title: ‘Investigating memory leaks’

description: ‘This document provides guidance on investigating memory leaks in V8.’

If you’re investigating a memory leak and wonder why an object is not garbage-collected, you can use %DebugTrackRetainingPath(object) to print the actual retaining path of the object on each GC.

This requires --allow-natives-syntax --track-retaining-path run-time flags and works both in release and debug modes. More info in the CL description.

Consider the following test.js:

  1. function foo() {
  2.   const x = { bar: 'bar' };
  3.   %DebugTrackRetainingPath(x);
  4.   return () => { return x; }
  5. }
  6. const closure = foo();
  7. gc();

Example (use debug mode or v8_enable_object_print = true for much more verbose output):

  1. $ out/x64.release/d8 --allow-natives-syntax --track-retaining-path --expose-gc test.js
  2. #################################################
  3. Retaining path for 0x245c59f0c1a1:
  5. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  6. Distance from root 6: 0x245c59f0c1a1 <Object map = 0x2d919f0d729>
  8. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  9. Distance from root 5: 0x245c59f0c169 <FixedArray[5]>
  11. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  12. Distance from root 4: 0x245c59f0c219 <JSFunction (sfi = 0x1fbb02e2d7f1)>
  14. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  15. Distance from root 3: 0x1fbb02e2d679 <FixedArray[5]>
  17. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  18. Distance from root 2: 0x245c59f0c139 <FixedArray[4]>
  20. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  21. Distance from root 1: 0x1fbb02e03d91 <FixedArray[279]>
  23. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  24. Root: (Isolate)
  25. -------------------------------------------------

Debugger support

While in a debugger session (e.g. gdb/lldb), and assuming you passed the above flags to the process (i.e. --allow-natives-syntax --track-retaining-path), you may be able to print isolate->heap()->PrintRetainingPath(HeapObject*) on an object of interest.