42 IBM BladeCenter PS703 and PS704 Technical Overview and Introduction2.2.3 Simultaneous multithreadingAn enhancement in the POWER7 processor is the addition of the SMT4 mode to enable fourinstruction threads to execute simultaneously in each POWER7 processor core. Thus, theinstruction thread execution modes of the POWER7 processor are as follows: SMT1: single instruction execution thread per core SMT2: two instruction execution threads per core SMT4: four instruction execution threads per coreSMT4 mode enables the POWER7 processor to maximize the throughput of the processorcore by offering an increase in processor-core efficiency. SMT4 mode is the latest step in anevolution of multithreading technologies introduced by IBM. Figure 2-4 shows the evolution ofsimultaneous multithreading.Figure 2-4 Evolution of simultaneous multithreadingThe various SMT modes offered by the POWER7 processor allow flexibility, enabling users toselect the threading technology that meets a combination of objectives (such as performance,throughput, energy use, and workload enablement).Intelligent threadsThe POWER7 processor features intelligent threads, which can vary based on the workloaddemand. The system either automatically selects (or the system administrator can manuallyselect) whether a workload benefits from dedicating as much capability as possible to a singlethread of work, or if the workload benefits more from having capability spread across two orfour threads of work. With more threads, the POWER7 processor can deliver more totalcapacity because more tasks are accomplished in parallel. With fewer threads, workloads thatneed fast individual tasks can get the performance they need for maximum benefit.Multi-threading EvolutionThread 1 ExecutingThread 0 ExecutingNo Thread ExecutingFX0FX1FP0FP1LS0LS1BRXCRL1995 Single thread out of orderFX0FX1FP0FP1LS0LS1BRXCRL1997 Hardware mutithreadFX0FX1FP0FP1LS0LS1BRXCRL2003 2 Way SMTFX0FX1FP0FP1LS0LS1BRXCRL2009 4 Way SMTThread 3 ExecutingThread 2 Executing
