java.1

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.B \f[CB]\-XX:LogFile=\f[R]\f[I]path\f[R]
Sets the path and file name to where log data is written.
By default, the file is created in the current working directory, and
it\[aq]s named \f[CB]hotspot.log\f[R].
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\f[B]Linux and macOS:\f[R] The following example shows how to set the log
file to \f[CB]/var/log/java/hotspot.log\f[R]:
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\f[CB]\-XX:LogFile=/var/log/java/hotspot.log\f[R]
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\f[B]Windows:\f[R] The following example shows how to set the log file to
\f[CB]C:/log/java/hotspot.log\f[R]:
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\f[CB]\-XX:LogFile=C:/log/java/hotspot.log\f[R]
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.B \f[CB]\-XX:+PrintClassHistogram\f[R]
Enables printing of a class instance histogram after one of the
following events:
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\f[B]Linux and macOS:\f[R] \f[CB]Control+Break\f[R]
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\f[B]Windows:\f[R] \f[CB]Control+C\f[R] (\f[CB]SIGTERM\f[R])
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By default, this option is disabled.
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Setting this option is equivalent to running the \f[CB]jmap\ \-histo\f[R]
command, or the \f[CB]jcmd\f[R] \f[I]pid\f[R] \f[CB]GC.class_histogram\f[R]
command, where \f[I]pid\f[R] is the current Java process identifier.
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.B \f[CB]\-XX:+PrintConcurrentLocks\f[R]
Enables printing of \f[CB]java.util.concurrent\f[R] locks after one of the
following events:
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\f[B]Linux and macOS:\f[R] \f[CB]Control+Break\f[R]
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\f[B]Windows:\f[R] \f[CB]Control+C\f[R] (\f[CB]SIGTERM\f[R])
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By default, this option is disabled.
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Setting this option is equivalent to running the \f[CB]jstack\ \-l\f[R]
command or the \f[CB]jcmd\f[R] \f[I]pid\f[R] \f[CB]Thread.print\ \-l\f[R]
command, where \f[I]pid\f[R] is the current Java process identifier.
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.B \f[CB]\-XX:+PrintFlagsRanges\f[R]
Prints the range specified and allows automatic testing of the values.
See \f[B]Validate Java Virtual Machine Flag Arguments\f[R].
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.B \f[CB]\-XX:+PerfDataSaveToFile\f[R]
If enabled, saves \f[B]jstat\f[R] binary data when the Java application
exits.
This binary data is saved in a file named
\f[CB]hsperfdata_\f[R]\f[I]pid\f[R], where \f[I]pid\f[R] is the process
identifier of the Java application that you ran.
Use the \f[CB]jstat\f[R] command to display the performance data contained
in this file as follows:
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\f[CB]jstat\ \-class\ file:///\f[R]\f[I]path\f[R]\f[CB]/hsperfdata_\f[R]\f[I]pid\f[R]
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\f[CB]jstat\ \-gc\ file:///\f[R]\f[I]path\f[R]\f[CB]/hsperfdata_\f[R]\f[I]pid\f[R]
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.B \f[CB]\-XX:+UsePerfData\f[R]
Enables the \f[CB]perfdata\f[R] feature.
This option is enabled by default to allow JVM monitoring and
performance testing.
Disabling it suppresses the creation of the \f[CB]hsperfdata_userid\f[R]
directories.
To disable the \f[CB]perfdata\f[R] feature, specify
\f[CB]\-XX:\-UsePerfData\f[R].
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.SH ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA
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These \f[CB]java\f[R] options control how garbage collection (GC) is
performed by the Java HotSpot VM.
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.B \f[CB]\-XX:+AggressiveHeap\f[R]
Enables Java heap optimization.
This sets various parameters to be optimal for long\-running jobs with
intensive memory allocation, based on the configuration of the computer
(RAM and CPU).
By default, the option is disabled and the heap sizes are configured
less aggressively.
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.B \f[CB]\-XX:+AlwaysPreTouch\f[R]
Requests the VM to touch every page on the Java heap after requesting it
from the operating system and before handing memory out to the
application.
By default, this option is disabled and all pages are committed as the
application uses the heap space.
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.B \f[CB]\-XX:ConcGCThreads=\f[R]\f[I]threads\f[R]
Sets the number of threads used for concurrent GC.
Sets \f[I]\f[CI]threads\f[I]\f[R] to approximately 1/4 of the number of
parallel garbage collection threads.
