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Computer Architecture Embedded Computing

Computer Architecture Embedded Computing. Recap: Multithreaded Processors. Simultaneous Multithreading. Multiprocessing. Superscalar. Fine-Grained. Coarse-Grained. Time (processor cycle). Thread 1. Thread 3. Thread 5. Thread 2. Thread 4. Idle slot. Embedded Computing. Sensor Nets.

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Computer Architecture Embedded Computing

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  1. Computer Architecture Embedded Computing

  2. Recap: Multithreaded Processors Simultaneous Multithreading Multiprocessing Superscalar Fine-Grained Coarse-Grained Time (processor cycle) Thread 1 Thread 3 Thread 5 Thread 2 Thread 4 Idle slot

  3. Embedded Computing Sensor Nets Cameras Games Set-top boxes Media Players Printers Robots Smart phones Routers Aircraft Automobiles

  4. What is an Embedded Computer? • A computer not used to run general-purpose programs, but instead used as a component of a larger system. Usually, user does not change the computer program (except for manufacturer upgrades). • Example applications: • Toasters • Cellphone • Digital camera (some have several processors) • Games machines • Set-top boxes (DVD players, personal video recorders, ...) • Televisions • Dishwashers • Car (some have dozens of processors) • Internet router (some have hundreds to thousands of processors) • Cellphone basestation • .... many more

  5. MIT Whirlwind, 1946-51 • Initially developed for real-time flight simulator • IBM later manufactured versions for SAGE air defence network, last used in 1983 • Intel 4004, 1971 • developed for Busicom 141-PF printing calculator • Intel engineers decided that building a programmable computer would be simpler and more flexible than hard-wired digital logic Early Embedded Computing Examples

  6. Reducing Cost of Transistors Drives Spread of Embedded Computing • When individuals could afford a single transistor, the “killer application” was the transistor radio • When individuals could afford thousands of transistors, the killer app was the personal computer • Now individuals can soon afford thousands of processors, what will be the killer apps? In 2007: • human population growth per day >200,000 • cellphones sold per day >2,000,000

  7. What is different about embedded computers? • Embedded processors usually optimized to perform one fixed task with software from system manufacturer • General-purpose processors designed to run flexible, extensible software systems with code from third-party suppliers • applications not known at design time • Note, many products contain both embedded and general-purpose processors • e.g., smartphone has embedded processors for radio baseband signal processing, and general-purpose processors to run third-party software applications

  8. Lesser emphasis on software portability in embedded applications • Embedded systems • can usually recompile/rewrite source code for different ISA, and/or use assembler code for new application-specific instructions • processor pipeline microarchitecture and memory capacity and hierarchy known to programmer/compiler • mix of tasks known to writer of each task, usually static: uses custom run-time system • each task usually “trusts” others, can run in same address space • General-purpose systems • must have standard binary interface for third-party software • compiler doesn’t know about this particular microarchitecture or memory capacity or hierarchy (compiled for general model) • unknown mix of tasks, tasks dynamically added and deleted from mix: uses general-purpose operating system • tasks written by various third-parties, mutually distrustful, need separate address spaces or protection domains

  9. Embedded application requirements & constraints • Real-time performance • hard real-time: if deadline missed, system has failed (car brakes!) • soft real-time: missing deadline degrades performance (skipping frames on DVD playback) • Real-world I/O with multiple concurrent events • sensor and actuators require continuous I/O (can’t batch process) • non-deterministic concurrent interactions with outside world • Cost • includes cost of supporting structures, particularly memory • static code size very important (cost of ROM/RAM) • often ship millions of copies (worth engineer time to optimize cost down) • Power • expensive package and cooling affects cost, system size, weight, noise, temperature

  10. What is Performance? • Latency (or response time, or execution time) • time to complete one task • Bandwidth (or throughput) • tasks completed per unit time

  11. Worst case rates Average rates Inputs C B A Processing Rate (Inputs/Second) Performance Measurement Average rate: A > B > C Worst-case rate: A < B < C Which is best for desktop performance? _______ Which is best for hard real-time task? _______

  12. Processors for real-time software • Simpler pipelines and memory hierarchies make it easier (possible?) to determine the worst-case execution time (WCET) of a piece of code • Would like to guarantee task completed by deadline • Out-of-order execution, caches, prefetching, branch prediction, make it difficult to determine worst-case run time • Have to pad WCET estimates for unlikely but possible cases, resulting in over-provisioning of processor (wastes resources)

  13. Power Measurement I • Energy measured in Joules • Power is rate of energy consumption measured in Watts (Joules/second) • Instantaneous power is Volts * Amps • Battery Capacity Measured in Joules • 720 Joules/gram for Lithium-Ion batteries • 1 instruction on Intel XScale processor takes ~1nJ  ~1 billion executed instructions weigh ~1mg V

