preparation GC data collection is used during a monitoring session. They are not present when collecting GC data using the
Total Bytes Allocated The total amount of space created for new objects over the lifetime of the application. This number represents the total amount of memory the program would have consumed had no garbage collection been performed. It is an abstract measure of the total work done by the application.
Number of GC Events The number of times the garbage collector was invoked during the program run.
Average Ideal Allocation Rate What the average memory allocation rate for new Java objects would have been had no garbage collection been necessary. It is a theoretical limit of a program's performance if GC time were driven to zero.
Residual Bytes Heap usage when the program ends.
Time spent in GC The total amount of wall clock time spent in garbage collection during the program run.
Percentage of Time in GC The percentage of the total amount of wall clock time spent in garbage collection during the program run. This value displays in red when 5% or more of clock time is spent in garbage collection. When this value shows red, the amount of garbage collection activity should be scrutinized.
Percentage of Time in Full GC The percentage of the total amount of wall clock time spent doing a full GC during the program run. This value displays in red when 5% or more of clock time is spent in a full garbage collection. When this value shows red, the amount of garbage collection activity should be scrutinized. See also “Comparison of Percentages” color bars.
Average Allocation Rate The actual average memory allocation rate for new Java objects. See also Average Ideal Allocation Rate.
Precise Data For rounded values given in MB or GB, mouse over the value to see the measure in precise bytes.
Process is Swapping Indicates whether or not process swapping is occurring during the measured period. To determine the amount of swap memory occupied when processes are being swapped, click the System Details tab, and locate the swap data in the System and Memory Details section.
Comparison of Percentages The color bars compare the percentage of time spent in Full GC or in System.gc calls as a percentage of total time spent doing garbage collection. However, if the total time spent in System.gc calls is greater than half the total time spent in Full GC, System.gc percentages will display instead of Full GC percentages.
Understanding the System Details Captured with GC Data
To view the system details for GC data, select the System Details tab. If the value in a table cell is too long, double click the data and a
The following image shows system details collected using the
The data on this tab is a summary of operating system attributes and the JVM options in effect at the time the data collection began. It can be useful to refer to this information when determining adjustments to make in the size of the heap and/or the memory spaces.
90 Analyzing Garbage Collection Data