The default value depends on the number of CPUs available to the JVM.
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For example, to set the number of threads for concurrent GC to 2,
specify the following option:
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\f[CB]\-XX:ConcGCThreads=2\f[R]
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.B \f[CB]\-XX:+DisableExplicitGC\f[R]
Enables the option that disables processing of calls to the
\f[CB]System.gc()\f[R] method.
This option is disabled by default, meaning that calls to
\f[CB]System.gc()\f[R] are processed.
If processing of calls to \f[CB]System.gc()\f[R] is disabled, then the JVM
still performs GC when necessary.
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.B \f[CB]\-XX:+ExplicitGCInvokesConcurrent\f[R]
Enables invoking of concurrent GC by using the \f[CB]System.gc()\f[R]
request.
This option is disabled by default and can be enabled only with the
\f[CB]\-XX:+UseG1GC\f[R] option.
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.B \f[CB]\-XX:G1AdaptiveIHOPNumInitialSamples=\f[R]\f[I]number\f[R]
When \f[CB]\-XX:UseAdaptiveIHOP\f[R] is enabled, this option sets the
number of completed marking cycles used to gather samples until G1
adaptively determines the optimum value of
\f[CB]\-XX:InitiatingHeapOccupancyPercent\f[R].
Before, G1 uses the value of
\f[CB]\-XX:InitiatingHeapOccupancyPercent\f[R] directly for this purpose.
The default value is 3.
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.B \f[CB]\-XX:G1HeapRegionSize=size\f[R]
Sets the size of the regions into which the Java heap is subdivided when
using the garbage\-first (G1) collector.
The value is a power of 2 and can range from 1 MB to 32 MB.
The default region size is determined ergonomically based on the heap
size with a goal of approximately 2048 regions.
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The following example sets the size of the subdivisions to 16 MB:
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\f[CB]\-XX:G1HeapRegionSize=16m\f[R]
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.B \f[CB]\-XX:G1HeapWastePercent=\f[R]\f[I]percent\f[R]
Sets the percentage of heap that you\[aq]re willing to waste.
The Java HotSpot VM doesn\[aq]t initiate the mixed garbage collection
cycle when the reclaimable percentage is less than the heap waste
percentage.
The default is 5 percent.
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.B \f[CB]\-XX:G1MaxNewSizePercent=\f[R]\f[I]percent\f[R]
Sets the percentage of the heap size to use as the maximum for the young
generation size.
The default value is 60 percent of your Java heap.
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This is an experimental flag.
This setting replaces the \f[CB]\-XX:DefaultMaxNewGenPercent\f[R] setting.
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.B \f[CB]\-XX:G1MixedGCCountTarget=\f[R]\f[I]number\f[R]
Sets the target number of mixed garbage collections after a marking
cycle to collect old regions with at most
\f[CB]G1MixedGCLIveThresholdPercent\f[R] live data.
The default is 8 mixed garbage collections.
The goal for mixed collections is to be within this target number.
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.B \f[CB]\-XX:G1MixedGCLiveThresholdPercent=\f[R]\f[I]percent\f[R]
Sets the occupancy threshold for an old region to be included in a mixed
garbage collection cycle.
The default occupancy is 85 percent.
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This is an experimental flag.
This setting replaces the
\f[CB]\-XX:G1OldCSetRegionLiveThresholdPercent\f[R] setting.
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.B \f[CB]\-XX:G1NewSizePercent=\f[R]\f[I]percent\f[R]
Sets the percentage of the heap to use as the minimum for the young
generation size.
The default value is 5 percent of your Java heap.
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This is an experimental flag.
This setting replaces the \f[CB]\-XX:DefaultMinNewGenPercent\f[R] setting.
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.B \f[CB]\-XX:G1OldCSetRegionThresholdPercent=\f[R]\f[I]percent\f[R]
Sets an upper limit on the number of old regions to be collected during
a mixed garbage collection cycle.
The default is 10 percent of the Java heap.
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.B \f[CB]\-XX:G1ReservePercent=\f[R]\f[I]percent\f[R]
Sets the percentage of the heap (0 to 50) that\[aq]s reserved as a false
ceiling to reduce the possibility of promotion failure for the G1
collector.
When you increase or decrease the percentage, ensure that you adjust the
total Java heap by the same amount.
By default, this option is set to 10%.
