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Flame Graphs

What's a flame graph useful for?

Flame graphs are a way of visualizing CPU time spent in functions. They can help you pin down where you spend too much time doing synchronous operations.

How to create a flame graph

You might have heard creating a flame graph for Node.js is difficult, but that's not true (anymore). Solaris vms are no longer needed for flame graphs!

Flame graphs are generated from perf output, which is not a node-specific tool. While it's the most powerful way to visualize CPU time spent, it may have issues with how JavaScript code is optimized in Node.js 8 and above. See perf output issues section below.

Use a pre-packaged tool

If you want a single step that produces a flame graph locally, try 0x

For diagnosing production deployments, read these notes: 0x production servers

Create a flame graph with system perf tools

The purpose of this guide is to show steps involved in creating a flame graph and keep you in control of each step.

If you want to understand each step better, take a look at the sections that follow where we go into more detail.

Now let's get to work.

  1. Install perf (usually available through the linux-tools-common package if not already installed)
  2. try running perf - it might complain about missing kernel modules, install them too
  3. run node with perf enabled (see perf output issues for tips specific to Node.js versions)
     perf record -e cycles:u -g -- node --perf-basic-prof app.js
  4. disregard warnings unless they're saying you can't run perf due to missing packages; you may get some warnings about not being able to access kernel module samples which you're not after anyway.
  5. Run perf script > perfs.out to generate the data file you'll visualize in a moment. It's useful to apply some cleanup for a more readable graph
  6. install stackvis if not yet installed npm i -g stackvis
  7. run stackvis perf < perfs.out > flamegraph.htm

Now open the flame graph file in your favorite browser and watch it burn. It's color-coded so you can focus on the most saturated orange bars first. They're likely to represent CPU heavy functions.

Worth mentioning - if you click an element of a flame graph a zoom-in of its surroundings will get displayed above the graph.

Using perf to sample a running process

This is great for recording flame graph data from an already running process that you don't want to interrupt. Imagine a production process with a hard to reproduce issue.

perf record -F99 -p `pgrep -n node` -g -- sleep 3

Wait, what is that sleep 3 for? It's there to keep the perf running - despite -p option pointing to a different pid, the command needs to be executed on a process and end with it. perf runs for the life of the command you pass to it, whether or not you're actually profiling that command. sleep 3 ensures that perf runs for 3 seconds.

Why is -F (profiling frequency) set to 99? It's a reasonable default. You can adjust if you want. -F99 tells perf to take 99 samples per second, for more precision increase the value. Lower values should produce less output with less precise results. Precision you need depends on how long your CPU intensive functions really run. If you're looking for the reason of a noticeable slowdown, 99 frames per second should be more than enough.

After you get that 3 second perf record, proceed with generating the flame graph with the last two steps from above.

Filtering out Node.js internal functions

Usually you just want to look at the performance of your own calls, so filtering out Node.js and V8 internal functions can make the graph much easier to read. You can clean up your perf file with:

sed -i \
  -e "/( __libc_start| LazyCompile | v8::internal::| Builtin:| Stub:| LoadIC:|\[unknown\]| LoadPolymorphicIC:)/d" \
  -e 's/ LazyCompile:[*~]\?/ /' \
  perfs.out

If you read your flame graph and it seems odd, as if something is missing in the key function taking up most time, try generating your flame graph without the filters - maybe you got a rare case of an issue with Node.js itself.

Node.js's profiling options

--perf-basic-prof-only-functions and --perf-basic-prof are the two that are useful for debugging your JavaScript code. Other options are used for profiling Node.js itself, which is outside the scope of this guide.

--perf-basic-prof-only-functions produces less output, so it's the option with least overhead.

Why do I need them at all?

Well, without these options you'll still get a flame graph, but with most bars labeled v8::Function::Call.

perf output issues

Node.js 8.x V8 pipeline changes

Node.js 8.x and above ships with new optimizations to JavaScript compilation pipeline in V8 engine which makes function names/references unreachable for perf sometimes. (It's called Turbofan)

The result is you might not get your function names right in the flame graph.

You'll notice ByteCodeHandler: where you'd expect function names.

0x has some mitigations for that built in.

For details see:

Node.js 10+

Node.js 10.x addresses the issue with Turbofan using the --interpreted-frames-native-stack flag.

Run node --interpreted-frames-native-stack --perf-basic-prof-only-functions to get function names in the flame graph regardless of which pipeline V8 used to compile your JavaScript.

Broken labels in the flame graph

If you're seeing labels looking like this

node`_ZN2v88internal11interpreter17BytecodeGenerator15VisitStatementsEPNS0_8ZoneListIPNS0_9StatementEEE

it means the Linux perf you're using was not compiled with demangle support, see https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1396654 for example

Examples

Practice capturing flame graphs yourself with a flame graph exercise!

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