Viewing the Detailed Information About the Garbage Collection
When you select a garbage collection, the Details section of the GC Events tab changes name to either Details - Old Collection or Details - Young Collection depending on the type of garbage collection you have selected (this is only possible when selecting one garbage collection). You will also see different sets of tabs that contain specific information about the garbage collection that you have selected (see Figure 6-6).
Figure 6-6 Tab differences when viewing old and young collections
Each one of these tabs are described here. As much of the information in the tabs are fairly self-explanatory, those types of details will not be covered in the documentation.
Viewing Information on the General Garbage Collection Tab
The General tab (see Figure 6-7) displays information such as start time and end time of the garbage collection.
Figure 6-7 The General garbage collection tab
Generation-Indicates whether the garbage collector performed an old or young collection (see the Memory Management user guide for more information on generational garbage collection). If a parallel garbage collector has been used, there will be only old collections in the Garbage Collections list.
Pause Time-the time in milliseconds that the garbage collector stops all threads in JRockit. This is not the same as end time-start time in the case of a concurrent garbage collector.
Start/End Time-the times when the garbage collection started and ended, counted in milliseconds from when JRockit started.
Heap Usage Before/After-the used heap size before or after the garbage collection.
Number of References-there are different types of references collected during a recording. For information on what a reference is, see the Memory Management user guide.
Committed Heap Size-the total size of the heap (used plus unused memory) after the garbage collection.
Viewing Information on the GC Method Call Tree Tab
The GC Method Call Tree tab (see Figure 6-8) shows an aggregation of the call traces of the threads triggering a garbage collection.
Figure 6-8 The GC Method Call Tree tab
Viewing Information on the Old/Young Collection Tab
The name of this tab is dynamically changed when you select a garbage collection instance in the Garbage Collections section. Here you find information about nursery, mark and sweep pause times, etc. (see Figure 6-9).
Figure 6-9 The Old/Young Collection tab
Nursery Size Before/After-indicates the size of the young space on the heap before and after the garbage collection (in some cases the nursery size can increase).
The information below is only valid for old collections:
Nursery Start/End Position-the starting and ending position in the memory address of nursery.
Mark/Sweep Phase Time-the time spent in the marking and sweep phases, measured in milliseconds.
Compacted Size-the size of the heap that has been compacted in the garbage collection.
Compaction Ratio-the ratio of heap size before and after the compaction, measured in percent.
Desired/Actual evacuation-the desired evacuation is the size of the area on the Java heap that you want to evacuate and the actual evacuation is the size of the area that JRockit managed to evacuate. The value for actual evacuation can be smaller than the desired due to temporarily pinned objects (objects that are not allowed to be moved during garbage collection). The evacuation takes place during compaction or shrinking of the Java heap.
GC Reason-indicates the reason for doing this garbage collection.
Viewing Information on the Cache Lists Tab (Only valid for old collections)
The Cache Lists tab (see Figure 6-10) the specification for the different cache lists. Each cache list contains settings for upper and lower cache size.
Figure 6-10 The Cache Lists tab
Index-this is the identification number for the cache list.
#free blocks-the number of free blocks in the cache list.
Cache size-the total size of this cache list.
Avg free block size-the average size of each free memory block in the cache list.
Low limit-the lower limit of a free memory block. There will be no smaller memory block than this in the selected cache list.
High limit-the upper limit of a free memory block. There will be no larger memory blocks than this in the selected cache list.
The Pause Time Tab
The information under the Pause Time tab is mainly intended for BEA JRockit internal use when you have sent a JRA recording for analysis to the JRockit engineering team.
GC Pause-this column displays the names of the pauses (the main entry in the tree structure). If you are running a parallel garbage collector, then there will only be one pause per garbage collection. For the concurrent garbage collector, there can be several pauses during one garbage collection. The pauses consists of pause parts that can help the JRockit engineering staff to analyze why certain pauses are longer than others.
Note:
During a pause, the application is standing still.
Length-this is the length, measured in milliseconds, of the pause.
Start/End-this is the absolute time, measured in milliseconds, since January 1, 1970.