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Purpose

Elements

Main View

The Start with menu provides a list from which you choose a base set of objects. This base set of objects appears in the gray area on the left; the view shows references to/from this base set of objects.

Squares represent objects, each colored uniquely by class.  A diamond represents a class object.  Squares come in two forms: a single square denotes a single instance, while twin squares represent multiple instances.  A node in the view can be a square (single or twin), a diamond, or a combination of the two, depending on the set of objects being represented.  Arrows between nodes denote one or more references between instances.  An arrow points to the object(s) being referenced.  Arrows run right to left by default.

Not all references are shown at once. Objects cascade from left to right as you reveal more and more references.  Hidden references are indicated by a "+" in a box to the left of a node.  Clicking on this box will expand the tree of references rightward, assuming there are further references.  If there are, the "+" changes to a "-", which you can click to recollapse that branch; if there are no further references, the box with the "+" will disappear entirely.  You can expand and collapse all nodes at a given level in the tree at once using the buttons marked "Depth" along the upper edge of the view.

A node for the same object(s) may appear more than once, since the view uses a tree to represent a network of object references.  A blue upward-pointing arrow identifies a repetition of the same node.  Clicking on this arrow will produce a long blue arrow leading to another occurrence of the same node.

To the right of each node appears a count of the number of objects the node represents and the class of the object(s).  The count and class name appear in red wherever there are old-generation objects; the number of old instances is then shown also.  Old-generation objects are those created prior to a certain point in the trace.  You can specify that point as you're taking the trace using the dump population tracing command.  Objects created after that point are new-generation objects.

A small circle appearing to the left of a node indicates that this object is referred to by a source other than a Java object.  The color of the circle shows the type of reference:  green for a Java stack, orange for a local JNI root, and red for a global JNI root.

You may select an object or a group of objects by left-clicking on the appropriate node. Use Control-left-click to select more than one object or group. Nodes will be highlighted in yellow if at least one of the objects represented by the node is currently selected in any view.

Menus

• Popup menus. Right clicking on a node reveals a popup menu that enumerates the set of objects, allows for ungrouping/regrouping them, and allows for selecting one of them.  The enumerated object information includes the instance number, the time the object was reported, and the number of bytes of memory it consumes.  An object that is selected is prefixed in this popup menu with *. An object that is pinned on a Java stack, or by a  JNI local or a global root will show this information as well.

Ungroup Objects changes the view to separate the node into individual nodes for each of the objects in the set.  Regroup Objects on any of the individual objects returns to the grouped representation.  When objects are ungrouped, left clicking on any one in the list selects it and tags it with an asterisk in this popup menu.

The remaining popup menu options are active only when current selections exist. Drill Down from Selected Items lets you open a view that is restricted to the current selections. Create a New Slice will create one or more new slices from the current selections. You can create either a single slice From Selected Items or one slice For Each Selected Item. Choosing Limit an Existing Slice presents the set of execution slices you can choose to limit to Selected Items.

• Start With menu. This menu lets you choose the base set of objects to appear in the gray area in the view.  The view then displays references to/from the base set of objects, out to the current depth setting.
• Selected Items -- the base set is the set of objects currently selected (highlighted in yellow) in this or some other view.
• Java and JNI Roots -- the base set is restricted to those objects directly referred to by any of the roots from which a mark-and-sweep garbage collection scan would start marking.  These include references from Java stacks, and references from native code using JNI.  Viewing references from this base set shows additional objects that cannot be garbage collected.
• All Class Objects -- the base set is restricted to all the class objects. Viewing references from this base set shows objects that are being held on to by class objects.
• Non-JDK and Non-Array Objects -- the base set is all the objects, except arrays and objects whose classes start with java/ or sun/.  These are typically objects of user-defined classes.  Sorting the view by number of references to these objects might help solve problems with excessive memory usage.

The next four choices are only applicable if your trace contains two memory snapshots: a dump population tracing command prior to the dump references tracing command.  These four choices restrict the base set to new generation objects, that is, those that are created between the above two tracing commands.  Typically, you expect objects in this new generation base set to be garbage collected.  Viewing old generation (red) references to these base set objects can show which references are preventing the garbage collection of the new generation objects.