  14. Power versus Energy • System A has higher peak power, but lower total energy • System B has lower peak power, but higher total energy Peak A Integrate power curve to get energy Peak B Power Time

  15. Power Impacts on Computer System • Energy consumed per task determines battery life • Second order effect is that higher current draws decrease effective battery energy capacity (higher power also lowers battery life) • Current draw causes IR drops in power supply voltage • Requires more power/ground pins to reduce resistance R • Requires thick&wide on-chip metal wires or dedicated metal layers • Switching current (dI/dt) causes inductive power supply voltage bounce  LdI/dt • Requires more pins/shorter pins to reduce inductance L • Requires on-chip/on-package decoupling capacitance to help bypass pins during switching transients • Power dissipated as heat, higher temps reduce speed and reliability • Requires more expensive packaging and cooling systems • Fan noise • Laptop/handheld case temperature

  16. Power Dissipation in CMOS Short-Circuit Current Primary Components: • Capacitor Charging (~85% of active power) • Energy is 1/2 CV2 per transition • Short-Circuit Current (~10% of active power) • When both p and n transistors turn on during signal transition • Subthreshold Leakage (dominates when inactive) • Transistors don’t turn off completely, getting worse with technology scaling • For Intel Pentium-4/Prescott, around 60% of power is leakage • Optimal setting for lowest total power is when leakage around 30-40% • Gate Leakage (becoming significant) • Current leaks through gate of transistor • Diode Leakage (negligible) • Parasitic source and drain diodes leak to substrate Diode Leakage Current CapacitorCharging Current CL Gate Leakage Current Subthreshold Leakage Current

  17. Reducing Switching Power Power  activity * 1/2 CV2 * frequency • Reduce activity • Reduce switched capacitance C • Reduce supply voltage V • Reduce frequency

  18. Clock Gating • don’t clock flip-flop if not needed • avoids transitioning downstream logic • Pentium-4 has hundreds of gated clocks Enable Global Clock Latch (transparent on clock low) Gated Local Clock D Q Reducing Activity • Bus Encodings • choose encodings that minimize transitions on average (e.g., Gray code for address bus) • compression schemes (move fewer bits) • Remove Glitches • balance logic paths to avoid glitches during settling • use monotonic logic (domino)

  19. C A B A B Shared bus driven by A or B when sending values to C Bus C Insert switch to isolate bus segment when B sending to C Reducing Switched Capacitance Reduce switched capacitance C • Different logic styles (logic, pass transistor, dynamic) • Careful transistor sizing • Tighter layout • Segmented structures

  20. Reducing Frequency • Doesn’t save energy, just reduces rate at which it is consumed • Some saving in battery life from reduction in rate of discharge

  21. Reducing Supply Voltage Quadratic savings in energy per transition – BIG effect • Circuit speed is reduced • Must lower clock frequency to maintain correctness

  22. Voltage Scaling for Reduced Energy • Reducing supply voltage by 0.5 improves energy per transition to 0.25 of original • Performance is reduced – need to use slower clock • Can regain performance with parallel architecture • Alternatively, can trade surplus performance for lower energy by reducing supply voltage until “just enough” performance Dynamic Voltage Scaling

  23. Run fast then stop Frequency Run slower and just meet deadline Time t=0 t=deadline “Just Enough” Performance • Save energy by reducing frequency and voltage to minimum necessary (usually done in O.S.)

  24. Voltage Scaling on Transmeta Crusoe TM5400

  25. Chip energy versus frequency for various supply voltages [ MIT Scale Vector-Thread Processor, TSMC 0.18µm CMOS process, 2006 ]

  26. Chip energy versus frequency for various supply voltages 2x Reduction in Supply Voltage 4x Reduction in Energy [ MIT Scale Vector-Thread Processor, TSMC 0.18µm CMOS process, 2006 ]

  27. Parallel Architectures Reduce Energy at Constant Throughput • 8-bit adder/comparator • 40MHz at 5V, area = 530 km2 • Base power, Pref • Two parallel interleaved adder/compare units • 20MHz at 2.9V, area = 1,800 km2 (3.4x) • Power = 0.36 Pref • One pipelined adder/compare unit • 40MHz at 2.9V, area = 690 km2 (1.3x) • Power = 0.39 Pref • Pipelined and parallel • 20MHz at 2.0V, area = 1,961 km2 (3.7x) • Power = 0.2 Pref Chandrakasan et. al. “Low-Power CMOS Digital Design”, IEEE JSSC 27(4), April 1992

  28. CS252 Administrivia • Next project meetings Nov 12, 13, 15 • Should have “interesting” results by then • Only three weeks left after this to finish project • Second midterm Tuesday Nov 20 in class • Focus on multiprocessor/multithreading issues • We’ll assume you’ll have worked through practice questions

