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Chapter 2. Java Virtual Machine settings

Java Virtual Machine tuning might be divided into sections like memory or GC. Below is a list of helpful configuration parameters and a guide how to adjust them.

2.1. Memory settings
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Adjusting memory size is one of the most crucial step in Red Hat Data Grid tuning. The most commonly used JVM flags are:

  • -Xms - Defines the minimum heap size allowed.
  • -Xmx - Defines the maximum heap size allowed.
  • -Xmn - Defines the minimum and maximum value for the young generation.
  • -XX:NewRatio - Define the ratio between young and old generations. Should not be used if -Xmn is enabled.

Using Xms equal to Xmx will prevent JVM from dynamically sizing memory and might decrease GC pauses caused by resizing. It is a good practice to specify Xmn parameter. This guaranteed proper behavior during load peak (in such case Red Hat Data Grid generates lots of small, short living objects).

2.2. Garbage collection
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The main goal is to minimize the amount of time when JVM is paused. Having said that, CMS is a suggested GC for Red Hat Data Grid applications.

The most frequently used JVM flags are:

  • -XX:MaxGCPauseMillis - Sets a target for the maximum GC pause time. Should be tuned to meet the SLA.
  • -XX:+UseConcMarkSweepGC - Enables usage of the CMS collector.
  • -XX:+CMSClassUnloadingEnabled - Allows class unloading when the CMS collector is enabled.
  • -XX:+UseParNewGC - Utilize a parallel collector for the young generation. This parameter minimizes pausing by using multiple collection threads in parallel.
  • -XX:+DisableExplicitGC - Prevent explicit garbage collections.
  • -XX:+UseG1GC - Turn on G1 Garbage Collector.

2.3. Other settings
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There are two additional parameters which are suggested to be used:

  • -server - Enables server mode for the JVM.
  • -XX:+ UseLargePages - Instructs the JVM to allocate memory in Large Pages. These pages must be configured at the OS level for this parameter to function successfully.

2.4. Example configuration
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In most of the cases we suggest using CMS. However when using the latest JVM, G1 might perform slightly better.

32GB JVM

-server
-Xmx32G
-Xms32G
-Xmn8G
-XX:+UseLargePages
-XX:+UseConcMarkSweepGC
-XX:+UseParNewGC
-XX:+DisableExplicitGC
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32GB JVM with G1 Garbage Collector

-server
-Xmx32G
-Xms32G
-Xmn8G
-XX:+UseG1GC
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