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The following example sets the reserved heap to 20%:
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\f[CB]\-XX:G1ReservePercent=20\f[R]
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.B \f[CB]\-XX:+G1UseAdaptiveIHOP\f[R]
Controls adaptive calculation of the old generation occupancy to start
background work preparing for an old generation collection.
If enabled, G1 uses \f[CB]\-XX:InitiatingHeapOccupancyPercent\f[R] for the
first few times as specified by the value of
\f[CB]\-XX:G1AdaptiveIHOPNumInitialSamples\f[R], and after that adaptively
calculates a new optimum value for the initiating occupancy
automatically.
Otherwise, the old generation collection process always starts at the
old generation occupancy determined by
\f[CB]\-XX:InitiatingHeapOccupancyPercent\f[R].
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The default is enabled.
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.B \f[CB]\-XX:InitialHeapSize=\f[R]\f[I]size\f[R]
Sets the initial size (in bytes) of the memory allocation pool.
This value must be either 0, or a multiple of 1024 and greater than 1
MB.
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate kilobytes,
\f[CB]m\f[R] or \f[CB]M\f[R] to indicate megabytes, or \f[CB]g\f[R] or
\f[CB]G\f[R] to indicate gigabytes.
The default value is selected at run time based on the system
configuration.
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The following examples show how to set the size of allocated memory to 6
MB using various units:
.IP
.nf
\f[CB]
\-XX:InitialHeapSize=6291456
\-XX:InitialHeapSize=6144k
\-XX:InitialHeapSize=6m
\f[R]
.fi
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If you set this option to 0, then the initial size is set as the sum of
the sizes allocated for the old generation and the young generation.
The size of the heap for the young generation can be set using the
\f[CB]\-XX:NewSize\f[R] option.
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.B \f[CB]\-XX:InitialRAMPercentage=\f[R]\f[I]percent\f[R]
Sets the initial amount of memory that the JVM will use for the Java
heap before applying ergonomics heuristics as a percentage of the
maximum amount determined as described in the \f[CB]\-XX:MaxRAM\f[R]
option.
The default value is 1.5625 percent.
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The following example shows how to set the percentage of the initial
amount of memory used for the Java heap:
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\f[CB]\-XX:InitialRAMPercentage=5\f[R]
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.B \f[CB]\-XX:InitialSurvivorRatio=\f[R]\f[I]ratio\f[R]
Sets the initial survivor space ratio used by the throughput garbage
collector (which is enabled by the \f[CB]\-XX:+UseParallelGC\f[R] option).
Adaptive sizing is enabled by default with the throughput garbage
collector by using the \f[CB]\-XX:+UseParallelGC\f[R] option, and the
survivor space is resized according to the application behavior,
starting with the initial value.
If adaptive sizing is disabled (using the
\f[CB]\-XX:\-UseAdaptiveSizePolicy\f[R] option), then the
\f[CB]\-XX:SurvivorRatio\f[R] option should be used to set the size of the
survivor space for the entire execution of the application.
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The following formula can be used to calculate the initial size of
survivor space (S) based on the size of the young generation (Y), and
the initial survivor space ratio (R):
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\f[CB]S=Y/(R+2)\f[R]
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The 2 in the equation denotes two survivor spaces.
The larger the value specified as the initial survivor space ratio, the
smaller the initial survivor space size.
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By default, the initial survivor space ratio is set to 8.
If the default value for the young generation space size is used (2 MB),
then the initial size of the survivor space is 0.2 MB.
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The following example shows how to set the initial survivor space ratio
to 4:
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\f[CB]\-XX:InitialSurvivorRatio=4\f[R]
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.B \f[CB]\-XX:InitiatingHeapOccupancyPercent=\f[R]\f[I]percent\f[R]
Sets the percentage of the old generation occupancy (0 to 100) at which
to start the first few concurrent marking cycles for the G1 garbage
collector.
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By default, the initiating value is set to 45%.
A value of 0 implies nonstop concurrent GC cycles from the beginning
until G1 adaptively sets this value.
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See also the \f[CB]\-XX:G1UseAdaptiveIHOP\f[R] and
\f[CB]\-XX:G1AdaptiveIHOPNumInitialSamples\f[R] options.
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The following example shows how to set the initiating heap occupancy to
75%:
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\f[CB]\-XX:InitiatingHeapOccupancyPercent=75\f[R]
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.B \f[CB]\-XX:MaxGCPauseMillis=\f[R]\f[I]time\f[R]
Sets a target for the maximum GC pause time (in milliseconds).
