ELEC 7770 Advanced VLSI Design Spring 2007 Power Aware Microprocessors

1. ELEC 7770Advanced VLSI DesignSpring 2007Power Aware Microprocessors Vishwani D. Agrawal James J. Danaher Professor ECE Department, Auburn University Auburn, AL 36849 vagrawal@eng.auburn.edu http://www.eng.auburn.edu/~vagrawal/COURSE/E7770_Spr07 ELEC 7770: Advanced VLSI Design (Agrawal)

2. SIA Roadmap for Processors (1999) Source: http://www.semichips.org ELEC 7770: Advanced VLSI Design (Agrawal)

3. Power Reduction in Processors • Just about everything is used. • Hardware methods: • Voltage reduction for dynamic power • Dual-threshold devices for leakage reduction • Clock gating, frequency reduction • Sleep mode • Architecture: • Instruction set • hardware organization • Software methods ELEC 7770: Advanced VLSI Design (Agrawal)

4. SPEC CPU2000 Benchmarks • Twelve integer and 14 floating point programs, CINT2000 and CFP2000. • Each program run time is normalized to obtain a SPEC ratio with respect to the run time of Sun Ultra 5_10 with a 300MHz processor. • CINT2000 and CFP2000 summary measurements are the geometric means of SPEC ratios. ELEC 7770: Advanced VLSI Design (Agrawal)

5. Reference CPU s: Sun Ultra 5_10 300MHz Processor ELEC 7770: Advanced VLSI Design (Agrawal)

6. CINT2000: 3.4GHz Pentium 4, HT Technology (D850MD Motherboard) SPECint2000_base = 1341 SPECint2000 = 1389 Source: www.spec.org ELEC 7770: Advanced VLSI Design (Agrawal)

7. Two Benchmark Results • Baseline: A uniform configuration not optimized for specific program: • Same compiler with same settings and flags used for all benchmarks • Other restrictions • Peak: Run is optimized for obtaining the peak performance for each benchmark program. ELEC 7770: Advanced VLSI Design (Agrawal)

8. CFP2000: 3.6GHz Pentium 4, HT Technology (D925XCV/AA-400 Motherboard) SPECfp2000_base = 1627 SPECfp2000 = 1630 Source: www.spec.org ELEC 7770: Advanced VLSI Design (Agrawal)

9. CINT2000: 1.7GHz Pentium 4(D850MD Motherboard) SPECint2000_base = 579 SPECint2000 = 588 Source: www.spec.org ELEC 7770: Advanced VLSI Design (Agrawal)

10. CFP2000: 1.7GHz Pentium 4 (D850MD Motherboard) SPECfp2000_base = 648 SPECfp2000 = 659 Source: www.spec.org ELEC 7770: Advanced VLSI Design (Agrawal)

11. Energy SPEC Benchmarks • Energy efficiency mode: Besides the execution time, energy efficiency of SPEC benchmark programs is also measured. Energy efficiency of a benchmark program is given by: 1/(Execution time) Energy efficiency = ──────────── joules consumed ELEC 7770: Advanced VLSI Design (Agrawal)

12. Energy Efficiency • Efficiency averaged on n benchmark programs: n Efficiency = (Π Efficiencyi)1/n i=1 where Efficiencyi is the efficiency for program i. • Relative efficiency: Efficiency of a computer Relative efficiency = ───────────────── Eff. of reference computer ELEC 7770: Advanced VLSI Design (Agrawal)

13. SPEC2000 Relative Energy Efficiency Always max. clock Laptop adaptive clk. Min. power min. clock ELEC 7770: Advanced VLSI Design (Agrawal)

14. Voltage Scaling • Dynamic: Reduce voltage and frequency during idle or low activity periods. • Static: Clustered voltage scaling • Logicon non-critical paths given lower voltage. • 47% power reduction with 10% area increase reported. • M. Igarashi et al., “Clustered Voltage Scaling Techniques for Low-Power Design,” Proc. IEEE Symp. Low Power Design, 1997. ELEC 7770: Advanced VLSI Design (Agrawal)

