HP jmeter Software for -UX manual Understanding the Summary Presentation of GC Data

Table 6-5 Common Garbage Collection Types Recognized by HPjmeter

Concurrent Mark-Sweep (CMS)	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.
Parallel Scavenge	Only objects from the young generation are collected using a
	multi-threaded garbage collector.
Scavenge	Objects from the young generation only are collected using a single
	thread.
Scavenge during CMS	Collection in the young generation occurs when a CMS operation is set
	to collect in the old generation. The pauses for the young generation
	collection and the old generation collection occur independently and
	cannot overlap.
Incomplete Concurrent Mark-Sweep	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.
Old Expanded Full GC	Necessary when the old area is expanded on the most recent scavenge.
	Typically, this happens when -Xmsand -Xmxare not the same.
Perm Full	Full GC necessary because the space reserved for metadata is full.
Train Full	Full GC necessary because intermediate space assigned to collection
	using the train algorithm is full.
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 without analyzing the heap.
Old 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.
System.gc	The application calls method System.gc() to force a full garbage
	collection.
Heap Dump Triggered GC	A full GC is performed as a result of a signal sent to the JVM to collect
	a heap dump.
JVMTI force GC	A garbage collection is performed as a result of invoking the
	ForceGarbageCollection function of JVMTI.
Other Full GC	A full garbage collection of unknown type and cause.
CMS First STW	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 Second 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.

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 -Xverbosegcoption and displayed under the Summary tab. Important aspects of the summary data are defined below the image.

88 Analyzing Garbage Collection Data