This is a soft goal, and the JVM will make its best effort to achieve
it.
The specified value doesn\[aq]t adapt to your heap size.
By default, for G1 the maximum pause time target is 200 milliseconds.
The other generational collectors do not use a pause time goal by
default.
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The following example shows how to set the maximum target pause time to
500 ms:
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\f[CB]\-XX:MaxGCPauseMillis=500\f[R]
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.B \f[CB]\-XX:MaxHeapSize=\f[R]\f[I]size\f[R]
Sets the maximum size (in byes) of the memory allocation pool.
This value must be a multiple of 1024 and greater than 2 MB.
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate kilobytes,
\f[CB]m\f[R] or \f[CB]M\f[R] to indicate megabytes, or \f[CB]g\f[R] or
\f[CB]G\f[R] to indicate gigabytes.
The default value is selected at run time based on the system
configuration.
For server deployments, the options \f[CB]\-XX:InitialHeapSize\f[R] and
\f[CB]\-XX:MaxHeapSize\f[R] are often set to the same value.
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The following examples show how to set the maximum allowed size of
allocated memory to 80 MB using various units:
.IP
.nf
\f[CB]
\-XX:MaxHeapSize=83886080
\-XX:MaxHeapSize=81920k
\-XX:MaxHeapSize=80m
\f[R]
.fi
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The \f[CB]\-XX:MaxHeapSize\f[R] option is equivalent to \f[CB]\-Xmx\f[R].
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.B \f[CB]\-XX:MaxHeapFreeRatio=\f[R]\f[I]percent\f[R]
Sets the maximum allowed percentage of free heap space (0 to 100) after
a GC event.
If free heap space expands above this value, then the heap is shrunk.
By default, this value is set to 70%.
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Minimize the Java heap size by lowering the values of the parameters
\f[CB]MaxHeapFreeRatio\f[R] (default value is 70%) and
\f[CB]MinHeapFreeRatio\f[R] (default value is 40%) with the command\-line
options \f[CB]\-XX:MaxHeapFreeRatio\f[R] and
\f[CB]\-XX:MinHeapFreeRatio\f[R].
Lowering \f[CB]MaxHeapFreeRatio\f[R] to as low as 10% and
\f[CB]MinHeapFreeRatio\f[R] to 5% has successfully reduced the heap size
without too much performance regression; however, results may vary
greatly depending on your application.
Try different values for these parameters until they\[aq]re as low as
possible yet still retain acceptable performance.
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\f[CB]\-XX:MaxHeapFreeRatio=10\ \-XX:MinHeapFreeRatio=5\f[R]
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Customers trying to keep the heap small should also add the option
\f[CB]\-XX:\-ShrinkHeapInSteps\f[R].
See \f[B]Performance Tuning Examples\f[R] for a description of using this
option to keep the Java heap small by reducing the dynamic footprint for
embedded applications.
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.B \f[CB]\-XX:MaxMetaspaceSize=\f[R]\f[I]size\f[R]
Sets the maximum amount of native memory that can be allocated for class
metadata.
By default, the size isn\[aq]t limited.
The amount of metadata for an application depends on the application
itself, other running applications, and the amount of memory available
on the system.
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The following example shows how to set the maximum class metadata size
to 256 MB:
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\f[CB]\-XX:MaxMetaspaceSize=256m\f[R]
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.B \f[CB]\-XX:MaxNewSize=\f[R]\f[I]size\f[R]
Sets the maximum size (in bytes) of the heap for the young generation
(nursery).
The default value is set ergonomically.
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.B \f[CB]\-XX:MaxRAM=\f[R]\f[I]size\f[R]
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics.
The default value is the maximum amount of available memory to the JVM
process or 128 GB, whichever is lower.
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The maximum amount of available memory to the JVM process is the minimum
of the machine\[aq]s physical memory and any constraints set by the
environment (e.g.
container).
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Specifying this option disables automatic use of compressed oops if the
combined result of this and other options influencing the maximum amount
of memory is larger than the range of memory addressable by compressed
oops.
See \f[CB]\-XX:UseCompressedOops\f[R] for further information about
compressed oops.
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The following example shows how to set the maximum amount of available
memory for sizing the Java heap to 2 GB:
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\f[CB]\-XX:MaxRAM=2G\f[R]
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.B \f[CB]\-XX:MaxRAMPercentage=\f[R]\f[I]percent\f[R]
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a percentage of the maximum
amount determined as described in the \f[CB]\-XX:MaxRAM\f[R] option.
