Understanding IBM zIIP: Unlocking Performance and Cost Efficiency

Disclaimer: This article share my personal working experience on IBM zIIP processing and it does not include any proprietary information and its for educational and awareness purpose only.

Introduction

IBM’s z Integrated Information Processor (zIIP) is a specialty processor designed to optimize workloads on IBM Z systems. By offloading specific processing tasks from general-purpose CPUs (GPs), zIIP helps reduce software costs and improve system efficiency. This blog explores zIIP’s capabilities, restrictions, prerequisites, and implementation process, providing insights for organizations looking to leverage zIIP for enhanced performance.

What is zIIP?

zIIP is a specialty processor introduced by IBM to offload specific workloads from traditional CPUs, thereby reducing software licensing costs. Unlike general processors, zIIP:

• Is enabled via a one-time licensing fee from IBM.
• Cannot execute TCB (Task Control Block) workloads; it primarily runs SRB (Service Request Block) tasks.
• Can be dedicated to an LPAR or shared across multiple LPARs for flexible workload distribution.
• Has a processing speed that varies based on the underlying hardware model.

Workloads Eligible for zIIP

IBM has restricted zIIP usage to specific workload types. Some of the key workloads that can run on zIIP include:

• Java workloads running on IBM Z.
• DB2 utility functions and remote DRDA access via TCP/IP (JDBC and ODBC).
• IMS Connect and IMS ODBM processing (from IMS v13 onwards).
• z/OS XML system services for processing XML data.
• Certain ISV-defined workloads, provided they are explicitly enabled for zIIP processing.

Why Did IBM Introduce zIIP?

The introduction of zIIP was IBM’s response to the increasing adoption of open systems, such as Java, XML, and Linux-based workloads. zIIP allows enterprises to:

• Reduce the load on general-purpose processors, freeing them for core transactional workloads.
• Lower Monthly License Charges (MLC) by shifting processing from traditional CPUs to zIIP.

Restrictions on zIIP Usage

IBM has implemented several restrictions to regulate zIIP usage:

• Only a portion of eligible workloads is redirected to zIIP (IBM’s “generosity factor”).
• The number of zIIP processors cannot exceed the number of general processors in the system.
• Workloads must run under an Enclave SRB to be eligible for zIIP processing.
• Independent Software Vendors (ISVs) cannot offload arbitrary customer programs to zIIP unless explicitly enabled.

Prerequisites for zIIP Utilization

To successfully leverage zIIP, organizations must meet the following prerequisites:

• Obtain a zIIP API license (typically under a non-disclosure agreement).
• Implement Service Request Blocks (SRBs) in an Enclave, as only SRB workloads can be offloaded.
• Understand and integrate with Workload Manager (WLM) services, which control workload dispatching to zIIP.

The zIIP API

IBM provides a zIIP API that is licensed separately and requires explicit enablement. Key details include:

• The macro IWM4EOCT, located in SYS2.*.MACLIB (depending on your installation), is essential for zIIP integration.
• The API is a component of z/OS Workload Manager (WLM) services and controls offload characteristics.

Service Request Blocks (SRB)

In z/OS environments, code execution can occur in TCB mode or SRB mode.

• SRB mode is optimized for high-performance, lightweight execution and is required for zIIP processing.
• SRBs are scheduled using the SCHEDULE or IEAMSCHD macros.

Enclaves in zIIP Processing

An enclave acts as an execution boundary for workloads across multiple address spaces and dispatchable units.

• Enclaves group SRBs and TCBs into a single logical unit for workload management.
• Enclave SRBs are pre-emptible, preventing them from monopolizing system resources.

Determining zIIP Availability

Before leveraging zIIP, organizations need to check its availability:

• Is zIIP available? Check CVTOSLV4.CVTZIIP—if the bit is set, a zIIP processor is present.
• How many zIIP processors are available? Retrieve the PCCA vector address from CVTPCCAT, then check each PCCAPROCCLASS_BYTE1 entry for x'04'.

What Workloads Are Dispatched to zIIP?

• Only SRB-mode workloads are eligible for zIIP processing.
• zIIP cannot execute long-running programs or issue SVCs (except Abends).
• Workloads must remain in supervisor state while executing on zIIP.

Will an SRB Always Run on zIIP?

No. zIIP does not guarantee execution on a specialty processor. If the zIIP is busy or unavailable, the workload is redirected to the first available general processor (GP).

Implementing zIIP: Step-by-Step Guide

1. Setting Up a zIIP-Eligible Enclave

To make a workload zIIP-eligible, follow these steps:

1. Connect to Workload Manager (WLM): Use the IWM4CON macro.
2. Create an Enclave: Call IWM4ECRE to define the execution boundary.
3. Set Offload Characteristics: Invoke IWM4EOCT (requires IBM NDA).
4. Delete the Enclave When Done: Use IWM4EDEL.
5. Disconnect from WLM: Call IWM4DIS to finalize cleanup.

2. Scheduling an SRB to a zIIP-Eligible Enclave

1. Join the Enclave (optional): Call IWMEJOIN for the parent task.
2. Schedule the SRB: Use IEAMSCHD to execute the workload.

Conclusion

IBM’s zIIP processor is a strategic tool for reducing software costs and optimizing IBM Z performance. By shifting eligible workloads to zIIP, organizations can free up CPU resources, reduce licensing costs, and improve system efficiency. However, strict eligibility criteria and workload restrictions must be considered to maximize benefits.

For more information, refer to IBM’s official documentation: ?? IBM Authorized Use Table for Machines: ftp://public.dhe.ibm.com/systems/support/warranty/pdfs/aut/Authorized_Use_Table_11-2015_en_US.pdf