Intel AS/400 RISC Server, 170 Servers, 7xx Servers manual Native Code, Garbage Collection

On i5/OS, IBM Technology for Java runs in i5/OS Portable Application Solutions Environment (i5/OS PASE) with either a 32-bit (for the 32-bit VM) or 64-bit (for the 64-bit VM) environment. Due to sophisticated memory management, both the 32-bit and 64-bit VMs provide a significant reduction in memory requirements over the Classic VM for most applications. Because the 32-bit VM uses only 4 bytes (instead of 8 bytes) for object references, applications will have an even smaller memory footprint with the 32-bit VM; however, the 32-bit address space leads to a maximum heap size of 2.5 - 3.25 GB, which may not be enough memory for some applications.

Because IBM Technology for Java shares a common implementation with IBM’s VMs on other platforms, the available tuning parameters are essentially the same on i5/OS as on other platforms. This will require some adjustment for users of the i5/OS Classic VM, but may be a welcome change for those who work with Java on multiple platforms.

Some of the key areas to be aware of when considering use of IBM Technology for Java are described below.

Native Code

Because IBM Technology for Java runs in i5/OS PASE, there is some additional overhead in calls to native ILE code. This may affect performance of certain applications which make calls to native ILE code through the Java Native Interface (JNI). Calls to certain operating system services, such as IFS file access and socket communication, may also have some additional overhead, although the overhead should be minimal for applications with a typical use of these services. Conversely, JNI calls to PASE native methods will have less overhead than they did with the Classic VM, offering a performance improvement for some applications.

The performance impact for JNI method calls to ILE will depend on the frequency of JNI calls and the complexity of the native methods. If the calls are infrequent, or if the native methods are very complex (and therefore take a long time to execute), the increased overhead may not make a big difference in the overall performance of the application. Applications which make frequent calls to simple native methods may see a more significant performance impact compared to the 64-bit Classic VM.

For some applications, it may be possible to port these native methods to run in i5/OS PASE rather than in ILE, greatly reducing the overhead of the native call. In other cases, it may be possible to modify the application to require fewer JNI calls.

Garbage Collection

Recommendations for Garbage Collector (GC) tuning with the i5/OS Classic VM have always been a bit different from tuning recommendations for Java VMs on other platforms. While the main GC tuning parameters (initial and max heap size) have the same names as the key parameters for other VMs, and are set in the same way when running Java from qsh (-Xms and -Xmx), the meaning of these parameters in the Classic 64-bit VM is significantly different. However, with IBM Technology for Java these parameters mean the same thing that they do in IBM VMs on other platforms. Many users will welcome this commonality; however, it does make the transition to the new VM a bit more complicated. The move from a 64-bit VM to a 32-bit VM also complicates matters somewhat, as the ideal heap size will be significantly lower in a 32-bit VM than in a 64-bit VM.

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
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