Understanding the Summary Presentation of GC Data |
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To view the summary presentation of GC data, double-click on the data
representation in the main panel. A window will open to display
the data.
The following image shows a summary of .vgc data collected using the -Xverbosegc option and displayed under the Summary tab. Important aspects of the summary data are defined below the
image.
Heap
Capacity Initial, final, and peak sizes allocated for various
organizational spaces in the heap.
Peak
Usage of Capacity The highest percentage of actual use by the application
of the heap at its final capacity configuration. A rule of thumb for
optimal utilization would be to have the eden space at 100% with the
survivor and old spaces showing some reserve capacity, depending on
the application.
GC Activity
Summary In this example, a comparison of scavenges and
full garbage collection of the old and permanent generations, showing
number of GC occurrences, duration, and memory usage for each GC type
displayed.
Duration
of the Measurement The approximate, elapsed, wall clock time for
the entire data collection.
Measurement
Enabled and Measurement Disabled “Measurement enabled” refers to the
total time during which zero preparation
GC data collection has been activated during the session thus
far. “Measurement disabled” refers to the total time during
which no data was collected (zero preparation GC data collection is
not in use). These values are visible on this tab when zero preparation
GC data collection is used during a monitoring session. They are not
present when collecting GC data using the -Xverbosegc or -Xloggc options on the command line.
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 |
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To view the system details for GC data, double-click on the data representation in the main panel. A window will open to display the data. Select
the System Details tab.
The following image shows system details collected
using the -Xverbosegc option and displayed under
the System Details tab.
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.
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 | NOTE: The Number of Localities, Heap
Pointer Mode, and UseNUMA fields only
display if JDK/JRE 6.0.08 or later is being used. |
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As with
any field whose value is too long for its column width, if
the JVM Arguments value shown at the bottom of the panel is extensive
and runs beyond its column, you can mouse over the value to display
a yellow pop-up box that shows all the JVM arguments. When you mouse
away from it, the yellow box disappears. However, if you double click
the JVM arguments value, a pop-up box displays and remains until you
close it: