Compiler Flags

With the +PrintCompilation flag, information about compilation of each method is print in the console:

java -XX:+PrintCompilation main.Main 10

Compilation information can otherwise be directed to a file with:

java -XX:+UnlockDiagnosticVMOptions -XX:+LogCompilation main.Main 5000

Available and used code cache sizes can be print with:

java -XX:+PrintCodeCache main.Main 10

Parameters for dynamic code cache allocation can be set with 3 flags : InitialCodeCacheSize, ReservedCodeCacheSize and CodeCacheExpansionSize.

java -XX:InitialCodeCacheSize=28m -XX:+PrintCodeCache main.Main 10

Flags -client, -server and -d64 can be used to specify whether to use the 32 bit or the 64 bit compiler. However this is not always relevant, depending on the OS and on the hardware.

Note that tiered compilation can be turned off with a -TieredCompilation flag. Execution it then a hundred percent interpreted. This is not usually of benefit in a production.

Here is how to display all of the jvm flags by default:

java -XX:+PrintFlagsFinal

Among the information that is obtained, the value of CICompilerCount is the number of threads available for compilation.

The following command

jps

outputs PIDs of processes running the jvm. Assuming one of those PIDs is stored as PID, here is another way to get the value of CICompilerCount:

jinfo -flag CICompilerCount $PID

Note that the minimum number of threads is 2 : one for the client compiler and another one for the server compiler.

The CompileThreshold flag sets the number of times a method is run before it is compiled with the C2 compiler.

Memory

The +PrintStringTableStatistics flag prints information about the String pool.

The following command turns the above flag on and also increases the number of keys of the String pool map to 120121:

java -XX:+PrintStringTableStatistics -XX:StringTableSize=120121 main.Main

Here is how to get information on the default VM parameters:

java -XX:+UnlockDiagnosticVMOptions -XX:+PrintFlagsFinal

Of relevance in the output are InitialHeapSize and MaxHeapSize. Options with them, e.g. -XX:InitialHeapSize=1g can be relevant in improving the performance of an application.

-XX:MaxHeapSize=256m can be shortened into -Xmx256m -XX:InitialHeapSize=1g can be shortened into -Xms1g

Application can be set to generate a heap dump when it crashes with a given error, for instance an OutOfMemoryError, using the following flags:

-XX:+HeapDumpOnOutOfMemoryError and -XX:HeapDumpPath=someFilePath.

Garbage Collection

The -verbose:gc option activates the logging of garbage collection information.

How large the old generation is compared with the young generation can be set with -XX:NewRatio=n

The ratio bewteen the total size of new generation space and the survivor spaces s0 and s1 can be set with -XX:SurvivorRatio=n

How many generations an object survives before being moved to the old generation space can be set with -XX:MaxTenuringThreshold=n

Types of garbage collectors:

• serial: can be selected with -XX:+UseSerialGC
• parallel: default in version up to 8 or else can be selected with -XX:+UseParallelGC
• mostly concurrent. Two possible options : -XX:+UseConcMarkSweepGC or -XX:+UseG1GC (the default from java 10).

Here are two possible flags for tuning the G1 GC: -XX:ConcGCThreads=n and -XX:InitiatingHeapOccupancyPercent=n.

The following option available only with G1 can be useful if many Strings duplicates occur which are not in the String pool: -XX:+UseStringDeDuplication.