Investigating memory leaks

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:

function foo() {
const x = { bar: 'bar' };
%DebugTrackRetainingPath(x);
return () => { return x; }
}
const closure = foo();
gc();

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

$ out/x64.release/d8 --allow-natives-syntax --track-retaining-path --expose-gc test.js
#################################################
Retaining path for 0x245c59f0c1a1:

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Distance from root 6: 0x245c59f0c1a1 <Object map = 0x2d919f0d729>

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Distance from root 5: 0x245c59f0c169 <FixedArray[5]>

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Distance from root 4: 0x245c59f0c219 <JSFunction (sfi = 0x1fbb02e2d7f1)>

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Distance from root 3: 0x1fbb02e2d679 <FixedArray[5]>

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Distance from root 2: 0x245c59f0c139 <FixedArray[4]>

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Distance from root 1: 0x1fbb02e03d91 <FixedArray[279]>

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Root: (Isolate)
-------------------------------------------------

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.