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Offline Compression for On-Chip RAM

John Regehr University of Utah Joint work with Nathan Cooprider. Offline Compression for On-Chip RAM. Microcontrollers (MCUs). $12.5 billion market in 2006 ~10 billion units / year MCU-based products typically highly sensitive to unit cost. On-Chip RAM is Small. Atmel AVR examples

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Offline Compression for On-Chip RAM

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  1. John Regehr University of Utah Joint work with Nathan Cooprider Offline Compression for On-Chip RAM

  2. Microcontrollers (MCUs) • $12.5 billion market in 2006 • ~10 billion units / year • MCU-based products typically highly sensitive to unit cost

  3. On-Chip RAM is Small • Atmel AVR examples • mega48 – 0.5 KB RAM – $1.50 • mega128 – 4 KB RAM – $8.75 • mega256 – 8 KB RAM – $10.66 • SRAM can dominate power consumption of a sleeping chip • On-chip flash memory is larger • 1 transistor / bit vs. 6 transistors / bit for SRAM

  4. Out of RAM – What Next? • Manually optimize RAM usage? • Remove features? • Buy MCUs with more RAM?

  5. Is RAM Used Efficiently? • Performed byte-level value profiling for TinyOS applications • Simulator counts distinct values stored in each byte of RAM over a number of executions • Result: Average byte stores four values! • These applications are already tuned for RAM usage

  6. RAM Compression • Use static analysis to find redundancy in scalars, pointers, structs, arrays in embedded C code • Automatically perform sub-word packing to save RAM

  7. Static Analysis • Input: C program • For example, output of nesC compiler • Output: Value set for each variable • E.g. {0,1} for a boolean • Analysis built on standard techniques • However: • Very aggressive compared to e.g. gcc • Many tricks to get good precision on TinyOS applications – several novel

  8. x ≝ variable that occupies n bits Vx≝ conservative estimate of value set log2|Vx| < n RAM compression possible Cx≝ another set such that |Cx| = |Vx| fx≝ bijection from Vx to Cx n - log2|Cx| bits saved through compression of x

  9. void (*function_queue[8])(void);

  10. void (*function_queue[8])(void); x n = size of a function pointer = 16 bits

  11. Vx x &function_A &function_B &function_C NULL

  12. n = 16 bits Vx x |Vx| = 4 log2|Vx| < n  2 < 16

  13. Vx Cx x fx≝Vx to Cx≝ compression 0 1 fx-1≝Cx to Vx≝ decompression 2 3

  14. ROM Cx Vx = { , , , } x 0 fx≝ compression table scan 1 2 fx-1≝ decompression table lookup 3

  15. ROM Cx Vx = { , , , } x 0 1 128 bits reduced to 16 bits 2 112 bits of RAM saved 3

  16. Implementation • Source-to-source transformation for C • Rewrite declaration, initializer, reads, writes • What about value sets of size 1? • log2 1 = 0 • No bits required – just propagate the constant • Optimizations • Avoid table-driven compression funcs when possible • Align hot compressed values on word boundaries • Merge redundant compression tables • Compile-time compression when storing constants

  17. RAM Compression Results *  simulator unavailable

  18. RAM Compression Results Constant Prop / DCE 10% RAM reduction 20% ROM reduction 5.9% duty cycle reduction *  simulator unavailable

  19. RAM Compression Results Constant Prop / DCE 10% RAM reduction 20% ROM reduction 5.9% duty cycle reduction Compression 22% RAM reduction 3.6% ROM reduction 29% duty cycle increase *  simulator unavailable

  20. Tuning RAM Compression • Can elect to not compress some compressible variables • But which ones? • For each compressible variable compute a cost / benefit ratio • Cost – estimated penalty in ROM or CPU cycles • Benefit – RAM savings • Sort compressible variables by ratio and compress until some threshold is met

  21. Cost/Benefit Ratio C ≝ access profile A,B ≝ platform-specific costs V ≝ cardinality of value set

  22. Cost/Benefit Ratio C ≝ access profile A,B ≝ platform-specific costs V ≝ cardinality of value set Su≝ original size Sc≝ compressed size

  23. Turning the RAM Knob 0%

  24. Turning the RAM Knob 10%

  25. Turning the RAM Knob 20%

  26. Turning the RAM Knob 30%

  27. Turning the RAM Knob 40%

  28. Turning the RAM Knob 50%

  29. Turning the RAM Knob 60%

  30. Turning the RAM Knob 70%

  31. Turning the RAM Knob 80%

  32. Turning the RAM Knob 90%

  33. Turning the RAM Knob 100%

  34. Turning the RAM Knob 95%

  35. Compression Spectrum

  36. Compression Spectrum 95%

  37. Compression Spectrum 95%

  38. Conclusion • RAM likely to remain scarce in low-cost, low-power MCUs • RAM is used inefficiently • Manually RAM optimization is unpleasant • Tradeoffs are useful • CComp implements RAM compression http://www.cs.utah.edu/~coop/research/ccomp/

  39. Analysis Times

  40. No RAM Compression * ⇒ simulator unavailable

  41. Full RAM Compression * ⇒ simulator unavailable

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