IBM JDK 1.4.2 heap

Heap size

The maximum heap size is controlled by the -Xmx option. If this option is not

specified, the default applies as follows:

Windows®®

Half the real storage with a minimum of 16 MB and a maximum of

2 GB -1.

OS/390®® and AIX®

64 MB.

Linux Half the real storage with a minimum of 16 MB and a maximum of

512 MB -1.

The initial size of the heap is controlled by the -Xms option. If this option is not

specified, the default applies as follows:

Windows, AIX, and Linux

4 MB

OS/390

1 MB

Some basic heap sizing problems

For the majority of applications, the default settings work well. The heap expands

until it reaches a steady state, then remains in that state, which should give a heap

occupancy (the amount of live data on the heap at any given time) of 70%. At this

level, the frequency and pause time of garbage collection should be acceptable.

For some applications, the default settings might not give the best results. Listed

here, are some problems that might occur, and some suggested actions that you

can take. Use verbosegc to help you monitor the heap.

The frequency of garbage collections is too high until the heap reaches a steady

state.

Use verbosegc to determine the size of the heap at a steady state and set -Xms

to this value.

The heap is fully expanded and the occupancy level is greater than 70%.

Increase the -Xmx value so that the heap is not more than 70% occupied, but

for best performance try to ensure that the heap never pages. The maximum

heap size should, if possible, be able to be contained in physical memory.

At 70% occupancy the frequency of garbage collections is too great.

Change the setting of -Xminf. The default is 0.3, which tries to maintain 30%

free space by expanding the heap. A setting of 0.4, for example, increases this

free space target to 40%, and reduces the frequency of garbage collections.

Pause times are too long.

Try using -Xgcpolicy:optavgpause. This reduces the pause times and makes

them more consistent when the heap occupancy rises. It does, however, reduce

throughput by approximately 5%, although this value varies with different

applications.

overview of garbage collection

Chapter 2. Understanding the Garbage Collector 9

Here are some useful tips:

v Ensure that the heap never pages; that is, the maximum heap size must be able

to be contained in physical memory.

v Avoid finalizers. You cannot guarantee when a finalizer will run, and often they

cause problems. If you do use finalizers, try to avoid allocating objects in the

finalizer method. A verbosegc trace shows whether finalizers are being called.

v Avoid compaction. A verbosegc trace shows whether compaction is occurring.

Compaction is usually caused by requests for large memory allocations. Analyze

requests for large memory allocations and avoid them if possible. If they are

large arrays, for example, try to split them into smaller arrays.