This section covers some additional JVM options:
When considering the JVM options covered in this section, be aware that their behavior is subject to change depending on the version, therefore the effect of each should be tested before being implemented in a production environment.
Compressed OOPs, covered briefly in Section 9.1, “32-bit vs. 64-bit JVM”
, implements a compression of the internal memory pointers within the JVM for objects and so reduces the heap. From JVM revision 1.6.0_20 and above compressed OOPs is on by default but if an earlier revision of the JDK is being used this option should be used for heap sizes 32GB and lower.
Aggressive optimizations, -XX:+AggressiveOpts, enables additional Hotspot JVM optimizations that are not enabled by default. The behavior enabled by this option is subject to change with each JVM release and so its effects should be tested prior to implementing it in production.
The final options are locking based options, -XX:+DoEscapeAnalysis, -XX:+UseBiasedLocking and -XX:+EliminateLocks. They are designed to work together to eliminate locking overhead. Their effect on performance is unpredictable for specific workloads and so require testing prior to being implemented. Reduced locking should improve concurrency and, on current multi-core hardware, improve throughput.
In summary, all these options should be considered on their merits. To accurately judge their impact on performance they should be tested independently of each other.