Operating Systems: AIX, HP-UX, Linux, Solaris, Windows, z/OS

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Modifying the JVM heap size

The Java virtual machine (JVM) heap size setting directly relates to how many server instances can be started within a dynamic cluster on a specific node. You might need to modify the JVM heap size setting based on your environment configuration. The default value is 256 MB. Modifying the heap size can affect application placement and the health management function.

 

Before you begin

Before modifying the JVM heap size read:

• Tuning Java virtual machines

 

About this task

The total value of all server JVM heap sizes within the dynamic cluster for a specific node must be less than half of the total RAM of that computer. To determine the maximum heap size setting for a single server instance, use the following equation:

total_RAM / 2 / number_of_servers = maximum_heap_size 

For example, to support three servers on a machine with 1.5 GB of RAM:

1.5 GB / 2 = 750 MB
750 MB / 3 = 250 MB

The maximum heap size is 250 MB for each server instance.

The following table contains some maximum heap size samples.

Table 1. Maximum heap size samples

WebSphere® Virtual Enterprise server total RAM in GB	Number of supported servers	Dynamic cluster size in MB	Approximate maximum heap size in MB
1.5	1	Less than 750	750
1.5	2	Less than 750	375
1.5	3	Less than 750	250
2	1	Less than 1000	1000
2	2	Less than 1000	500
2	3	Less than 1000	333

When a dynamic cluster creates server instances, a server template is used to set initial values for the newly created server instance. The following steps outline the procedure for modifying JVM heap size within the server template. Newly created server instances can then use the maximum heap size you specified.

 

Procedure

1. In the administrative console click Servers > Dynamic clusters > dynamic_cluster_name > Server template > Java and process management > Process definition > Java virtual machine.

2. Modify the heap size accordingly. Typically, the total value of all server instance JVM heap sizes on a specific node must be less than half of the total RAM of that computer. The default maximum heap size value is 256 MB.

3. Click OK.

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Related tasks

Monitor and tuning health managementConfigure the health controllerEnabling and disabling health managementConfigure the autonomic managers