Heap has multiple regions. Old Generation is one of them, where long-living objects (like cache, connection pool...) are placed. When a Full GC event runs, it removes unreferenced objects (i.e. garbage) from Old Gen.
Say suppose JVM has allocated 100mb size for your Old Gen. But your application has only 75mb of objects in Old Gen. Now GC event ran, cleared garbage and got the Old Gen's size to 25mb. Then in this 'Old gen' graph, we have plotted 100mb (as 'allocated space'), 75mb (as 'before GC'), 25mb (as 'after GC').
100mb - allocated space, the space allocated by JVM for your Old generation
75mb - before GC, Allocated space will be the addition of free space and Live + unreferenced objects (Used space)
25mb - after GC, it removes unreferenced objects from the young generation such that the allocated space will be the addition of free space and Live objects.
Steps to use the Old Generation graph:
1. In the GCeasy report, You can see the old generation graph in the Interactive Graphs section.
2. From the left side, select the Old Gen button such that it will show the corresponding graph, where X-axis indicates the time at which the GC event ran and Y-axis indicates the number of bytes.
3. Click on legends in the graph to highlight and see a particular trend line as shown below.
4. If you want to see any one of the graphs (either allocated space, before GC, or after GC) you can click either allocated space, before GC, or after GC below the X-axis. It will change as shown below.
5. You can zoom in to see a particular area of the graph. Here is the video that explains how to zoom gceasy graphs.
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