Group by Dominators
The Group by Dominators view allows you to understand how memory is retained in your app. For example, you know that the major part of memory in your app is consumed by strings. Nevertheless, most likely, the subject of your optimizations are not these strings by themselves but data structures that store them. The Group by Dominators view answers the "Who retains the memory" question which is extremely important when analyzing ineffective memory usage. To answer this question properly, you should be acquainted with the concept of dominators.
What is a dominator?
The object A dominates the object B if every path to B from application's roots goes through A. In other words, the object B is retained in memory exclusively by the object A: If A is garbage collected, B is also garbage collected. For example, an array is a dominator for its elements (in case there are no other references to array elements). If there are more than one path to an object from app's roots, it is considered as not dominated or not exclusively retained object. For example, if the object C is retained by the objects A and B and A is garbage collected, C will stay in memory (as it is still referenced by B). That's why it is important to understand the difference between the domination path (any object on the path is retained only by one other object) and the retention path (an object on the path may be retained by any number of other objects).
- A tree list of dominators on the left.
- Sunburst chart on the right: a graphical representation of the tree list.
The tree can work in two modes that can be selected with the corresponding buttons in the tree header:
- Tree: All dominator nodes are shown. The dominators that take part in a domination path for the selected tree node are shown in bold.
- Path: Only the dominators that take part in a domination path for the selected tree node are shown.
The list of dominators consists of the following columns:
Sunburst chart on the left is a graphical representation of the dominators tree. The more memory a dominator retains, the larger the central angle.
The analysis of the dominators tree can be quite complicated as you have to expand its nodes and interpret the numbers in the Retained bytes column. In contrast, just a glance at the sunburst chart allows you to determine main dominators and largest structures in your app. A click on a dominator you're interested in will show you its domination path in the tree.
Consider the example below for better understanding of how the sunburst chart displays data from the dominators tree.
Zooming in and Out
If you want to take a more detailed look at a certain dominator subtree, you can change the scale of the chart.
To zoom in on a dominator:
To zoom back out:
- Click the center of the chart.
Selecting Objects for Further Analysis
- Objects retained by a particular dominator;
- Objects exclusively retained (dominated) by the whole object set.
To open objects retained by a particular dominator:
- Do one of the following:
- Hold Alt and click the dominator in the tree or on the chart.
- Right-click the dominator and select Open objects retained via this domination path in the context menu.
To open the dominator object set:
- Do one of the following:
- Hold Shift and click the dominator in the tree or on the chart.
- Right-click the dominator and select Open this object set in the context menu.