The default value is 25 percent.
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Specifying this option disables automatic use of compressed oops if the
combined result of this and other options influencing the maximum amount
of memory is larger than the range of memory addressable by compressed
oops.
See \f[CB]\-XX:UseCompressedOops\f[R] for further information about
compressed oops.
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The following example shows how to set the percentage of the maximum
amount of memory used for the Java heap:
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\f[CB]\-XX:MaxRAMPercentage=75\f[R]
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.B \f[CB]\-XX:MinRAMPercentage=\f[R]\f[I]percent\f[R]
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a percentage of the maximum
amount determined as described in the \f[CB]\-XX:MaxRAM\f[R] option for
small heaps.
A small heap is a heap of approximately 125 MB.
The default value is 50 percent.
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The following example shows how to set the percentage of the maximum
amount of memory used for the Java heap for small heaps:
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\f[CB]\-XX:MinRAMPercentage=75\f[R]
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.B \f[CB]\-XX:MaxTenuringThreshold=\f[R]\f[I]threshold\f[R]
Sets the maximum tenuring threshold for use in adaptive GC sizing.
The largest value is 15.
The default value is 15 for the parallel (throughput) collector.
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The following example shows how to set the maximum tenuring threshold to
10:
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\f[CB]\-XX:MaxTenuringThreshold=10\f[R]
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.B \f[CB]\-XX:MetaspaceSize=\f[R]\f[I]size\f[R]
Sets the size of the allocated class metadata space that triggers a
garbage collection the first time it\[aq]s exceeded.
This threshold for a garbage collection is increased or decreased
depending on the amount of metadata used.
The default size depends on the platform.
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.B \f[CB]\-XX:MinHeapFreeRatio=\f[R]\f[I]percent\f[R]
Sets the minimum allowed percentage of free heap space (0 to 100) after
a GC event.
If free heap space falls below this value, then the heap is expanded.
By default, this value is set to 40%.
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Minimize Java heap size by lowering the values of the parameters
\f[CB]MaxHeapFreeRatio\f[R] (default value is 70%) and
\f[CB]MinHeapFreeRatio\f[R] (default value is 40%) with the command\-line
options \f[CB]\-XX:MaxHeapFreeRatio\f[R] and
\f[CB]\-XX:MinHeapFreeRatio\f[R].
Lowering \f[CB]MaxHeapFreeRatio\f[R] to as low as 10% and
\f[CB]MinHeapFreeRatio\f[R] to 5% has successfully reduced the heap size
without too much performance regression; however, results may vary
greatly depending on your application.
Try different values for these parameters until they\[aq]re as low as
possible, yet still retain acceptable performance.
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\f[CB]\-XX:MaxHeapFreeRatio=10\ \-XX:MinHeapFreeRatio=5\f[R]
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.PP
Customers trying to keep the heap small should also add the option
\f[CB]\-XX:\-ShrinkHeapInSteps\f[R].
See \f[B]Performance Tuning Examples\f[R] for a description of using this
option to keep the Java heap small by reducing the dynamic footprint for
embedded applications.
.RE
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.B \f[CB]\-XX:MinHeapSize=\f[R]\f[I]size\f[R]
Sets the minimum size (in bytes) of the memory allocation pool.
This value must be either 0, or a multiple of 1024 and greater than 1
MB.
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate kilobytes,
\f[CB]m\f[R] or \f[CB]M\f[R] to indicate megabytes, or \f[CB]g\f[R] or
\f[CB]G\f[R] to indicate gigabytes.
The default value is selected at run time based on the system
configuration.
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The following examples show how to set the mimimum size of allocated
memory to 6 MB using various units:
.IP
.nf
\f[CB]
\-XX:MinHeapSize=6291456
\-XX:MinHeapSize=6144k
\-XX:MinHeapSize=6m
\f[R]
.fi
.PP
If you set this option to 0, then the minimum size is set to the same
value as the initial size.
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.B \f[CB]\-XX:NewRatio=\f[R]\f[I]ratio\f[R]
Sets the ratio between young and old generation sizes.
By default, this option is set to 2.
The following example shows how to set the young\-to\-old ratio to 1:
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\f[CB]\-XX:NewRatio=1\f[R]
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.B \f[CB]\-XX:NewSize=\f[R]\f[I]size\f[R]
Sets the initial size (in bytes) of the heap for the young generation
(nursery).
