node-heapdump

node-heapdump

Make a dump of the V8 heap for later inspection.

Install

npm install heapdump

Build

node-gyp configure build

Usage

Load the add-on in your application:

var heapdump = require('heapdump');

The module exports a single writeSnapshot([filename], [callback]) function that writes out a snapshot. filename defaults to heapdump-<sec>.<usec>.heapsnapshot when omitted.

heapdump.writeSnapshot('/var/local/' + Date.now() + '.heapsnapshot');

The function also takes an optional callback function which is called upon completion of the heap dump.

heapdump.writeSnapshot(function(err, filename) {
  console.log('dump written to', filename);
});

The snapshot is written synchronously to disk. When the JS heap is large, it may introduce a noticeable "hitch".

On UNIX platforms, you can force a snapshot by sending the node.js process a SIGUSR2 signal:

$ kill -USR2 <pid>

The SIGUSR2 signal handler is enabled by default but you can disable it by setting NODE_HEAPDUMP_OPTIONS=nosignal in the environment:

$ env NODE_HEAPDUMP_OPTIONS=nosignal node script.js

Catch the SIGUSR2 signal to write the snapshot to a custom location:

if (!/nosignal/.test(process.env.NODE_HEAPDUMP_OPTIONS)) {
  process.on("SIGUSR2", function() {
    heapdump.writeSnapshot('/var/local/' + Date.now() + '.heapsnapshot');
  });
}

Inspecting the snapshot

Open Google Chrome and press F12 to open the developer toolbar.

Go to the Memory tab, right-click in the tab pane and select Load profile....

Select the dump file and click Open. You can now inspect the heap snapshot at your leisure. Some snapshots may take a long time to load, on the order of minutes or even hours.

Note that Chrome will refuse to load the file unless it has the .heapsnapshot extension.

Caveats

On UNIX systems, the rule of thumb for creating a heap snapshot is that it requires memory twice the size of the heap at the time of the snapshot. If you end up with empty or truncated snapshot files, check the output of dmesg; you may have had a run-in with the system's OOM killer or a resource limit enforcing policy, like ulimit -u (max user processes) or ulimit -v (max virtual memory size).