IPS - Instructions Per Second

Enhanced Definition

IPS (Instructions Per Second) is a fundamental metric representing the raw number of machine instructions a CPU can execute in one second. While a basic measure of processing speed, in the mainframe z/OS environment, it's often a foundational concept leading to more sophisticated metrics like MIPS and especially MSU, which are more commonly used for capacity planning and billing.

Key Characteristics

- Raw Processing Speed: Represents the absolute count of instructions executed per second, providing a baseline measure of a processor's potential.
- Hardware Dependent: The IPS value is intrinsic to the specific CPU architecture and clock speed of the mainframe processor.
- Instruction Mix Sensitivity: The actual IPS achieved can vary based on the mix of instructions being executed (e.g., simple arithmetic vs. complex floating-point operations).
- Precursor to MIPS: IPS is typically expressed in millions, leading to the term MIPS (Millions of Instructions Per Second), which was historically a common, though often misleading, mainframe performance metric.
- Less Relevant than MSU on z/OS: While a basic measure, IPS (and MIPS) are generally not used for z/OS capacity planning or software licensing; MSU (Millions of Service Units) is the standard.

Use Cases

- Historical Performance Comparison: Used in the early days of computing to compare the raw speed of different processor models or generations.
- Theoretical Peak Performance: Can be cited by hardware vendors to indicate the maximum theoretical instruction execution capability of a new mainframe processor.
- Benchmarking Low-Level Operations: In highly specialized scenarios, it might be used to benchmark the efficiency of specific instruction sequences or microcode changes.
- Understanding Architectural Improvements: Helps illustrate the gains in raw processing power between different generations of IBM zSystems architecture.

Related Concepts

IPS is a foundational concept that directly leads to MIPS (Millions of Instructions Per Second), which was a widely used but often problematic measure of mainframe capacity. However, for modern z/OS environments, MSU (Millions of Service Units) has largely superseded MIPS. MSU normalizes CPU consumption across different mainframe models and generations, providing a more consistent and reliable metric for capacity planning, workload management, and software licensing. While IPS measures raw instruction execution, MSU measures the *work* done in a way that accounts for varying instruction complexities and processor efficiencies, making it a more accurate reflection of a system's ability to process business workloads.

Best Practices:

Prioritize MSU for z/OS: For capacity planning, performance analysis, and software licensing on z/OS, always rely on MSU rather than IPS or MIPS due to MSU's normalized and consistent nature.
Understand Context: Recognize that IPS is a raw, hardware-specific measure and does not directly translate to business throughput or workload completion rates.
Avoid Direct Comparisons: Do not directly compare IPS values between different mainframe architectures or even different vendors, as instruction sets and complexities vary greatly.
Focus on Workload Metrics: For actual system performance assessment, focus on metrics like CPU utilization, transaction rates, response times, and service units consumed by workloads, which provide a more accurate picture of system effectiveness.