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate kilobytes,
\f[CB]m\f[R] or \f[CB]M\f[R] to indicate megabytes, or \f[CB]g\f[R] or
\f[CB]G\f[R] to indicate gigabytes.
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The young generation region of the heap is used for new objects.
GC is performed in this region more often than in other regions.
If the size for the young generation is too low, then a large number of
minor GCs are performed.
If the size is too high, then only full GCs are performed, which can
take a long time to complete.
It is recommended that you keep the size for the young generation
greater than 25% and less than 50% of the overall heap size.
.PP
The following examples show how to set the initial size of the young
generation to 256 MB using various units:
.IP
.nf
\f[CB]
\-XX:NewSize=256m
\-XX:NewSize=262144k
\-XX:NewSize=268435456
\f[R]
.fi
.PP
The \f[CB]\-XX:NewSize\f[R] option is equivalent to \f[CB]\-Xmn\f[R].
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.B \f[CB]\-XX:ParallelGCThreads=\f[R]\f[I]threads\f[R]
Sets the number of the stop\-the\-world (STW) worker threads.
The default value depends on the number of CPUs available to the JVM and
the garbage collector selected.
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For example, to set the number of threads for G1 GC to 2, specify the
following option:
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\f[CB]\-XX:ParallelGCThreads=2\f[R]
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.B \f[CB]\-XX:+ParallelRefProcEnabled\f[R]
Enables parallel reference processing.
By default, this option is disabled.
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.B \f[CB]\-XX:+PrintAdaptiveSizePolicy\f[R]
Enables printing of information about adaptive\-generation sizing.
By default, this option is disabled.
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.B \f[CB]\-XX:+ScavengeBeforeFullGC\f[R]
Enables GC of the young generation before each full GC.
This option is enabled by default.
It is recommended that you \f[I]don\[aq]t\f[R] disable it, because
scavenging the young generation before a full GC can reduce the number
of objects reachable from the old generation space into the young
generation space.
To disable GC of the young generation before each full GC, specify the
option \f[CB]\-XX:\-ScavengeBeforeFullGC\f[R].
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.B \f[CB]\-XX:SoftRefLRUPolicyMSPerMB=\f[R]\f[I]time\f[R]
Sets the amount of time (in milliseconds) a softly reachable object is
kept active on the heap after the last time it was referenced.
The default value is one second of lifetime per free megabyte in the
heap.
The \f[CB]\-XX:SoftRefLRUPolicyMSPerMB\f[R] option accepts integer values
representing milliseconds per one megabyte of the current heap size (for
Java HotSpot Client VM) or the maximum possible heap size (for Java
HotSpot Server VM).
This difference means that the Client VM tends to flush soft references
rather than grow the heap, whereas the Server VM tends to grow the heap
rather than flush soft references.
In the latter case, the value of the \f[CB]\-Xmx\f[R] option has a
significant effect on how quickly soft references are garbage collected.
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The following example shows how to set the value to 2.5 seconds:
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\f[CB]\-XX:SoftRefLRUPolicyMSPerMB=2500\f[R]
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.B \f[CB]\-XX:\-ShrinkHeapInSteps\f[R]
Incrementally reduces the Java heap to the target size, specified by the
option \f[CB]\-XX:MaxHeapFreeRatio\f[R].
This option is enabled by default.
If disabled, then it immediately reduces the Java heap to the target
size instead of requiring multiple garbage collection cycles.
Disable this option if you want to minimize the Java heap size.
You will likely encounter performance degradation when this option is
disabled.
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See \f[B]Performance Tuning Examples\f[R] for a description of using the
\f[CB]MaxHeapFreeRatio\f[R] option to keep the Java heap small by reducing
the dynamic footprint for embedded applications.
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.B \f[CB]\-XX:StringDeduplicationAgeThreshold=\f[R]\f[I]threshold\f[R]
Identifies \f[CB]String\f[R] objects reaching the specified age that are
considered candidates for deduplication.
An object\[aq]s age is a measure of how many times it has survived
garbage collection.
This is sometimes referred to as tenuring.
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\f[B]Note:\f[R] \f[CB]String\f[R] objects that are promoted to an old heap
region before this age has been reached are always considered candidates
for deduplication.
The default value for this option is \f[CB]3\f[R].
See the \f[CB]\-XX:+UseStringDeduplication\f[R] option.
