Hyperthread
While `Hyperthread` is a term commonly associated with Intel processors, the analogous technology in IBM z/Architecture is called **Simultaneous Multithreading (SMT)**. SMT allows a single physical processor core to execute instructions from multiple independent threads concurrently, effectively presenting multiple logical processors to the operating system (typically two per core, known as SMT-2). This improves processor utilization by allowing threads to use execution units that would otherwise be idle.
Key Characteristics
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- Increased Throughput: SMT aims to improve overall system throughput by keeping the processor busy with multiple instruction streams, especially during memory latency or other stalls.
- Resource Sharing: Threads on the same physical core share most execution resources (e.g., caches, execution units, instruction pipelines) but maintain separate architectural states (e.g., registers, program counter).
- z/Architecture Implementation (SMT-2): IBM's implementation in z/Architecture typically supports two hardware threads per core (SMT-2), meaning each physical core can dispatch instructions from two independent threads.
- Logical Processor Presentation: To the operating system (like z/OS) and
LPARs, an SMT-enabled physical core appears as multiple logical processors, increasing the available dispatchable units. - Dynamic Dispatching: The hardware dynamically dispatches instructions from the active threads based on resource availability, optimizing for concurrent execution.
- Not "Hyperthread": It's crucial to note that "Hyperthread" is an Intel trademark; IBM's term for this capability in its zSystems is Simultaneous Multithreading (SMT).
Use Cases
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- Workload Consolidation: Running multiple, diverse workloads (e.g., transaction processing, batch, Linux on Z) on fewer physical cores, maximizing hardware utilization and potentially reducing software licensing costs.
- Batch Processing: Enhancing the execution speed of parallelizable batch jobs by allowing more threads to run concurrently, reducing overall batch window times.
- Transaction Processing: Improving the responsiveness and capacity of high-volume transaction processing systems (like CICS or IMS) by enabling more concurrent transactions to be processed within the same core.
- Linux on Z (LoZ): Particularly beneficial for Linux workloads running in
LPARsorKVMguests, as Linux kernels are generally well-optimized to leverage SMT capabilities.
Related Concepts
SMT directly impacts how Central Processors (CPs) are utilized; a single physical CP core with SMT enabled can appear as multiple logical CPs to the operating system. These logical CPs are then allocated to Logical Partitions (LPARs), increasing the number of dispatchable units available within an LPAR. The Workload Manager (WLM) in z/OS plays a critical role in managing the dispatching priorities of work across these SMT-enabled logical processors, optimizing resource allocation based on defined service goals. The z/OS Dispatcher leverages the SMT