JVM Arguments

2 minute read

  • -XX means it’s an advanced option
  • a plus or a minus indicates the option to be switched on or off
  • and then the option name in SentenceCase

Jshell

Pass in the PID for the running Java progam along with the flag to know its current settings. To find the process id for the java program run jps

❯ jps
❯ jinfo -flag NewRatio 46615
-XX:NewRatio=2

❯ jinfo -flag SurvivorRatio 46615
-XX:SurvivorRatio=8

Get all the possible flags/options -XX:+PrintFlagsFinal :

java -XX:+PrintFlagsFinal

Check the following flags CICompilerCount - how many threads are available to run the compiling process

CompileThreshold - the number of times a method/code needs to run before it is natively compiled

Compilation

-XX:+PrintCompilation - provides insight into the compilation process of methods by the JVM, including information about their optimization levels and status

-XX:-TieredCompilation: This option disables tiered compilation, meaning that only the C2 compiler will be used.

-client: This option instructs the JVM to use the client compiler (C1) as the default compiler. Prevent’s C2 compiler to kick in if needed.

-server: This option instructs the JVM to use the server compiler (C2) as the default compiler.

-d64: This option specifies that the JVM should run in 64-bit mode, utilizing the larger address space available on 64-bit architectures.

-XX:CICompilerCount=n

-XX:CompileThreshold=n

-XX:+PrintCodeCache : If the code cache is full, the warning message is code cache is full, compiler has been disabled.

InitialCodeCacheSize is the size of the code cache when the application starts. The default size varies based on available memory, but it’s often around about 160kB.

-XX:ReservedCodeCacheSize=150M : is the maximum size of the code cache. In other words, the code cache can grow over time up to the size of the reserved code cache.

CodeCacheExpansionSize dictates how quickly the code cache should grow as it gets full. How much extra space should be added each time the code cache is grown

Diagnostic and Troubleshooting

-XX:+UnlockDiagnosticVMOptions: Unlocks diagnostic VM options for troubleshooting.

-XX:+HeapDumpOnOutOfMemoryError: Generates heap dump on OutOfMemoryError.

-XX:HeapDumpPath=<path>: Specifies heap dump file path where it needs to be saved.

-XX:+UnlockDiagnosticVMOptions -XX:+LogCompilation : creates a log file like hotspot_pid1234.log

String Pool Management

XX:+PrintStringTableStatistics : Prints statistics about the String pool.

XX:StringTableSize=120121: Sets the size of the String table at the beginning of the JVM start

Heap Management

-XX:MaxHeapSize=1g or -Xmx1g: Sets maximum heap size.

-XX:InitialHeapSize=4g or -Xms4g: Sets initial heap size.

-Xss for stack

-XX:+HeapDumpOnOutOfMemoryError

-XX:HeapDumpPath=<>

Garbage Collections

-Xmx10m -verbose:gc : Spits out the garbage collection logs on console

-XX:-UseAdaptiveSizePolicy : Turned on by default for most JVM’s. It dynamically adapts the size of the S0 & S1 space during runtime.

-XX:NewRatio=n : how many times should the old generation be in camparson to young gen

-XX:SurvivorRatio=n : how much of the young generation should be taken up by the survivor spaces S0 and S1.

-XX:MaxTenuringThreshold=n : how many generations should an object survive before it becomes part of the old generation

-XX:+UseSerialGC : Single Threaded GC Algo

-XX:+UseParallelGC : Parallel

-XX:+UseConcMarkSweepGC : Mark Sweep Collector - General Algorithm that GC uses

-XX:+UseG1GC : G1 Garbage Collector - default java 10 onwards

-XX:ConcGCThreads=n : with G1GC : specify the number of concurrent threads available for the smaller regional collections.

-XX:InitiatingHeapOccupancyPercent=n : G1 runs whenever the entire heap gets to 45% full. Change this from the 45% default by using this flag.

-XX:UseStringDeDuplication : G1 Garbage Collector - Allows GC to make more space if it finds duplicate strings in the heap.

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