WebSphere Application Server V8.5 Administration and Configuration Guide for the Full Profile

Chapter 17. Performance tuning 607

17.4 64-bit considerations

This section provides a comprehensive overview of 64-bit mode and its impact on the

application server’s performance. The 64-bit mode was available in WebSphere Application

Server V6. Since WebSphere Application Server V7, the installation defaulted to 64-bit mode

because 31-bit mode was deprecated. Consider reconfiguring application servers that were

migrated from previous releases to 64-bit mode.

17.4.1 Enabling 64-bit mode

Ensure that z/OS page set allocations are sufficient and that JCL and Automation procedures

do not specify the AMODE=31 parameter on the start command before you switch to 64-bit

mode. If the detected mode differs from the AMODE setting, an error message of BBOO0336E

is produced, and the server fails to start.

Refer to the following website for further information:

http://pic.dhe.ibm.com/infocenter/wasinfo/v8r5/index.jsp?topic=%2Fcom.ibm.webspher

e.nd.multiplatform.doc%2Fae%2Fcrun_z64bit.html

To change to 64-bit mode using the administrative console:

1. Click Servers  Server types  WebSphere application servers  servername.

2. On the Configuration tab, select Run in 64 bit JVM mode, as shown in Figure 17-2.

Figure 17-2 Switch to 64-bit mode

3. Click Apply.

4. For a Network Deployment configuration, click Review  Synchronize changes with

nodes, and click Save. Alternatively, click System administration  Nodes, and click

Synchronize with the appropriate node or nodes selected, or select the Synchronize

changes with Nodes option in System administration  Console Preferences.

5. Click Restart for the target server in the Servers  Server types  WebSphere

Application servers view.

608 WebSphere Application Server V8.5 Administration and Configuration Guide for the Full Profile

For application development purposes, JVM provides the com.ibm.vm.bitmode programmatic

API, to determine the bit mode setting in which an application server is running.

17.4.2 Effects of switching to 64-bit mode

Running WebSphere Application Server in 64-bit mode allows for heap relief. It grants

WebSphere Application Server the ability to run heaps larger than 1 GB and provides access

for up to 16 EB of virtual memory. Alternatively, the cost of using 64-bit object references can

enlarge your heap by as much as 40%. The inherently bigger objects also affect data locality,

and thus contribute to higher translation look-aside buffer (TLB) and data cache miss rates.

These higher rates slow dynamic address translation (DAT) and affect application

performance.

The compressed references and large page support features can provide relief for reduced

throughput and memory footprint growth incurred when migrating from 31-bit JVM to 64-bit

JVM. With these two JVM properties turned on, 64-bit environments can match and out

perform previous 31-bit environments. These features are not turned on by default because

they have software and hardware requirements.

Compressed references

Compressed references is a method for managing object pointers with the JVM. Some

workloads in 64-bit environments have shown objects increased by up to 45% because the

object header and object references doubles in width. Compressed references mode is

usable for JVM heaps of up to 30 GB on z/OS. It reduces the size of the 64-bit object pointer

to 4 bytes (word).

In compressed references mode, object header referenced data (such as class-related data

and thread data) are allocated below the 2 GB bar. This allocation enables all the references

to be 32-bit and no padding to occur in the object header, as illustrated in Figure 17-3.

Figure 17-3 Header and object references in 31-bit and 64-bit mode

Tip: The administrative console uses the following command to facilitate the bit mode

change to 64-bit addressing inside the XML configuration files:

AdminTask.setJVMMode('[-nodeName nodename -serverName servername -mode

bitmode]').

Class Flags Monitor

Int

field

Object

field

Object

field

Class Flags Pad Monitor

Int

field

Pad

Object

field

Object

field

Class Flags Monitor

Int

field

Object

field

Object

field

Word

(4 bytes)

• 31-bit object (24 bytes)

• 64-bit object (48 bytes)

• 64-bit compressed references (24 bytes)