Find Memory Leaks Not Reported by Default

By default, memlab reports only high-confidence memory leaks identified by its built-in leak detector. If memory leaks exist but memlab does not report them, this tutorial shows alternative ways to surface suspicious heap objects.

Start by running the following command without specifying a leak filter in the scenario file. MemLab will apply its built-in leak detectors, which find detached DOM elements and unmounted React Fiber nodes. In some cases, this may not detect all (or any) memory leaks.

memlab run --scenario ~/memlab/scenarios/unbound-object.js

Now let's run:

memlab find-leaks --trace-all-objects

This tells memlab to treat every object allocated by the target interaction as a memory leak. In this case, there are many objects that were not released after the target interaction.

memlab clusters the retainer traces of the leaked objects and prints them in decreasing order based on the aggregated retained sizes of leak clusters.

MemLab found 46 leak(s)
--Similar leaks in this run: 4--
--Retained size of leaked objects: 8.3MB--
[Window] (native) @35847 [8.3MB]
  --20 (element)--->  [InternalNode] (native) @130981728 [8.3MB]
  --8 (element)--->  [InternalNode] (native) @130980288 [8.3MB]
  --1 (element)--->  [EventListener] (native) @131009888 [8.3MB]
  --1 (element)--->  [V8EventListener] (native) @224808192 [8.3MB]
  --1 (element)--->  [eventHandler] (closure) @168079 [8.3MB]
  --context (internal)--->  [<function scope>] (object) @181905 [8.3MB]
  --bigArray (variable)--->  [Array] (object) @182925 [8.3MB]
  --elements (internal)--->  [(object elements)] (array) @182929 [8.3MB]


--Similar leaks in this run: 79--
--Retained size of leaked objects: 16.9KB--
[Window] (native) @35847 [8.3MB]
  --17 (element)--->  [InternalNode] (native) @224820352 [0 byte]
  --3 (element)--->  [InternalNode] (native) @224766112 [0 byte]
  --1 (element)--->  [InternalNode] (native) @224771072 [0 byte]
  --1 (element)--->  [InternalNode] (native) @224723840 [540 bytes]
  --1 (element)--->  [InternalNode] (native) @224723680 [540 bytes]
  --1 (element)--->  [InternalNode] (native) @224818752 [84 bytes]


--Similar leaks in this run: 62--
--Retained size of leaked objects: 12.8KB--
[Window] (native) @35847 [8.3MB]
  --4 (element)--->  [HTMLDocument] (native) @35845 [6KB]
  --part of key -> value pair in ephemeron table (internal)--->  [HTMLDocument] (object) @167199 [28 bytes]
  --__proto__ (property)--->  [HTMLDocument] (object) @173029 [144 bytes]
  --properties (internal)--->  [(object properties)] (array) @182697 [76 bytes]

...

One way to reduce the verbosity is to apply a leak filter or use --trace-object-size-above. The following command shows only traces whose retainedSize is greater than 1MB:

memlab find-leaks --trace-object-size-above 1000000

The result will look like:

MemLab found 1 leak(s)
--Similar leaks in this run: 4--
--Retained size of leaked objects: 8.3MB--
[Window] (native) @33651 [8.3MB]
  --20 (element)--->  [InternalNode] (native) @216691968 [8.3MB]
  --8 (element)--->  [InternalNode] (native) @216691168 [8.3MB]
  --1 (element)--->  [EventListener] (native) @216563936 [8.3MB]
  --1 (element)--->  [V8EventListener] (native) @216563776 [8.3MB]
  --1 (element)--->  [eventHandler] (closure) @160711 [8.3MB]
  --context (internal)--->  [<function scope>] (object) @176463 [8.3MB]
  --bigArray (variable)--->  [Array] (object) @176465 [8.3MB]
  --elements (internal)--->  [(object elements)] (array) @176489 [8.3MB]

Another option is --ignore-leak-cluster-size-below, which ignores memory leak clusters with an aggregated retained size smaller than the specified threshold. To learn more about the find-leaks command, run memlab find-leaks -h.