• Selected Items in New Generation -- the base set is all the new generation objects in the set of objects currently selected (highlighted in yellow) in this or some other view.
• Class Objects in New Generation -- the base set is restricted to all the new generation class objects.
• Non-JDK and Non-Array Objects in New Generation -- the base set is all the new generation objects, except arrays and objects whose classes start with java/ or sun/.
• All Objects in New Generation -- the base set is all the new generation objects.
• Options menu. This menu lets you restrict, sort, or output the visible reference information. Expand to Old and Class Objects only shows sets of references that include either an old generation object or a class object. This can be useful in searching for the reason why an object has not been freed.  The current depth of the view is respected in carrying out the search for such objects.

Sort by Number of References determines how the base set, or each group of references to it, is sorted in the view.  If this option is chosen (checked), then nodes with the highest number of references appear at the top of the view.  If not chosen, then nodes are sorted alphabetically.

Print Report to data.ctl outputs all visible reference information to a file called data.ctl in the current directory. This file provides a record of the visualization should you want to study it.

A final option, Font Scaling, will make text grow and shrink just like other elements of the view when it's zoomed.  Text remains a constant size by default.
 

• Zoom menu. These options help you to navigate around the Execution View, by zooming and by varying the time dimension in the view. Home returns the view to the original scale and scroll position.

Zooming can be done in several ways. One is to drag the mouse over a rectangular area to zoom in on. Another is to use the plus and minus keys to zoom in and out. Additional menu options remember previous zoom positions you have traversed, and allow you to return to them. Toggle Zoom alternates between the two most recent zoom positions viewed. If you use Mark Zoom to save the current zoom position, then you can go to it later using the option Zoom To Mark. Also, a partial history of zoom positions is kept, using the most recent branch taken. Thus you can move back and forth along this zoom path history, with Zoom Back and Zoom Forward.
 

• Selected menu. These options are active only when current selection(s) exist. Drill Down from Selected Items lets you open a view that is restricted to the current selections. Create a New Slice will create one or more new slices from the current selections. You can create either a single slice From Selected Items or one slice For Each Selected Item. Choosing Limit an Existing Slice presents the set of execution slices you can choose to limit to Selected Items.

 
• Help menu. Selecting Help in a view attempts to use your browser to display the Reference Manual documentation for that view. The browser program location is specified in the environment variable JINSIGHT_BROWSER, in the shell script named jinsight that invokes the visualizer.

 

Bottom Panel

The leftmost button toggles the direction of reference arrows.  They are drawn right-to-left by default, and in that case the button is marked ">>>"; pressing it rearranges the view so that reference arrows are drawn left-to-right.  Pressing the button (now marked "<<<") again returns the arrows to their default direction.  Use right-to-left to find out who refers to the base set, and use left-to-right to see what the base set is holding on to.

The status line displays specific information about the object(s) underneath the mouse pointer, including

• the number and class of the object(s)
• the total memory consumed by the object(s)
• the number of references to/from the object(s).

See also Using Slices in the Reference Pattern View for a discussion of the effect of workspace filtering on this view.

Interpretation

Debugging

The view can reveal the causes of a memory leak, an object that cannot be garbage collected because of other references to it.  You can choose an appropriate base set of objects, and examine the references to/from them.  The view can show unexpected memory references which prevent garbage collection.

With tracing commands, you can delineate objects into two groups: an old generation set of objects that exist before a particular application activity, and a new generation set of objects that appear during the application activity.  The view can then show the new generation objects that were not garbage collectible after that application activity.  The view also identifies the old generation objects (labeled in red) still holding references to them and preventing their collection.

Program Understanding

You can see and explore data structures by following references between objects.  For example, a list will manifest itself as a linear succession of links between objects of the same type.  A tree will show up explicitly, and it will be easy to tell whether it's balanced or not.  Class objects reveal references from static members.

Tuning

The view can reveal excessive or redundant references, although judging what is "excessive" usually calls for insight into the program's design and implementation.

Tracing Considerations

If you suspect a memory leak related to a particular activity in your application, you should introduce a generation boundary, using the dump population tracing command (Ctrl-Brk dp), prior to invoking the target activity.  Then issue a Ctrl-Brk dr command after the target activity completes to take the snapshot of references. 

Related Views

Once you have selected an object in this view, you can use the the Execution Pattern view to study the interactions that produced the object's references.

See also Understanding memory usage and data structures in the Jinsight How-To Guide.

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