JVM Flags

Remarks#

It is strongly recommended that you use these options only:

• If you have a thorough understanding of your system.
• Are aware that, if used improperly, these options can have negative effect on the stability or performance of your system.

Information gathered from official Java documentation.

-XXaggressive

-XXaggressive is a collection of configurations that make the JVM perform at a high speed and reach a stable state as soon as possible. To achieve this goal, the JVM uses more internal resources at startup; however, it requires less adaptive optimization once the goal is reached. We recommend that you use this option for long-running, memory-intensive applications that work alone.

Usage:

-XXaggressive:<param>

-XXallocClearChunks

This option allows you to clear a TLA for references and values at TLA allocation time and pre-fetch the next chunk. When an integer, a reference, or anything else is declared, it has a default value of 0 or null (depending upon type). At the appropriate time, you will need to clear these references and values to free the memory on the heap so Java can use- or reuse- it. You can do either when the object is allocated or, by using this option, when you request a new TLA.

Usage:

-XXallocClearChunks

-XXallocClearChunks=<true | false>

The above is a boolean option and is generally recommended on IA64 systems; ultimately, its use depends upon the application. If you want to set the size of chunks cleared, combine this option with -XXallocClearChunkSize. If you use this flag but do not specify a boolean value, the default is true.

-XXallocClearChunkSize

When used with -XXallocClearChunkSize, this option sets the size of the chunks to be cleared. If this flag is used but no value is specified, the default is 512 bytes.

Usage:

-XXallocClearChunks -XXallocClearChunkSize=<size>[k|K][m|M][g|G]

-XXcallProfiling

This option enables the use of call profiling for code optimizations. Profiling records useful runtime statistics specific to the application and can—in many cases—increase performance because JVM can then act on those statistics.

Note: This option is supported with the JRockit JVM R27.3.0 and later version. It may become default in future versions.

Usage:

java -XXcallProfiling myApp

This option is disabled by default. You must enable it to use it.

-XXdisableFatSpin

This option disables the fat lock spin code in Java, allowing threads that block trying to acquire a fat lock go to sleep directly.

Objects in Java become a lock as soon as any thread enters a synchronized block on that object. All locks are held (that is, stayed locked) until released by the locking thread. If the lock is not going to be released very fast, it can be inflated to a “fat lock.” “Spinning” occurs when a thread that wants a specific lock continuously checks that lock to see if it is still taken, spinning in a tight loop as it makes the check. Spinning against a fat lock is generally beneficial although, in some instances, it can be expensive and might affect performance. -XXdisableFatSpin allows you to turn off spinning against a fat lock and eliminate the potential performance hit.

Usage:

-XXdisableFatSpin

-XXdisableGCHeuristics

This option disables the garbage collector strategy changes. Compaction heuristics and nursery size heuristics are not affected by this option. By default, the garbage collection heuristics are enabled.

Usage:

-XXdisableFatSpin

-XXdumpSize

This option causes a dump file to be generated and allows you to specify the relative size of that file (that is, small, medium, or large).

Usage:

-XXdumpsize:<size>

-XXexitOnOutOfMemory

This option makes JRockit JVM exit on the first occurrence of an out of memory error. It can be used if you prefer restarting an instance of JRockit JVM rather than handling out of memory errors. Enter this command at startup to force JRockit JVM to exit on the first occurrence of an out of memory error.

Usage:

-XXexitOnOutOfMemory