This inspector is a browser-based application for inspecting GC heap snapshots generated by notifyutil -p com.apple.WebKit.dumpGCHeap and saved as a .json file. For a detailed description of how to save a snapshot, see the opening screen of the inspector.
In this directory, execute
python -m SimpleHTTPServer
then in a web browser, go to http://localhost:8000/gc-heap-inspector.html, then drag and drop a snapshot file onto the indicated box.
Once a snapshot is loaded, the inspector displays an overview of the objects in the snapshot. The overview is divided into the following sections:
This section is initially empty. It can be populated with tools showing detailed information about certain objects, such as: - All paths from roots to a particular node. - A node explorer for navigating the edges of the object graph around a particular node. - Dominator summary of instances of a particular type.
This section lists all instances of Window and HTMLDocument. These two types are chosen because of their importance in the typical memory graph. Accidentally retaining them can retain a large amount of memory.
In this section as well as everywhere else in the inspector, an object is displayed as its class name and the node ID in the graph, followed by other details. For example:
Window#543 (retains 0.5K) [Show all paths] ... etc
The node ID is a unique identifier of the object in the memory snapshot. The same object appearing elsewhere in the inspector will show the same node ID. A different instance of the same class will have a different node ID. Clicking the node ID adds the Node Explorer tool (explained in a dedicated section below) for that object to the Tools section.
Clicking an object in this section expands it into the shortest path to it from a GC root. The path is presented as a list of objects with the GC root at the top and the window or the document at the bottom.
The Show all paths button adds a tool to the Tools section that displays all (as opposed to the shortest) paths to the object from GC roots.
Lists all types whose instances are registered as GC roots, with the number of root instances of each type. Each type entry expands into a list of instances.
This section lists all types, with the number of instances and their combined memory footprint. Expanding a type lists the individual instances.
The memory footprint of a type is followed by the Dominators button. The button adds an immediate dominator summary for this type to the Tools section. See below for the explanation of the dominator summary.
This tool is added by clicking the Show all paths button in an object. It lists all possible paths to the object from GC roots. Each element of the list represents one path. Expanding a path shows its individual steps.
A node explorer can be opened on any object by clicking the node ID. The explorer features three expandable entries:
The dominator summary tool goes through a list of instances of a type and collects information about their immediate dominators. For each dominator, it counts how many instances it dominates. The dominators are then listed in the reverse order of the counts.
Dominator information can be useful in finding GC object leaks that happen because objects are added to a registry of some kind and never removed. If a large group of unexplainably live objects have the same dominator, the dominator is a prime suspect for being the reason those objects are staying alive.
The inspector presents dominator information in two forms. One is the dominator summary. The other is a list of “interesting” dominators of an object. Interesting dominators are dominators other than the object itself and a GC root. If an object has interesting dominators, they are always displayed when displaying the object.