JVM Heap Tuning Flags

Some of the most commonly used JVM Heap tuning flags are mentioned below.

Heap size

Ø  Initial Heap size: -Xms   e.g. -Xms2048m

Ø  Maximum Heap size: -Xmx e.g. -Xmx2048m

Ø  To set ratio of young generation to old generation: -XX:NewRatio

Ø  To set initial size of the young generation: -XX:NewSize

Ø  To set Maximum size of the young generation: -XX:MaxNewSize

Ø  To set permgen space initial size:

-XX:PermSize=N (JDK 7 only) OR -XX:MetaspaceSize=N (JDK 8 only)

Ø  To set maximum size of Permgen space:

-XX:MaxPermSize=N (JDK 7 only) OR -XX:MaxMetaspaceSize=N (JDK 8 only)

Ø  Instruct to take heap dump on out of memory error: -XX:+HeapDumpOnOutOfMemoryError

Ø  To specify path for Heap dump file: -XX:HeapDumpPath=<path>

Garbage Collection

Ø  To instruct use of Serial Garbage collector:     -XX:+UseSerialGC

Ø  To instruct use of parallel Garbage collector:  -XX:+UseParallelGC

Ø  To instruct use of Concurrent Mark & Sweep collector:   

-XX:+UseConcMarkSweepGC -XX:+UseParNewGC

Ø  To instruct use of G1 GC collector: -XX:+UseG1GC

Ø  To set no of threads (in case of parallel GC): -XX:ParallelGCThreads=N

Ø  To enable GC logging: -verbose:gc

Ø  To specify separate log file name for GC log instead of standard output:

-Xloggc:<path>

Ø  To enabled detailed GC logging: -XX:+PrintGCDetails

Ø  To print relative timestamp for each GC event: -XX:+PrintGCTimeStamps

Ø  To set the amount of the young generation set aside for survivor spaces: -XX:InitialSurvivorRatio=N

o    survivor_space_size = new_size / (initial_survivor_ratio + 2)

o    For the default initial survivor ratio of 8, each survivor space will occupy 10% of the young generation.

Ø  To specify initial number of GC cycles the JVM attempts to keep an object in the survivor spaces: -XX:InitialTenuringThreshold=N

o    The default is 7 for the throughput and G1 collectors, and 6 for CMS.JVM may change this value on his own.

Ø  To specify maximum  number of GC cycles the JVM attempts to keep an object in the survivor spaces:

o    -XX:MaxTenuringThreshold=N

o    For the throughput and G1 collectors, the default maximum threshold is 15, and for CMS it is 6.

 CMS & G1 specific

Ø  To specify when CMS/G1 should begin background scanning of the old generation: -XX:CMSInitiatingOccupancyFraction=N

               Can try to reduce this value if there are concurrent mode failures.

Ø  To set the number of threads to use for CMS/G1 background scanning: -XX:ConcGCThreads=N