.

First we can use the --stacks-only flag to purely capture stacks.

0x --stacks-only my-app.js  #0x on the server 

Press ctrl+c when ready, this will create the usual profile folder, holding one file, that stacks.$PID.out file.

Now we need to transfer the stacks file from our production server to our local dev machine.

Let's say the pid was 7777, we can generate the flamegraph locally with

0x -c gen stacks.7777.out # 0x locally 

Now the hard work is done away from production, ensuring we avoid any service-level problems.

Memory Issues

As your stack grows you may have memory issues with both Node and your browser.

For Node, run with the following flag


For Chrome, run with the following flag

--js-flags="--stack-size 8024"

Where 8024 is the megabytes of RAM required to run load stack. Adjust this as needed and confirm you have it to spare.

0x Flags

--help | -h

Print usage info

--output-dir | -o

Specify artifact output directory Default: '${process.cwd()}/profile-${PID}(-${})?'


Generates an flamegraph.svg file in the artifact output directory, in addition to the flamegraph.html file.


Generates an SVG file, prerenders SVG inside HTML and outputs a PNG to the terminal (if possible) Depends on imagemagick (brew install imagemagick) If using iTerm 2.9+ image will be output to terminal Warning - depending on the amount of stacks this option can take tens of seconds

Default: false

--delay | -d

Milliseconds. Delay before tracing begins (or before stacks are processed in the Linux case), allows us to ignore initialisation stacks (e.g. module loading).

Example: 0x -d 2000 my-app.js

Default: 300

--langs | -l

Color code the stacks by JS and C

Example: 0x -l my-app.js

Default: false

--tiers | -t

A comma separated list

Overrides langs, Color code frames by type

Examples: 0x -t my-app.js

Default: false

--exclude | -x

Exclude tiers or langs, comma seperated list

Options: v8, regexp, nativeC, nativeJS, core, deps, app, js, c

Examples: 0x -x v8,nativeC,core my-app.js 0x -x c my-app.js

Default: v8


Include tiers, Overwrites exclude. Really only useful for including the v8 tier (which is excluded by default).

Options: v8, regexp, nativeC, nativeJS, core, deps, app, js, c

Example: 0x --include v8 my-app.js

Default: false


Dark or Light theme

Options: dark | light

Example: 0x --theme light my-app.js

Default: dark


Don't generate the flamegraph, only create the stacks output. If assigned to '-' stacks output will come through stdout. Use this in combination with the -c gen argument to generate the flamegraph from raw stacks.

Options: false | true | -


0x --stacks-only my-app.js

0x --stacks-only=- my-app.js

Default: false


Show output from dtrace or perf tools

Default: false

--cmd | -c

Run a "0x command", possible commands are help and gen.

0x -c help

outputs advanced usage (i.e. the commands).

0x -c gen

The gen command will generate the flamegraph from a stacks.out file.

Example: 0x -c gen [flags] profile-$PID/stacks.$PID.out > flamegraph.html

Flags include all the flags that can be passed to 0x


Adds the current timestamp to the Profile Folder's name minimizing collisions for in containerized environments

Example: profile-3866-

The Profile Folder

By default, a profile folder will be created and named after the PID, e.g. profile-3866 (we can set this name manually using the --output-dir flag).

The Profile Folder can contain the following files

  • flamegraph-small.png - the preview image output to terminal
  • flamegraph.svg - an SVG rendering of the flamegraph
  • stacks.3866.out - the traced stacks (run through perf-sym on OS X)
  • flamegraph.html - the interactive flamegraph
  • stacks.3866.json - a JSON tree generated from the stacks, this tree is rendered by d3.js in the flamegraph.html

The is helpful, because there's other things you can do with stacks output. For instance, checkout cpuprofilify and traceviewify.


Want to try it out? 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.

Now try some other options, e.g.

0x -t examples/rest-api

Babel (ES6 Transpile) Examples

See ./examples/babel for an example. Note the babel require hook is not currently supported. Notes on using the babel-cli instead can be found in the babel example readme.


Don't use v1, it was an experiment and is non functional Should have be v0...


Yes please!


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



0x is generously sponsored by nearForm

This tool is essentially a mashup from various info and code sources, and therefore would have taken much longer without the following people and their Open Source/Info Sharing efforts


MIT and Apache (depending on the code, see