Estimating the Worst-Case Energy Consumption of Embedded Software

1
Estimating the Worst-Case Energy Consumption of
Embedded Software
Ramkumar Jayaseelan Tulika Mitra Xianfeng
Li School of Computing National University of
Singapore
2
Motivation

• Conventional scheduling techniques give timing
  guarantees
• Processor cycles is the critical resource
• WCET of the tasks are required input
• Battery life is equally important for mobile
  devices
• Scheduling technique have to give energy
  guarantees
• Worst-Case Energy Consumption (WCEC) of the tasks
  are required input

3
Remotely Deployed Systems
Local Station
Sensor Network

• Available energy unevenly distributed among nodes
• Spatio-temporal scheduling benefits from WCEC

4
Energy-Based Guarantees

• Scheduling critical and non-critical tasks in a
  battery-operated system
• Non-critical tasks can be run only if energy
  constraints for critical tasks are satisfied
• Worst-case energy estimation is crucial

5
Reward-Based Scheduling

• Energy consumption ? Voltage
• Delay ? (1 / Voltage)
• Reward-based scheduling attempts to satisfy
  constraints on energy and timing
• Energy guarantee only if worst-case energy
  consumption of tasks are known

6
Outline

• Background
• Relation between WCET and Worst-case energy
  consumption
• Estimation technique Simplified model
• Instruction cache and speculation
• Experimental results
• Conclusion

7
Background

• Power and energy are often used interchangeably
• Power is energy consumed per unit time
• Energy consumed during program execution E P
  t
• Approximation as P is also a function of time

8
EPT is an approximation

• In reality when a program executes
• Energy is the area under the curve
• E ?P(t)dt

9
WCEC versus WCET
Full Input Space Expansion for a 5-element
Insertion Sort program
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Cannot Estimate WCEC from WCET
Benchmark WCETavg_power µJ Observed µJ
isort 489.92 525.88
fft 12106.49 10260.86
fdct 138.20 105.57
ludcmp 131.76 119.33
matsum 972.03 1154.31
minver 93.61 80.80
bsearch 3.84 3.07
des 724.05 643.75
matmult 178.12 166.88
qsort 54.79 43.73
qurt 23.80 17.65
Possible underestimation using WCECWCET power
11
WCEC versus WCET

• WCEC path need not be the same as the WCET path
• WCEC cannot be directly estimated from the WCET
  value

12
A closer look at Power

• Dynamic Power Power Consumption due to
  switching of transistors
• Leakage Power Power consumed independent of
  switching activity
• Dynamic power forms the bulk of power consumption
  in todays processors

13
Dynamic Power

• Dynamic Power
• P(1/2) A V2 C f
• V is supply voltage
• C is the capacitance of the circuit
• f is the frequency
• A is the activity factor
• V, C, f are independent of program execution
• Variation in P is due to the variation in A

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Variation in Activity Factor (A)

• Not all parts of the processor are used in every
  cycle
• e.g., data-cache is used only for loads/stores
• Clock gating disables unused components
• Activity factor (A) varies during the execution
  of the program
• Model variation in A through static analysis

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Switch-off Energy

• An inactive component cannot be fully switched
  off
• A certain portion of the peak energy is consumed
  even in idle cycles
• Switch-off energy is proportional to the number
  of idle cycles

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Clock Energy and Leakage Energy

• Clock power power consumed in clock distribution
  network
• Leakage power power consumed due to leakage in
  transistors
• Clock energy and leakage energy are directly
  proportional to the execution time

17
Energy Components Summary

• Dynamic Energy
• Switching of transistors during execution
• Independent of execution time
• Switch-off Energy
• Energy consumed in unused components
• Depends on idle cycles
• Clock and Leakage energy
• Directly proportional to execution time

18
WCEC versus WCET
Full Input Space Expansion for a 5-element
Insertion Sort program
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Our Analysis Overview

• Operate on the control flow graph
• Estimate worst-case energy of basic blocks
• Formulate estimation for whole program as an
  integer linear programming (ILP) problem

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ILP Formulation

• Input Control flow graph of the program
• Objective function
• Need to estimate Worst-Case Energy Consumption(
  WCECB) for each basic block

Worst Case Energy ? WCECB ? countB
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Flow Constraints
Inflow Basic Block Execution Count
Outflow Bounds on maximum loop iterations

• E0,1 B0 1
• E2,3 E1,3 B3 1
• E0,1 E2,1 E1,2 E1,3 B1
• E1,2 E2,3 E2,1 B2
• Loop bound E2,1 lt 100

B0
B1
B2
B3
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Worst-Case Energy of a Basic Block

• Processor Model
• Energy Components
• Instruction Specific Energy
• Pipeline Specific Energy

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Processor Model
I1
IF
IBUF
I
ID
I-1
I-4
ISSUE
EX
I-2
I-3
WB
ALU
MULT
CM
FPU
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Pipelined Execution of Instructions
ADD R1,R2,R3 MUL R4,R5,R6 SUB R7,R8,R9
1 2 3 4 5 6 7 8
CC
IF ID IS EX WB CM
MUL
Difficult to statically predict the energy
consumption in each cycle
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Pipelined Execution of Instructions
ADD R1,R2,R3 MUL R4,R5,R6 SUB R7,R8,R9
1 2 3 4 5 6 7 8
CC
IF ID IS EX WB
MUL
Difficult to statically predict the energy
consumption in each cycle
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Our Approach

• Determine the maximum energy consumed on a
  component by component basis
• Static analysis to determine the maximum energy
  consumed by a component in a specified interval

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Execution of Instruction
IF
ID
ISSUE
EX
WB
CM
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Instruction Specific Energy

