HPjmeter 4.2 User's Guide

Table 6-5 Common Garbage Collection Types Recognized by HPjmeter
A garbage collection performed in the old area of the heap as a
background thread that does its work with few pauses in the application
run.
Concurrent Mark-Sweep (CMS)
Only objects from the young generation are collected using a
multi-threaded garbage collector.
Parallel Scavenge
Objects from the young generation only are collected using a single
thread.
Scavenge
Collection in the young generation occurs when a CMS operation is set
to collect in the old generation. The pauses for the young generation
Scavenge during CMS
collection and the old generation collection occur independently and
cannot overlap.
Occurs when a background thread is performing a garbage collection
in the old area, but is interrupted when the JVM determines that a full
GC is needed.
Incomplete Concurrent Mark-Sweep
Necessary when the old area is expanded on the most recent scavenge.
Typically, this happens when -Xms and -Xmx are not the same.
Old Expanded Full GC
Full GC necessary because the space reserved for metadata is full.Perm Full
Full GC necessary because intermediate space assigned to collection
using the train algorithm is full.
Train Full
Full GC performed when the garbage collector determines that space
for old objects is insufficient to support a successful scavenge. This
determination is reached without analyzing the heap.
Old Too Full
Full GC performed when the garbage collector determines that space
for old objects is insufficient to support a successful scavenge. This
determination is reached by analyzing the heap.
Old Full
The application calls method System.gc() to force a full garbage
collection.
System.gc
A full GC is performed as a result of a signal sent to the JVM to collect
a heap dump.
Heap Dump Triggered GC
A garbage collection is performed as a result of invoking the
ForceGarbageCollection function of JVMTI.
JVMTI force GC
A full garbage collection of unknown type and cause.Other Full GC
Mark the first pause during a concurrent collection cycle. Called the
initial mark, this identifies the initial set of live objects directly reachable
from the application code.
CMS First STW
Mark the second pause during a concurrent collection cycle. Called the
re-mark, this finalizes marking by revisiting any objects that were
modified during the concurrent marking phase.
CMS Second STW
See also Data Collection with -Xverbosegc (page 80).
For an explanation of Java memory terminology, see the Sun Developer Network publication
Memory Management in the Java HotSpot™ Virtual Machine (.pdf)
Understanding the Summary Presentation of GC Data
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.
88 Analyzing Garbage Collection Data