  29. Embedded memory hierarchies • Scratchpad RAMs often used instead, or as well as, caches • RAM has predictable access latency, simplifies execution time analysis for real-time applications • RAM has lower energy/access (no tag access or comparison/multiplexing logic) • RAM is cheaper than same size cache (no tags or cache logic) • Typically no memory protection or translation • Code uses physical addresses • Often embedded processors will not have direct access to off-chip memory (only on-chip RAM) • Often no disk or secondary storage (but printers, iPods, digital cameras, sometimes have hard drives) • No swapping or demand-paged virtual memory • Often, flash EEPROM storage of application code, copied to system RAM/DRAM at boot

  30. Reconfigurable lockable caches • Many embedded systems allow cache lines to be locked in cache to provide RAM-like predictable access • Lock by set • E.g., in an 8KB direct-mapped cache with 32B lines (213/25=28=256 sets), lock half the sets, leaving a 4KB cache with 128 sets • Have to flush entire cache before changing locking by set • Lock by way • E.g., in a 2-way cache, lock one way so it is never evicted • Can quickly change amount of cache that is locked (doesn’t change cache index function) • Can be used in both instruction and data caches • Lock instructions for interrupt handlers • Lock data used by handlers

  31. Code Size • Cost of memory big factor in cost of many embedded systems • RISC core about same size as 16KB of SRAM Techniques to reduce code size: • Variable length and complex instructions • Compressed Instructions • Compressed in memory then uncompressed in cache • compressed in cache 32KB 32KB Intel Xscale (2001) 16.8mm2 in 180nm

  32. Embedded Processor Architectures • Wide variety of embedded architectures, but mostly based on combinations of techniques originally pioneered in supercomputers • VLIW instruction issue • SIMD/vector instructions • Multithreading • VLIW more popular here than in general-purpose computing • Binary code compatibility not as important, recompile for new configuration OK • Memory latencies are more predictable in embedded system hence more amenable to static scheduling • Lower cost and power compared to out-of-order ILP core.

  33. System-on-a-Chip Environment • Often, a single chip will contain multiple embedded cores with multiple private or shared memory banks, and multiple hardware accelerators for application-specific tasks • Multiple dedicated memory banks provide high bandwidth, predictable access latency • Hardware accelerators can be ~100x higher performance and/or lower power than software for certain tasks • Off-chip I/O ports have autonomous data movement engines to move data in and out of on-chip memory banks • Complex on-chip interconnect to connect cores, RAMs, accelerators, and I/O ports together

  34. Block Diagram of Cellphone SoC(TI OMAP 2420)

  35. AReg 7 X Mem Y Mem AReg 1 Off-chip memory AReg 0 Addr X Addr Y Multiply ALU Single 32-bit DSP instruction: Acc. A AccA += (AR1++)*(AR2++) Acc. B Equivalent to one multiply, three adds, and two loads in RISC ISA! “Classic” DSP Processors

  36. TI C6x VLIW DSP VLIW fetch of up to 8 operations/instruction Dual symmetric ALU/Register clusters (each 4-issue)

  37. TI C6x regfile/ALU datapath clusters 32b/40b arithmetic 32b arithmetic, 32b/40b shifts 16b x 16b multiplies 32b arithmetic, address generation

  38. MicroEngine 15 MicroEngine 0 MicroEngine 1 Microcode RAM Microcode RAM Microcode RAM Register File Register File Register File PC0 PC0 PC0 PC1 PC1 PC1 Eight threads per microengine Eight threads per microengine Eight threads per microengine ALU ALU ALU PC7 PC7 PC7 Scratchpad Data RAM Scratchpad Data RAM Scratchpad Data RAM Intel IXP Network Processors RISC Control Processor Network 10Gb/s Buffer RAM DRAM0 Buffer RAM DRAM1 Buffer RAM DRAM2 Buffer RAM SRAM0 Buffer RAM Buffer RAM SRAM1 Buffer RAM SRAM2 SRAM3 16 Multithreaded microengines

  39. Programming Embedded Computers • Embedded applications usually involve many concurrent processes and handle multiple concurrent I/O streams • Microcontrollers, DSPs, network processors, media processors usually have complex, non-orthogonal instruction sets with specialized instructions and special memory structures • poor compiled code quality (% peak with compiled code) • high static code efficiency • high MIPS/$ and MIPS/W • usually assembly-coded in critical routines • Worth one engineer year in code development to save $1 on system that will ship 1,000,000 units • Assembly coding easier than ASIC chip design • But much room for improvement…

  40. Discussion: Memory consistency models • Discussion: Memory consistency models • Tutorial on consistency models + Mark Hill’s position paper • Conflict between simpler memory models and simpler/faster hardware

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