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Though physically identical to a Central Processor, the zIIP engine is microcoded at installation time to run specific types of workloads. The Central Processor continues to handle the operating system, I/O interrupts and timer interrupts, job initiations, and user interactions with the operating system. The zIIP concentrates on CPU intensive workloads, leaving the Central Processor more time to absorb otherwise queued workloads, thereby achieving some overall performance improvement across all mainframe activity.

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This section describes requirements for FOCUS use of the zIIP processor.

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SET ZIIP={ON[/SIMMAXZIIP]|OFF}

where:

ON

Configures FOCUS to offload processing to the zIIP engine.

FOCUS determines if the zIIP processor is accessible to the LPAR in which the job is running. If FOCUS determines that the zIIP is not accessible, it issues a message describing the reason and continues in ZIIP=OFF mode, which forwards all subsequent work to the Central Processor.

SIMMAXZIIP

Allows the job to run with processing on the zIIP and simulates full zIIP accessibility to see how much zIIP access the job would have had if the zIIP were 100 percent available for FOCUS usage. Use this option for simulation purposes only.

OFF

Configures FOCUS not to offload processing to the zIIP engine. OFF is the default value.

Note: Turn off zIIP enablement only when you know for sure that a job will not gain any advantage from using the zIIP processor or if the system operator or administrator requires that you turn it off.

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• All load libraries invoked on the zIIP processor must be APF authorized. If any load module in your STEPLIB allocation is not authorized, FOCUS will set ZIIP=OFF and start in an unauthorized state; the job will then run on the Central Processor.

  If FOCUS has already started running in an authorized state and encounters a load library that is not authorized, it abends. The FOCUS module is shipped as authorized (the AC flag is set to 1).

  Note:

  • You must contact your System Support staff to have them APF authorize all load libraries that will be executed on the zIIP processor or called by any program running on the zIIP processor.
  • If you have been running in an authorized state for a while without problems and you suddenly get the 306 message, you have called a new module that was not called in prior runs and this module is not authorized.

  Note that user function libraries as well as the FOCUS function libraries must be authorized.

  If you run FOCUS under TSO, your System Support staff must set up the FOCUS command as a TSO authorized command by adding the name FOCUS to the AUTHPGM and AUTHCMD lists. If you execute FOCUS from ISPF option 6, you will not be able to issue the TSO ISPF or IEDIT commands from within FOCUS.

• BM Workload Manager (WLM) prioritizes workloads among the Central Processors and zIIP processors at your site based on a complex set of goals and rules established by the system administrator. These rules apply to all workloads from all sources, not just FOCUS. These goals combine to influence the decision to direct FOCUS requests to the zIIP engine at any particular moment.

  IBM Workload Manager must have a policy and goals declared to allow FOCUS to access the zIIP with sufficient priority compared to other software to derive a quantifiable benefit from CPU redirection.

• Maximize the blocksizes of data sources that are read or written by FOCUS to reduce the number of I/Os required to access the file. This will reduce the number of switches to non-zIIP mode that FOCUS has to make, thus permitting a greater percentage of zIIP contribution to the request.
• Move or rewrite functions developed at your site since FOCUS must switch to non-zIIP mode for each call to such routines. You may be able to use one of the following possible solutions:
  • Move the routines from DEFINEs to COMPUTEs to reduce the number of times they are referenced. This tactic must be applied carefully, and only when report results would not change.
  • Rewrite the routines using DEFINE FUNCTION, which executes on the zIIP processor.
  • Confine the routine to a pre-step run with ZIIP=OFF which collects its calculated results, then use those calculations in the next step with ZIIP=ON.

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FOCUS diverts eligible workload to the zIIP engine by switching from TCB (Task Control Block) mode for workloads that can run only on a Central Processor to SRB (Service Request Block) mode for execution of enabled workloads on the zIIP engine.

Types of FOCUS processing that are offloaded to the zIIP engine include:

• Computations.
• Aggregation.
• Screening.
• Report formatting and styling.
• Transaction Processing.

The FOCUS zIIP Monitor detects situations in which the overhead cost of zIIP usage is exceeding the CPU benefits gained. When this threshold is reached, FOCUS may decide to suspend use of the zIIP for the duration of that command. It then resets to make the zIIP processor accessible to the next command.

TABLE, MATCH, and MORE requests may suspend and resume more than once as they progress through logical phases of execution.

In every case, FOCUS attempts to optimize the use of the zIIP and minimize chargeable CPU costs.

Applications that perform significant database I/O, high-volume sorting, or the use of third party tools or user functions during processing require switching out of SRB (zIIP) mode into TCB (non-zIIP) mode to communicate, and then back again to continue processing. Although each switch is miniscule, the cumulative effect can absorb measurable amounts of CPU time on both the zIIP engine and the Central Processor.

In order to diminish this effect, FOCUS buffers the collection of records passed to the system sort utility and some adapters rather than passing one record at a time, thus reducing the number of switches between TCB and SRB modes.

These third party products may themselves be zIIP enabled and may offload some or all of their processing to the zIIP engine independent of FOCUS. FOCUS always calls these products from the Central Processor because it cannot know whether they will perform any processing that is prohibited on the zIIP.

Even though zIIP usage occurs more frequently on non-optimized requests to a relational data source, optimized requests are still inherently more efficient and, therefore, may incur less CPU time. Being zIIP enabled, DB2 may also take advantage of the zIIP processor for FOCUS requests based on the local configuration of DB2 relative to FOCUS.

Requests against some types of data sources whose I/O can be buffered gain a lot of advantage from zIIP enablement. Data sources that gain the most benefit from zIIP processing due to buffered I/O include:

• Blocked flat files.
• FOCUS.
• XFOCUS.
• VSAM.
• DB2.

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In order to evaluate FOCUS zIIP processing in a session, you can query three Dialogue Manager variables that accumulate statistics about FOCUS processing:

• &FOCCPU accumulates the time spent on a Central Processor. This is an existing variable that precedes zIIP enablement.
• &FOCZIIPCPU accumulates the time actually spent on the zIIP processor (in SRB mode). This is the normalized CPU value using the same scale as &FOCCPU.
• &FOCZIIPONCP accumulates the time that processing could have been offloaded to the zIIP processor but was diverted to the Central Processor by the system.

  Note:

  • &FOCCPU includes the value of &FOCZIIPONCP.
  • The sum of &FOCZIIPCPU and &FOCCPU represents the total CPU utilized by the job.
  • If you set ZIIP=OFF, the zIIP variables do not accumulate further but are not reset to zero. If you later set ZIIP=ON, they resume accumulating statistics.

The FOCLOG and SiteAnalyzer products that monitor FOCUS usage also capture zIIP statistics for a session.

If you do not have a zIIP engine installed yet at your site, you can still see the effect that a zIIP engine would have on the workload of your application. In SYS1.PARMLIB for your LPAR, insert the following parameter:

PROJECTCPU=YES

This parameter makes all zIIP-enabled software products including FOCUS think that a zIIP engine is present and causes them to function accordingly. Since the zIIP engine actually is not present, all zIIP-eligible workload will be diverted to the Central Processor. Thus all of that CPU utilization will be recorded in &FOCZIIPONCP. This is the amount of workload that would have run on the zIIP engine, and would have appeared in &FOCZIIPCPU, had the zIIP been present and 100% accessible to FOCUS work.