Migrating Java Applications to HP-UX

Java Heap Size and Garbage Collection Behavior

The behavior of the Java heap and garbage collector is critical to achieving good application

performance. Tuning the heap is easy, especially when using HPjmeter. When migrating a Java

application from another platform to HP-UX, make sure that the heap parameters are comparable to

your previous settings, and that you are using an equivalent or similar GC policy.

Garbage Collection in HP's Hotspot JVM

Basic Generational Garbage Collection in HP Hotspot

The following is a brief description of the basic garbage collector in the HP Hotspot JVM and the GC

policies supported.

The HP Hotspot JVM supports a generational garbage collector. The Java heap is divided into multiple

spaces or generations:

• New Space for newly created objects

• Old Space for longer-lived objects

• Perm Space for reflective data or "permanent" objects

The New Space is further divided into 3 regions: eden and two survivor spaces (to and from). Newly

created objects get allocated into the eden space. When eden becomes full, a scavenge GC is

triggered. Live objects in the eden space are identified and copied into the survivor space (to), and

garbage is removed from eden. To space then becomes the from space. The process starts again,

with newly created objects going into eden and scavenge GCs getting triggered when eden is full.

Live objects are then copied from eden and from into to. After objects have survived enough

scavenges, they get migrated to old space, or tenured. The tenuring threshold determines how many

scavenges an object survives before getting migrated to old space.

Eventually, when the old space is determined to be too full, a Full GC is performed. A Full GC

collects both the new and old spaces, examining all objects in the entire heap.

Scavenge GCs are usually quick, whereas Full GC's are very time-consuming. The principle behind

generational garbage collection is that many objects in an application are short-lived and will be

garbage-collected while in the New Space. Therefore, overall GC time is reduced for the most

common case.

GC Policies Supported in HP's Hotspot

There are multiple types of garbage collectors that operate on the different subspaces of the Java

heap.

• Basic "serial" collector (described in the previous section)

This collector is a single-threaded, stop-the-world collector. On the new space, a fast copying

collector is used to perform the scavenge GC. On the old space, a mark-compact copying

collector is used to perform the Full GC. The Full GC requires scanning and processing the

entire heap and is very time-consuming.

• Parallel scavenge of New space (high-throughput collector)

This collector uses multiple threads to perform the scavenge GC on the New space, thereby

reducing the total stop-the-world pause time. To enable parallel scavenge, use the JVM