HPjmeter 4.3 User's Guide

4. Click the file name that you want to compare against; or, if choosing more than one file, hold
the Ctrl key while selecting each desired file.
5. When the files are highlighted, click the Compare button.
A new viewer opens that presents the selected data sets in comparison to one another.
NOTE: For alternate access when you have multiple viewers open, click the button to bring the
console to the top. The console lists all open sessions and data sets. For data sets, click the data
representation to bring the already open viewer to the top.
If the viewer has been closed, but the data representation is still listed in the console, double-clicking
the data representation opens a new viewer.
Basic Garbage Collection Concepts
A basic principle behind the design of the garbage collector is that objects tend to be either
short-lived or else persist for the lifetime of an application run. By separating persistent objects
from short-lived objects and moving them to designated spaces, the garbage collector can free
memory for use by the application (improving efficiency of memory use), and avoid examining
every object each time a collection is done (reduce garbage collection overhead).
Through a system of identification and classification, an object ages each time it survives a garbage
collection event. After surviving a certain number of garbage collection events, the object is
considered old — at which point, it is moved from the young to the old area of the heap.
A scavenge is a garbage collection event where only short-lived, unused objects are collected from
the young heap area. Typically, scavenges are significantly faster than a full garbage collection,
which involves examining all objects in the entire heap.
Key to Garbage Collection Types Recognized by HPjmeter
HPjmeter reports numerous types of garbage collection. You may see references in HPjmeter to
GC types in data summaries or visualizers, so it helps to become familiar with them.
Table 13 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
Basic Garbage Collection Concepts 103