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.B \f[CB]\-XX:SurvivorRatio=\f[R]\f[I]ratio\f[R]
Sets the ratio between eden space size and survivor space size.
By default, this option is set to 8.
The following example shows how to set the eden/survivor space ratio to
4:
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\f[CB]\-XX:SurvivorRatio=4\f[R]
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.B \f[CB]\-XX:TargetSurvivorRatio=\f[R]\f[I]percent\f[R]
Sets the desired percentage of survivor space (0 to 100) used after
young garbage collection.
By default, this option is set to 50%.
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The following example shows how to set the target survivor space ratio
to 30%:
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\f[CB]\-XX:TargetSurvivorRatio=30\f[R]
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.B \f[CB]\-XX:TLABSize=\f[R]\f[I]size\f[R]
Sets the initial size (in bytes) of a thread\-local allocation buffer
(TLAB).
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate kilobytes,
\f[CB]m\f[R] or \f[CB]M\f[R] to indicate megabytes, or \f[CB]g\f[R] or
\f[CB]G\f[R] to indicate gigabytes.
If this option is set to 0, then the JVM selects the initial size
automatically.
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The following example shows how to set the initial TLAB size to 512 KB:
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\f[CB]\-XX:TLABSize=512k\f[R]
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.B \f[CB]\-XX:+UseAdaptiveSizePolicy\f[R]
Enables the use of adaptive generation sizing.
This option is enabled by default.
To disable adaptive generation sizing, specify
\f[CB]\-XX:\-UseAdaptiveSizePolicy\f[R] and set the size of the memory
allocation pool explicitly.
See the \f[CB]\-XX:SurvivorRatio\f[R] option.
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.B \f[CB]\-XX:+UseG1GC\f[R]
Enables the use of the garbage\-first (G1) garbage collector.
It\[aq]s a server\-style garbage collector, targeted for multiprocessor
machines with a large amount of RAM.
This option meets GC pause time goals with high probability, while
maintaining good throughput.
The G1 collector is recommended for applications requiring large heaps
(sizes of around 6 GB or larger) with limited GC latency requirements (a
stable and predictable pause time below 0.5 seconds).
By default, this option is enabled and G1 is used as the default garbage
collector.
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.B \f[CB]\-XX:+UseGCOverheadLimit\f[R]
Enables the use of a policy that limits the proportion of time spent by
the JVM on GC before an \f[CB]OutOfMemoryError\f[R] exception is thrown.
This option is enabled, by default, and the parallel GC will throw an
\f[CB]OutOfMemoryError\f[R] if more than 98% of the total time is spent on
garbage collection and less than 2% of the heap is recovered.
When the heap is small, this feature can be used to prevent applications
from running for long periods of time with little or no progress.
To disable this option, specify the option
\f[CB]\-XX:\-UseGCOverheadLimit\f[R].
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.B \f[CB]\-XX:+UseNUMA\f[R]
Enables performance optimization of an application on a machine with
nonuniform memory architecture (NUMA) by increasing the
application\[aq]s use of lower latency memory.
By default, this option is disabled and no optimization for NUMA is
made.
The option is available only when the parallel garbage collector is used
(\f[CB]\-XX:+UseParallelGC\f[R]).
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.B \f[CB]\-XX:+UseParallelGC\f[R]
Enables the use of the parallel scavenge garbage collector (also known
as the throughput collector) to improve the performance of your
application by leveraging multiple processors.
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By default, this option is disabled and the default collector is used.
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.B \f[CB]\-XX:+UseSerialGC\f[R]
Enables the use of the serial garbage collector.
This is generally the best choice for small and simple applications that
don\[aq]t require any special functionality from garbage collection.
By default, this option is disabled and the default collector is used.
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.B \f[CB]\-XX:+UseSHM\f[R]
\f[B]Linux only:\f[R] Enables the JVM to use shared memory to set up
large pages.
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See \f[B]Large Pages\f[R] for setting up large pages.
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.B \f[CB]\-XX:+UseStringDeduplication\f[R]
Enables string deduplication.
By default, this option is disabled.
To use this option, you must enable the garbage\-first (G1) garbage
collector.
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String deduplication reduces the memory footprint of \f[CB]String\f[R]
objects on the Java heap by taking advantage of the fact that many
\f[CB]String\f[R] objects are identical.
Instead of each \f[CB]String\f[R] object pointing to its own character
array, identical \f[CB]String\f[R] objects can point to and share the same
character array.
