Java 7 for Absolute Beginners

CHAPTER 13 ■ GARBAGE COLLECTION

Table 13-2. Garbage Collection Settings

Switch Effect

-XX:NewRatio=3 Sets the ratio of objects in the young generation to objects in the tenured

generation. This is a key setting for optimization, which we'll explore later

in this chapter.

-XX:NewSize=32M Sets the minimum size of the new generation area. This is a key setting for

optimization, which we'll explore later in this chapter.

-XX:MaxNewSize=32M Sets the maximum size of the new generation area. This is a key setting for

optimization, which we'll explore later in this chapter.

-XX:MaxHeapFreeRatio=70 If the free memory after a scavenge or collection is less than the indicated

percentage of total available memory, set total available memory to that

value. This setting lets memory grow into the free space again. Its default

value is 70. Remember that the JVM grows or shrinks the heap at each

collection in order to preserve this ratio (within certain bounds).

-XX:MinHeapFreeRatio=40 If the free memory after a scavenge or collection is less than the indicated

percentage of total available memory, this sets the total available memory

to the value given in the argument. This setting keeps some memory

immediately available in situations where the garbage collector has

recovered a lot of memory. Its default value is 40.

-XX:MaxPermSize=64m Sets the size of the permanent generation. If your program has a lot of

classes, you may gain better performance by increasing this value. Its

default is 64M (that is, 64 megabytes).

-

XX:+ScavengeBeforeFullGC

Before trying to do a full collection, this scavenges first. If the result of the

scavenge puts free memory in the desired range, a full collection isn’t

done.

-XX:-UseParallelGC Uses parallel garbage collection for scavenges. Specifically, it uses one

thread per processor on the machine. As a rule, for a single processor

machine, this results in a loss of performance. On a two-processor

machine, it's roughly equivalent. On machines with more than two

processors, it's faster. The opposite setting is -XX:-UseSerialGC

-XX:-UseParallelOldGC (^) Uses parallel garbage collection for both scavenges and full collections.
The same rules for efficiency on multiple processors applies.
-XX:-UseSerialGC Uses serial garbage collection for both scavenges and full collections. This
setting is more efficient on machines with only one processor (or machines
with multiple processors that are running your Java program on just one
processor).