1.0.8 • Public • Published

node-memwatch: Leak Detection and Heap Diffing for Node.JS

node-memwatch is here to help you detect and find memory leaks in Node.JS code. It provides:

  • A stats event, emitted on full MarkSweepCompact GCs giving you data describing your heap usage and trends over time.

  • A HeapDiff class that lets you compare the state of your heap between two points in time, telling you what has been allocated, and what has been released.


  • npm install @ardatan/node-memwatch


There are a growing number of tools for debugging and profiling memory usage in Node.JS applications, but there is still a need for a platform-independent native module that requires no special instrumentation. This module attempts to satisfy that need.

To get started, import node-memwatch like so:

var memwatch = require('@ardatan/node-memwatch');

Leak Detection

Currently unsupported while we explore heuristics

Heap Usage

The best way to evaluate your memory footprint is to look at heap usage right after V8 performs garbage collection. memwatch does exactly this - it checks heap usage only after GC to give you a stable baseline of your actual memory usage.

When V8 performs a garbage collection (technically, we're talking about a full GC with heap compaction), memwatch will emit a stats event.

memwatch.on('stats', function(stats) { ... });

The stats data will look something like this:

  gcScavengeCount: 1,
  gcScavengeTime: 1100880, // ns
  gcMarkSweepCompactCount: 2,
  gcMarkSweepCompactTime: 21157231, // ns
  gcIncrementalMarkingCount: 0,
  gcIncrementalMarkingTime: 0, //ns
  gcProcessWeakCallbacksCount: 0,
  gcProcessWeakCallbacksTime: 0, // ns
  total_heap_size: 16097280, // bytes
  total_heap_size_executable: 3670016, // bytes
  total_physical_size: 10741880, // bytes
  total_available_size: 1487689928, // bytes
  used_heap_size: 5691584, // bytes
  heap_size_limit: 1501560832, // bytes
  malloced_memory: 8192,
  peak_malloced_memory: 1185464,
  gc_time: 4587251 // ns

V8 has its own idea of when it's best to perform a GC, and under a heavy load, it may defer this action for some time. To aid in speedier debugging, memwatch provides a gc() method to force V8 to do a full GC and heap compaction.

Heap Diffing

For leak isolation, it provides a HeapDiff class that takes two snapshots and computes a diff between them. For example:

// Take first snapshot
var hd = new memwatch.HeapDiff();

// Take the second snapshot and compute the diff
var diff = hd.compare()

// Update the base snapshot

// do some things ...

// compare() and destory
var diff = hd.end();

The contents of diff will look something like:

  "before": { "nodes": 11625, "size_bytes": 1869904, "size": "1.78 mb" },
  "after":  { "nodes": 21435, "size_bytes": 2119136, "size": "2.02 mb" },
  "change": { "size_bytes": 249232, "size": "243.39 kb", "freed_nodes": 197,
    "allocated_nodes": 10007,
    "details": [
      { "what": "String",
        "size_bytes": -2120,  "size": "-2.07 kb",  "+": 3,    "-": 62
      { "what": "Array",
        "size_bytes": 66687,  "size": "65.13 kb",  "+": 4,    "-": 78
      { "what": "LeakingClass",
        "size_bytes": 239952, "size": "234.33 kb", "+": 9998, "-": 0

The diff shows that during the sample period, the total number of allocated String and Array classes decreased, but Leaking Class grew by 9998 allocations. Hmmm.

You can use HeapDiff in your on('stats') callback; even though it takes a memory snapshot, which triggers a V8 GC, it will not trigger the stats event itself. Because that would be silly.

Future Work

Please see the Issues to share suggestions and contribute!






npm i @ardatan/node-memwatch

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