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.B \f[CB]\-XX:+UseTLAB\f[R]
Enables the use of thread\-local allocation blocks (TLABs) in the young
generation space.
This option is enabled by default.
To disable the use of TLABs, specify the option \f[CB]\-XX:\-UseTLAB\f[R].
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.B \f[CB]\-XX:+UseZGC\f[R]
Enables the use of the Z garbage collector (ZGC).
This is a low latency garbage collector, providing max pause times of a
few milliseconds, at some throughput cost.
Pause times are independent of what heap size is used.
Supports heap sizes from 8MB to 16TB.
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.B \f[CB]\-XX:ZAllocationSpikeTolerance\f[R]=\f[I]factor\f[R]
Sets the allocation spike tolerance for ZGC.
By default, this option is set to 2.0.
This factor describes the level of allocation spikes to expect.
For example, using a factor of 3.0 means the current allocation rate can
be expected to triple at any time.
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.B \f[CB]\-XX:ZCollectionInterval\f[R]=\f[I]seconds\f[R]
Sets the maximum interval (in seconds) between two GC cycles when using
ZGC.
By default, this option is set to 0 (disabled).
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.B \f[CB]\-XX:ZFragmentationLimit\f[R]=\f[I]percent\f[R]
Sets the maximum acceptable heap fragmentation (in percent) for ZGC.
By default, this option is set to 25.
Using a lower value will cause the heap to be compacted more
aggressively, to reclaim more memory at the cost of using more CPU time.
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.B \f[CB]\-XX:+ZProactive\f[R]
Enables proactive GC cycles when using ZGC.
By default, this option is enabled.
ZGC will start a proactive GC cycle if doing so is expected to have
minimal impact on the running application.
This is useful if the application is mostly idle or allocates very few
objects, but you still want to keep the heap size down and allow
reference processing to happen even when there are a lot of free space
on the heap.
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.B \f[CB]\-XX:+ZUncommit\f[R]
Enables uncommitting of unused heap memory when using ZGC.
By default, this option is enabled.
Uncommitting unused heap memory will lower the memory footprint of the
JVM, and make that memory available for other processes to use.
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.B \f[CB]\-XX:ZUncommitDelay\f[R]=\f[I]seconds\f[R]
Sets the amount of time (in seconds) that heap memory must have been
unused before being uncommitted.
By default, this option is set to 300 (5 minutes).
Committing and uncommitting memory are relatively expensive operations.
Using a lower value will cause heap memory to be uncommitted earlier, at
the risk of soon having to commit it again.
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.SH DEPRECATED JAVA OPTIONS
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These \f[CB]java\f[R] options are deprecated and might be removed in a
future JDK release.
They\[aq]re still accepted and acted upon, but a warning is issued when
they\[aq]re used.
.TP
.B \f[CB]\-Xfuture\f[R]
Enables strict class\-file format checks that enforce close conformance
to the class\-file format specification.
Developers should use this flag when developing new code.
Stricter checks may become the default in future releases.
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.B \f[CB]\-Xloggc:\f[R]\f[I]filename\f[R]
Sets the file to which verbose GC events information should be
redirected for logging.
The \f[CB]\-Xloggc\f[R] option overrides \f[CB]\-verbose:gc\f[R] if both are
given with the same java command.
\f[CB]\-Xloggc:\f[R]\f[I]filename\f[R] is replaced by
\f[CB]\-Xlog:gc:\f[R]\f[I]filename\f[R].
See Enable Logging with the JVM Unified Logging Framework.
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Example:
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\f[CB]\-Xlog:gc:garbage\-collection.log\f[R]
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.B \f[CB]\-XX:+FlightRecorder\f[R]
Enables the use of Java Flight Recorder (JFR) during the runtime of the
application.
Since JDK 8u40 this option has not been required to use JFR.
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.B \f[CB]\-XX:InitialRAMFraction=\f[R]\f[I]ratio\f[R]
Sets the initial amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a ratio of the maximum amount
determined as described in the \f[CB]\-XX:MaxRAM\f[R] option.
The default value is 64.
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Use the option \f[CB]\-XX:InitialRAMPercentage\f[R] instead.
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.B \f[CB]\-XX:MaxRAMFraction=\f[R]\f[I]ratio\f[R]
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a fraction of the maximum
amount determined as described in the \f[CB]\-XX:MaxRAM\f[R] option.
The default value is 4.
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