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WebSphere Application Server for z/OS and zAAP: Manage Costs While Gaining Benefits

Take advantage of QoS features such as reliability, availability, scalability, and serviceability

To set these options for WAS for z/OS you have to access the administration console: Select Servers > Application Servers > Server Name > Process Definition > Control/Servant > Java Virtual Machine > Advanced JVM Settings, and put the options into the Generic JVM arguments text input field as shown in Figure 5.

Figure 5 WAS for z/OS zAAP option set-up z/OS V1R6 provides the ability to run Java applications on zAAPs. RMF in z/OS V1R6 supports zAAPs by extending CPU Activity, Partition Data, and Workload Activity reports in Monitor I and Monitor III. The reports can be used for zAAP consumption assessment.

The following SMF record types are extended to support zAAPs:

  • SMF record 70 subtype 1 (CPU activity)
  • SMF record 72 subtype 2 (Workload activity)
  • SMF record 79 subtype 1 and 2 (Address Space State and Resource data)
More information on how to interpret these reports can be found in zSeries Application Assist Processor (zAAP) Implementation SG24-6386-00.

I would like to clear up one thing about zAAP performance. Currently zAAP itself doesn't have any built-in mechanism to speed up Java code execution. In other word, the Java code doesn't run any faster than it does on a general CP. (On a z890, the zAAP speed is different from the general CP; the situation is a little bit more complicated.). If your system is short of CPU cycles, you will see the application response time becomes shorter when you off-load the Java workload to zAAPs. However, it doesn't mean that the Java code runs faster. It's just because some of the CPU waits are eliminated. IBM may do something for zAAP to speed up Java code execution in the future.

Once you turn on the zAAPs, the WAS for z/OS performance tuning gets more complicated. I'm going to list some of the zAAP tuning options here to help Java developers get a basic understanding of zAAP tuning to ease the conversation for system people.

The options are specified in the IEAOPTxx member of the SYS1.PARMLIB:

  • IFACrossOver=YES|NO
  • IFAHonorPriority=YES|NO
The IFACrossOver with a value NO means that there's no zAAP-eligible work that can run on the general CPs. Otherwise zAAP-eligible work can run on general CPs.

The IFAHonorPriority with a value NO means a general CP only selects zAAP-eligible work when the general CP isn't busy. Otherwise a general CP selects the general CP work and the zAAP-eligible work in dispatch priority order.

IFACrossOver=YES and IFAHornorPriority=YES provide the best performance for high-priority work regardless of the workload type. But it probably reduces the estimated amount of work running on the zAAPs.

IFACrossOver=YES and IFAHonorPriority=NO let all processors to run ready work, but give the zAAP a better chance of running Java work and help achieve the estimated offload of Java work.

IFACrossOver=NO provides the maximum offload of Java work to a zAAP but might cause Java work to queue to the zAAP when a general CP is available.

zWLM and LPAR management have been enhanced to handle zAAPs. More information can be found in zSeries Application Assist Processor (zAAP) Implementation SG24-6386-00.

Not all the applications running in WAS for z/OS are good candidates for exploiting zAAPs. Java-intensive applications are good candidates. Low switch-rate applications are good candidates since they minimize the software cost. The Java eligible time per switch might be a good indicator of the application's applicability for a zAAP.

Since using a zAAP is controlled at the JVM level, it implies that it's preferable to deploy applications with similar Java eligible time per switch to the same WAS for z/OS server. If the applications running in a WAS for z/OS server have long Java eligible time per switch, the JVM should be configured to leverage the zAAP. Hence it's necessary to analyze the applications for zAAP utilization using the Projection Tool described above.

However, I haven't discussed the limitations of zAAP in this article because most of them aren't of interest to application development folks. Information about zAAP limitations can be found ar IBM's zAAP resource web site.

Conclusion
zAAP is a specialized processing unit which can be used to lower the overall cost of running WAS for z/OS and other Java workload on z/OS. This article explained the basic concepts of how zAAP works and how to exploit and use them.

Resources
IBM zAAP resource Web site www-03.ibm.com/servers/eserver/zseries/zaap/resource.html

More Stories By Linfeng Yu

Linfeng Yu is a software architect with ISO, Inc. He has extensive experiences in developing large-scale, complex enterprise-wide architectures and corss platform software development. He has been working with WebSphere for both distributed platform and z/OS since version 3.

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