A Memory Leak in v8 and how Jade is affected

Demo: https://alubbe.github.io/memoryleak/

Some time between 3.24.35.22 and 3.26.33, v8 introduced a performance regression, where

for(var i = 0; i < 500000; i++) {
  (function() {
    var array = [{a: 1, b: 'r'}];
    array.push({a:2, b:'s'});
  })()
}

suddenly causes a memory leak. Presumably, v8 doesn't recognize the shared hidden class {lineno: Integer, filename: String} behind the objects.

Jade is affected because, when the compileDebug flag is enabled, as it is by default, it generates these lines, which fall prey to the above v8 memory leak:

var jade_debug = [{ lineno: 1, filename: "/home/andreas/code/sampleexpress/views/fortunes/index.jade" }];
jade_debug.unshift({ lineno: 0, filename: "/home/andreas/code/sampleexpress/views/layout.jade" });

'Fixing' the bug

There is an outstanding fix to Jade and an issue at v8.

Until then, it is highly recommended to explicitly disable compileDebug outside of your development environment:

// app is the instance of your express app
if (process.env.NODE_ENV && process.env.NODE_ENV !== 'development') {
  app.locals.compileDebug = false;
}

Reproducing the bug

This repo contains a small express app and a very small jade file, taken from Techempowers Framework benchmarks.

There are two ways to reproduce this bug for v8 and three ways to reproduce it for jade:

v8

1. Open Chrome's dev tools and taskmanager. Watch the RAM grown as you run for(var i = 0; i < 500000; i++) {(function() {var array = [{a: 1, b: 'r'}];array.push({a:2, b:'s'});})()}.
2. Run /usr/bin/time -v node -e "for(var i = 0; i < 500000; i++) (function(){var array = [{a: 1, b: 'r'}]; array.push({a:2, b:'s'});})()" on your shell.

jade

1. Run the browser file and watch its RAM usage as you click the buttons.
2. Run the express app (index.js), hit it with wrk or ab and watch your mem explode. Then uncomment line 9, restart the server and watch the leak disappear.
3. Run the following commands from your command line. Also, run them in both 0.10/0.11.13 vs 0.11.14/0.12/iojs to see the performance regression caused by v8:

/usr/bin/time -v node -e "for(var i = 0; i < 300000; i++) require('./compiledWithDebug')();"
/usr/bin/time -v node -e "for(var i = 0; i < 300000; i++) require('./compiledWithoutDebug')();"
/usr/bin/time -v node -e "for(var i = 0; i < 300000; i++) require('./fixedMemoryLeak')();"

Affected versions

This bug affects:

• node 0.11.14+
• node 0.12.*
• all iojs releases