• Energy consumed due to the sub-tasks associated
  with execution of an instruction
• e.g., register file access, ALU usage, etc.
• Depends on the type of executed instruction
• No correlation with execution time

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Pipeline Specific Energy

• During program execution energy is consumed due
  to
• Switch-off power (idle cycles)
• Leakage power (every cycle)
• Clock network power (every cycle)
• Cannot be attributed to any instruction
• Energy consumed even in idle cycles

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Energy Components

• Observation Energy consumed can be separated out
  as
• Instruction Specific energy
• Energy associated with the execution of a
  particular instruction
• Independent of execution time
• Pipeline Specific energy
• Energy consumed in other components such as clock
  network, leakage etc.
• Related to execution time

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Worst-case Energy of a Basic block

• dynamicBB Instruction-Specific Energy for BB
• switchoffBB , leakageBB and clockBB are energy
  consumed in unused components, leakage and clock
  network during WCETBB

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Instruction Specific Energy

• Energy consumed due to switching activity
  generated by the instructions in BB
• Sum of energy consumed by individual instructions
  in BB

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Switch-off Energy

• Unused units consume 10 of peak energy
• Switch-off energy for a specific component (C)
• Switch-off energy for basic block BB

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Clock Energy and Leakage Energy

• Clock Energy
• Leakage Energy

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Overlap among basic blocks
Time
t1
B1
B2
B1
t2
t3
BB
WCETBB
t4
B3
t5
B3
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Switch-off Energy

• Unused units consume 10 of peak energy
• Switch-off energy for a specific component (C)
• Switch-off energy for basic block BB

37
Instruction Cache Modeling

• Context based ILP formulation used in WCET
  analysis Li et al RTSS 2004
• Basic block divided into memory blocks
• A context comprises of mapping each of these
  memory blocks to hit/miss
• Estimate the worst-case energy of each context
  taking into account main memory access energy

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Modeling Branch miss-prediction
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Objective function

• count(c,?) is the number of times the basic block
  Bi is executed with path from Bj and the branch
  is predicted correctly
• count(m,?) is similarly defined where the branch
  is miss-predicted
• In a similar manner energy(c,?) and energy(m,?)
  are defined
• The ILP problem is solved to generate values for
  count using constraints similar to WCET analysis

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Results

• Platform Simplescalar toolset
• Modified WCET analysis tool Li et al RTSS 2004
  to estimate worst-case energy
• Energy values for processor components derived
  from parameterized models in Wattch
• ILP problem is solved using CPLEX

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Results

• Compare estimated WCEC against the observed
  values for eleven benchmarks
• Observed values are obtained using Wattch power
  simulator
• Actual inputs producing WCEC is unknown
• Manually select inputs that might produce WCEC

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Styles of Clock Gating

• Simple Peak power is consumed even if there is
  one access to a specific component
• Ideal Power consumed is proportional to the
  number of ports accessed
• Realistic Same as ideal but unused components
  consume switch-off power

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Results
Ideal Clock Gating
Simple Clock Gating
Benchmarks
isort
fft
fdct
ludcmp
matsum
minver
bsearch
des
matmult
qsort
qurt
Est(µJ) Obs(µJ) Ratio
468.85 422.76 1.11
9600.99 8586.49 1.12
89.92 83.63 1.08
98.75 92.77 1.06
1012.83 929.94 1.09
63.66 59.61 1.07
2.54 2.40 1.06
546.41 518.22 1.05
149.70 132.08 1.13
34.90 31.16 1.12
13.98 11.91 1.17
Est(µJ) Obs(µJ) Ratio
524.95 455.94 1.15
11057.50 9185.39 1.20
99.31 88.79 1.11
115.39 100.32 1.15
1227.37 994.11 1.23
74.91 64.15 1.17
3.51 3.07 1.14
613.16 553.74 1.10
172.39 136.93 1.26
39.50 33.84 1.17
16.36 12.97 1.26

• Results for ideal clock gating more accurate than
  simple because of distribution of accesses

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Results
Realistic Clock Gating
Ideal Clock Gating
Benchmarks
isort
fft
fdct
ludcmp
matsum
minver
bsearch
des
matmult
qsort
qurt
Est(µJ) Obs(µJ) Ratio
596.93 525.88 1.14
13631.21 10260.86 1.33
121.65 105.57 1.15
139.75 119.33 1.17
1397.72 1154.31 1.21
90.95 80.80 1.13
3.81 3.07 1.24
715.58 643.75 1.11
212.94 166.88 1.28
49.84 43.73 1.14
21.95 17.65 1.24
Est(µJ) Obs(µJ) Ratio
468.85 422.76 1.11
9600.99 8586.49 1.12
89.92 83.63 1.08
98.75 92.77 1.06
1012.83 929.94 1.09
63.66 59.61 1.07
2.54 2.40 1.06
546.41 518.22 1.05
149.70 132.08 1.13
34.90 31.16 1.12
13.98 11.91 1.17

• Results for ideal clock gating more accurate than
  realistic because of conservative WCET estimation

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Conclusion

• Static worst-case energy estimation technique
  that takes into account pipelining, instruction
  cache and branch prediction
• Future work
• Validation using commercial processors
• Explore the possibility of providing thermal
  guarantees

46
Execution of an Add Instruction
I-Cache Access
IF
ADD
Instruction Decode Rename Logic
ID
ADD
Wakeup Selection logic
ADD
ISSUE
Register File Read Add unit access
ADD
EX
Result Bus
ADD
WB
ROB-retire Register file Update
CM
ADD
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Instruction Specific Energy

• Each Component Accessed once
• Selection logic maybe accessed multiple times
• Instruction Specific Energy is