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RecommendedJVMArgs.md

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JVM Arguments for better tunability

The following are the recommended arguments for better JVM tuning.

— JVM CodeCache
CodeCache is the storage for JIT generated compiled code. If CodeCache is filled, compilation is done unless the code cache is flushed.
Flag: -XX:InitialCodeCacheSize=SIZE
Purpose: Sets the initial size of the JVM CodeCache. Set this to a reasonable size to avoid dynamic adjustment overhead.
JVM default: 2MB
Pega Recommendation:Set to 256M for large scale deployments and JVM default for others.

Flag: -XX:ReservedCodeCacheSize=SIZE
Purpose: Sets the maximum size to which the JVM CodeCache can grow, after which the cache is attempted to be flushed.
JVM default: 250MB
Pega Recommendation:Set to 512M for large scale deployments and JVM default for others.

— JVM Metaspace
Metaspace is a non-heap memory that stores the class metadata such as class structure, virtual method hierarchy, etc. This is the Java8+ equivalent of PermGen space. This space is unbounded by default, in the sense that it can endlessly expand to the system Native space.
Flag: -XX:MetaspaceSize=SIZE
Purpose: Sets the initial size of Metaspace at which point the JVM triggers Metaspace induced Garbage Collection (GC), in order to free Metaspace and prevent "Metaspace OutOfMemoryError". This is not the minimum Metaspace allocated. The setting allows you to set a higher Metaspace trigger size in order to avoid an initial GC that may not be warranted when the deployment starts up.
JVM default: 20MB
Recommendation:Set to 512M for large scale deployments and JVM default for others.

Flag: -XX:MaxMetaspaceSize=SIZE
Purpose: Sets the maximum value the JVM Metaspace can occupy. Beyond this "Metaspace OutOfMemoryError" is thrown.
JVM default: 18446744073709486080 Bytes or Unbounded
Pega Recommendation: Unbounded(default), to ensure that "Metaspace OutOfMemoryError" exceptions do not occur.

Flag: -XX:+UseStringDeduplication
Purpose: Maintains a single copy of String literals aka Deduplication, in case multiple Strings have the same literal value
JVM default: UseStringDeduplication=false equivalent to -XX:-UseStringDeduplication
Pega Recommendation: Set to use the StringDeduplication.

Flag: -XX:+DisableExplicitGC
Purpose: Disables force GC, which can happen through external invocation of System.gc() call.
JVM default: DisableExplicitGC=false equivalent to -XX:-DisableExplicitGC
Pega Recommendation: Set to disable explicit GC
Note: This JVM argument is hardcoded in the Docker image with the recommended value. You cannot explicitly overwrite this argument.

Flag: -Djava.security.egd=file:///dev/urandom
Purpose: Allows generation of random numbers in non-blocking mode, in case the entropy is exhausted.
JVM default: dev/random
Pega Recommendation: Set to use dev/urandom
Note: This JVM argument is hardcoded in the Docker image with the recommended value. You cannot explicitly overwrite this argument.

Flag: -XX:+ExitOnOutOfMemoryError
Purpose: Exits the JVM when an OutOfMemoryError occurs. Exiting is required, as allowing the JVM to run after OutOfMemoryError leads to inconsistent system state.
JVM default: ExitOnOutOfMemoryError=false equivalent to -XX:-ExitOnOutOfMemoryError
Pega Recommendation: Set to Exit the JVM in case of OutOfMemoryError
Note: This JVM argument is hardcoded in the Docker image with the recommended value. You cannot explicitly overwrite this argument.

Flag: -Xlog:gc*,gc+heap=debug,gc+humongous=debug:file=/usr/local/tomcat/logs/gc.log:uptime,pid,level,time,tags:filecount=3,filesize=2M
Purpose: Dumps the GC logs with the provided tags into local log file.
JVM default: No GC logs will be emitted by default.
Pega Recommendation: Set to allow GC logs to be collected. Currently, the log rotation policy is set to 3 log files with maximum of 2MB each.

Few more arguments:

Flag: -XX:MaxGCPauseMillis=n
Purpose: Sets the peak pause time for GC to happen.
JVM default: 200 milliseconds
Pega Recommendation: Set to higher value for BATCH pods for better throughput.

Flag: -XX:StringTableSize=n
Purpose: Sets the number of hash-buckets to be used in string pool for accommodating Strings
JVM default: 65536, the default Hash bucket size of the JVM String pool
Pega Recommendation: Tune this based on requirements.