15. Pipeline Gating • A pipeline processor uses speculative execution. • Incorrect branch prediction results in pipeline stalls and wasted energy. • Idea: Stop fetching instructions if a branch hazard is expected: • If the count (M) of incorrect predictions exceeds a pre-specified number (N), then suspend fetching instruction for some k cycles. • Ref.: S. Manne, A. Klauser and D. Grunwald, “Pipeline Gating: Speculation Control for Energy Reduction,” Proc. 25th Annual International Symp. Computer Architecture, June 1998. ELEC 7770: Advanced VLSI Design (Agrawal)

16. Slack Scheduling • Application: Superscalar, out-of-order execution: • An instruction is executed as soon as data and resources it needs become available. • A commit unit reorders the results. • Delay the execution of instructions whose result is not immediately needed. • Example of RISC instructions: • add r0, r1, r2; (A) • sub r3, r4, r5; (B) • and r9, x1, r9; (C) • or r5, r9, r10; (D) • xor r2, r10, r11; (E) J. Casmira and D. Grunwald, “Dynamic Instruction Scheduling Slack,” Proc. ACM Kool Chips Workshop, Dec. 2000. ELEC 7770: Advanced VLSI Design (Agrawal)

17. Slack Scheduling Example ELEC 7770: Advanced VLSI Design (Agrawal)

18. Slack Scheduling Re-order buffer Scheduling logic Low-power execution units Slack bit ELEC 7770: Advanced VLSI Design (Agrawal)

19. Clock Distribution clock ELEC 7770: Advanced VLSI Design (Agrawal)

20. Clock Power Pclk = CLVDD2f + CLVDD2f / λ + CLVDD2f / λ2 + . . . stages – 1 1 = CLVDD2f Σ ─ n = 0 λn where CL = total load capacitance λ = constant fanout at each stage in distribution network Clock consumes about 40% of total processor power. ELEC 7770: Advanced VLSI Design (Agrawal)

21. Clock Network Examples D. W. Bailey and B. J. Benschneider, “Clocking Design and Analysis for a 600-MHz Alpha Microprocessor,” IEEE J. Solid-State Circuits, vol. 33, no. 11, pp. 1627-1633, Nov. 1998. ELEC 7770: Advanced VLSI Design (Agrawal)

22. Power Reduction Example • Alpha 21064: 200MHz @ 3.45V, power dissipation = 26W • Reduce voltage to 1.5V, power (5.3x) = 4.9W • Eliminate FP, power (3x) = 1.6W • Scale 0.75→0.35μ, power (2x) = 0.8W • Reduce clock load, power (1.3x) = 0.6W • Reduce frequency 200→160MHz, power (1.25x) = 0.5W • J. Montanaro et al., “A 160-MHz, 32-b, 0.5-W CMOS RISC Microprocessor,” IEEE J. Solid-State Circuits, vol. 31, no. 11, pp. 1703-1714, Nov. 1996. ELEC 7770: Advanced VLSI Design (Agrawal)

23. Parallel Architecture Processor Processor Input Output Output f/2 Input Processor f f Capacitance = C Voltage = V Frequency = f Power = CV2f Capacitance = 2.2C Voltage = 0.6V Frequency = 0.5f Power = 0.396CV2f f/2 ELEC 7770: Advanced VLSI Design (Agrawal)

24. Pipeline Architecture Processor ½ Proc. ½ Proc. Input Output Input Output Register Register Register f f Capacitance = 1.2C Voltage = 0.6V Frequency = f Power = 0.432CV2f Capacitance = C Voltage = V Frequency = f Power = CV2f ELEC 7770: Advanced VLSI Design (Agrawal)

25. Approximate Trend G. K. Yeap, Practical Low Power Digital VLSI Design, Boston: Kluwer Academic Publishers, 1998. ELEC 7770: Advanced VLSI Design (Agrawal)

26. For More on Microprocessors • T. D. Burd and R. W. Brodersen, Energy Efficient Microprocessor Design, Springer, 2002. • R. Graybill and R. Melhem, Power Aware Computing, New York: Plenum Publishers, 2002. ELEC 7770: Advanced VLSI Design (Agrawal)