Battery Aware Task Scheduling for a SystemOnAChip Using VoltageClock Scaling

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Battery Aware Task Scheduling for a
System-On-A-Chip Using Voltage/Clock Scaling

• Princey Chowdhury
• Chaitali Chakrabarti
• - IEEE SPS 02 -

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Motivation Goal

• Task Scheduling
• How to sequence load profile for long battery
  lifetime?
• Goal
• Battery aware scheduling algorithm is guided by
  heuristics that are derived from the properties
  of the battery model.

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Background (I)

• Battery Behavior

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Background (II)

• Peukerts Formula
• C(I) k / Ia
• Battery Model
• a 35220, ß 0.637

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Background (III)

• Battery Performance Curve

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Background (IV)

• Cost function
• Q a s
• Q is the instantaneous charge slack
• Objective minimize the value of the sum s
• Constraints
• Deadline constraints
• Dependency constraints
• Endurance constraints

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Scheduling Algorithm(I)

• Theorem 1
• For a fixed voltage assignment, sequencing tasks
  in the non-increasing order of their currents is
  optimal.
• Theorem 2
• If a battery fails during some task K, it is
  always cheaper to repair it by down-scaling its
  voltage than by inserting an off-line period
  before K.
• Theorem 3
• Given a pair of two identical tasks in the
  profile and a delay slack to be utilized by
  voltage down-scaling, it is always better to use
  the slack on the later task than on earlier task.

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Scheduling Algorithm (II)

• Scheduling algorithm

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Scheduling on asingle processor system (I)
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Scheduling on asingle processor system (II)
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Scheduling on a multiprocessor system (I)

• The charge that the battery has to deliver is
  proportional to the sum of the current loads of
  the processors.
• The assignment of loads among the processors
  should be such that the resulting load profile is
  the steepest.

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Scheduling on a multiprocessor system (II)
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Scheduling on a multiprocessor system (III)

• Phase 1
• Permissible schedule assuming no battery failure
• Task scheduling with list based
• ( task voltages are assigned to the highest )
• Phase 2
• Slack Utilization

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Scheduling on a multiprocessor system (IV)
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Scheduling on a multiprocessor system (V)
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Conclusion

• System load profile affects the battery lifetime
• Controlling the discharge current shape is
  critical to battery lifetime improvement
• Voltage / clock scaling is a effective way of
  increasing battery lifetime