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

Garbage Collector command-line options

Use these Garbage Collector command-line options to control garbage collection.

You might need to read The collector and the allocator to understand some of the references that are given here.

The -verbose:gc option detailed in Verbose garbage collection logging is the main diagnostic aid that is available for runtime analysis of the Garbage Collector. However, additional command-line options are available that affect the behavior of the Garbage Collector and might aid diagnostics data collection.

For options that take a <size> parameter, suffix the number with "k" or "K" to indicate kilobytes, "m" or "M" to indicate megabytes, or "g" or "G" to indicate gigabytes.

For options that take a <percentage> parameter, use a number from 0 to 1, for example, 50% is 0.5.

• -Xalwaysclassgc
  Always perform dynamic class unloading checks during global collection.
• -Xclassgc
  Enables dynamic unloading of classes by the JVM.
• -Xcompactexplicitgc
  Enables full compaction each time System.gc() is called.
• -Xcompactgc
  Compacts on all garbage collections (system and global).
• -Xconcurrentbackground
  Specifies the number of low-priority background threads attached to assist the mutator threads in concurrent mark.
• -Xconcurrentlevel
  Specifies the allocation "tax" rate.
• -Xconcurrentslack
  Attempts to keep the specified amount of the heap space free in concurrent collectors by starting the concurrent operations earlier.
• -Xconmeter
  This option determines the usage of which area, LOA (Large Object Area) or SOA (Small Object Area), is metered and hence which allocations are taxed during concurrent mark.
• -Xdisableexcessivegc
  Disables the throwing of an OutOfMemory exception if excessive time is spent in the GC.
• -Xdisableexplicitgc
  Disables System.gc() calls.
• -Xdisablestringconstantgc
  Prevents strings in the string intern table from being collected.
• -Xenableexcessivegc
  If excessive time is spent in the GC, the option returns null for an allocate request and thus causes an OutOfMemory exception to be thrown.
• -Xenablestringconstantgc
  Enables strings from the string intern table to be collected.
• -Xgc
  Options that change the behavior of the Garbage Collector.
• -Xgc:splitheap (Windows 32-bit JVM only)
  Allocates the new and old areas of the generational Java heap in separate areas of memory.
• -Xgcpolicy
  Controls the behavior of the Garbage Collector.
• -Xgcthreads
  Sets the number of threads that the Garbage Collector uses for parallel operations.
• -Xgcworkpackets
  Specifies the total number of work packets available in the global collector.
• -Xloa
  Allocates a large object area (LOA).
• -Xloainitial
  Specifies the initial percentage (between 0 and 0.95) of the current tenure space allocated to the large object area (LOA).
• -Xloamaximum
  Specifies the maximum percentage (between 0 and 0.95) of the current tenure space allocated to the large object area (LOA).
• -Xloaminimum
  Specifies the minimum percentage (between 0 and 0.95) of the current tenure space allocated to the large object area (LOA).
• -Xmaxe
  Sets the maximum amount by which the garbage collector expands the heap.
• -Xmaxf
  Specifies the maximum percentage of heap that must be free after a garbage collection.
• -Xmaxt
  Specifies the maximum percentage of time to be spent in Garbage Collection.
• -Xmca
  Sets the expansion step for the memory allocated to store the RAM portion of loaded classes.
• -Xmco
  Sets the expansion step for the memory allocated to store the ROM portion of loaded classes.
• -Xmine
  Sets the minimum amount by which the Garbage Collector expands the heap.
• -Xminf
  Specifies the minimum percentage of heap to be left free after a garbage collection.
• -Xmint
  Specifies the minimum percentage of time to spend in Garbage Collection.
• -Xmn
  Sets the initial and maximum size of the new area to the specified value when using -Xgcpolicy:gencon.
• -Xmns
  Sets the initial size of the new area to the specified value when using -Xgcpolicy:gencon.
• -Xmnx
  Sets the maximum size of the new area to the specified value when using -Xgcpolicy:gencon.
• -Xmo
  Sets the initial and maximum size of the old (tenured) heap to the specified value when using -Xgcpolicy:gencon.
• -Xmoi
  Sets the amount the Java heap is incremented when using -Xgcpolicy:gencon.
• -Xmos
  Sets the initial size of the old (tenure) heap to the specified value when using -Xgcpolicy:gencon.
• -Xmox
  Sets the maximum size of the old (tenure) heap to the specified value when using -Xgcpolicy:gencon.
• -Xmr
  Sets the size of the Garbage Collection "remembered set".
• -Xmrx
  Sets the remembered maximum size setting.
• -Xms
  Sets the initial Java heap size.
• -Xmx
  Sets the maximum memory size (-Xmx >= -Xms).
• -Xnoclassgc
  Disables dynamic class unloading.
• -Xnocompactexplicitgc
  Disables compaction on System.gc() calls.
• -Xnocompactgc
  Disables compaction on all garbage collections (system or global).
• -Xnoloa
  Prevents allocation of a large object area; all objects are allocated in the SOA.
• -Xnuma:none
  Use this option to turn off Non-Uniform Memory Architecture (NUMA) awareness when using the balanced garbage collection policy.
• -Xsoftmx
  This option sets a "soft" maximum limit for the initial size of the Java heap.
• -Xsoftrefthreshold
  Sets the value used by the garbage collector to determine the number of garbage collections after which a soft reference is cleared if its referent has not been marked.
• -Xtgc
  Provides garbage collection tracing options.
• -Xverbosegclog
  Causes -verbose:gc output to be written to a specified file.

Parent: Command-line options

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