IBM User Guide for Java V7 on Windows > Reference > Command-line options > Garbage Collector command-line options

-Xsoftmx

This option sets a "soft" maximum limit for the initial size of the Java™ heap.

-Xsoftmx<size>

Use the -Xmx option to set a "hard" limit for the maximum size of the heap. By default, -Xsoftmx is set to the same value as -Xmx. The value of -Xms must be less than, or equal to, the value of -Xsoftmx. See the introduction to this topic for more information about specifying <size> parameters.

You can set this option on the command line, then modify it at run time by using the MemoryMXBean.setMaxHeapSize() method in the com.ibm.lang.management API. By using this API, Java applications can dynamically monitor and adjust the heap size as required. This function can be useful in virtualized or cloud environments, for example, where the available memory might change dynamically to meet business needs. When you use the API, you must specify the value in bytes, such as

2147483648 instead of

2g.

For example, you might set the initial heap size to 1 GB and the maximum heap size to 8 GB. You might set a lower value, such as 2 GB, for -Xsoftmx, to limit the heap size that is used initially:

–Xms1g –Xsoftmx2g –Xmx8g

You can then use the com.ibm.lang.management API from within a Java application to increase the -Xsoftmx value during run time, as load increases. This change allows the application to use more memory than you specified initially.

If you reduce the -Xsoftmx value, the garbage collector attempts to respect the new limit. However, the ability to shrink the heap depends on a number of factors. There is no guarantee that a decrease in the heap size will occur. If or when the heap shrinks below the new limit, the heap will not grow beyond that limit.

When the heap shrinks, the garbage collector might release memory. The ability of the operating system to reclaim and use this memory varies based on the capabilities of the operating system.

Notes:

• When using -Xgcpolicy:gencon, -Xsoftmx applies only to the non-nursery portion of the heap. In some cases the heap grows above the -Xsoftmx value because the nursery portion grows, pushing the heap size above the limit set. See -Xmn for limiting the nursery size.
• When using -Xgcpolicy:metronome, -Xsoftmx is ignored because the Metronome garbage collector does not support contraction or expansion of the heap.

Parent: Garbage Collector command-line options

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