CS M151B EE M116C Computer Systems Architecture Glenn Reinman 4731G Boelter Hall reinman cs ucla edu 1 7 The Power Wall Power Trends In CMOS IC technology Power Capacitive load Voltage 2 Frequency Chapter 1 Computer Abstractions and Technology 2 Reducing Power Suppose a new CPU has 85 of capacitive load of old CPU 15 voltage and 15 frequency reduction Pnew Cold 0 85 Vold 0 85 2 Fold 0 85 4 0 85 0 52 2 Pold Cold Vold Fold The power wall We can t reduce voltage further We can t remove more heat How else can we improve performance Chapter 1 Computer Abstractions and Technology 3 1 8 The Sea Change The Switch to Multiprocessors Uniprocessor Performance Constrained by power instruction level parallelism memory latency Chapter 1 Computer Abstractions and Technology 4 Multiprocessors Multicore microprocessors More than one processor per chip Requires explicitly parallel programming Compare with instruction level parallelism Hardware executes multiple instructions at once Hidden from the programmer Hard to do Programming for performance Load balancing Optimizing communication and synchronization Chapter 1 Computer Abstractions and Technology 5 SPEC CPU Benchmark Programs used to measure performance Standard Performance Evaluation Corp SPEC Supposedly typical of actual workload Develops benchmarks for CPU I O Web SPEC CPU2006 Elapsed time to execute a selection of programs Negligible I O so focuses on CPU performance Normalize relative to reference machine Summarize as geometric mean of performance ratios CINT2006 integer and CFP2006 floating point n n Execution time ratio i i 1 Chapter 1 Computer Abstractions and Technology 6 CINT2006 for Intel Core i7 920 Chapter 1 Computer Abstractions and Technology 7 Improving an aspect of a computer and expecting a proportional improvement in overall performance Timproved Example multiply accounts for 80s 100s Taffected Tunaffected improvemen t factor 1 10 Fallacies and Pitfalls Pitfall Amdahl s Law How much improvement in multiply performance to get 5 overall 80 Can t be done 20 20 n Corollary make the common case fast Chapter 1 Computer Abstractions and Technology 8 Pitfall MIPS as a Performance Metric MIPS Millions of Instructions Per Second Doesn t account for Differences in ISAs between computers Differences in complexity between instructions MIPS Instruction count Execution time 10 6 Instruction count Clock rate 6 Instruction count CPI CPI 10 6 10 Clock rate CPI varies between programs on a given CPU Chapter 1 Computer Abstractions and Technology 9 Cost performance is improving Hierarchical layers of abstraction In both hardware and software Instruction set architecture Due to underlying technology development 1 9 Concluding Remarks Concluding Remarks The hardware software interface Execution time the best performance measure Power is a limiting factor Use parallelism to improve performance Chapter 1 Computer Abstractions and Technology 10