Tiny OS Optimistic Lightweight Interrupt Handler

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Tiny OS Optimistic Lightweight Interrupt Handler

• Simon Yau (smyau_at_cs.berkeley.edu)
• Alan Shieh (ashieh_at_hkn.eecs.berkeley.edu)

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Tiny OS Overview

• Small footprint system
• Used in small, low-power, embedded devices (e.g.,
  temperature sensors)
• Event-based programming model

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Tiny OS Primer

• Program consist of state machines
• All threads run to completion, unless preempted
  by hardware events
• Event filtering

Radio Bit
Radio Tran- ceiver
Radio Byte
Packet Object
Applic- ation
Active Mess- age
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Interrupt Handling in Tiny OS

• Context switch during interrupts are most
  expensive primitive
• Interrupt lost is power lost

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A Bag of tricks

• Software simulated register window
• Lightweight Interrupt handler
• Optimistic (Lazy) interrrupt handler

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Software Register Window

• Register windows useful for low-overhead
  interrupt handling
• Providing this support in hardware may not be
  cost-effective
• Additional area can be used for other I/O
  optimizations

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Software Register Window (implementation)

• Two versions of gcc
• User-mode and Interrupt-mode gcc
• Each allowed to use approximately half of
  register file
• Calling conventions restricted
• Post-compilation function rename

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Software Register Window
R31
R30
R29
R28
R27
R26
R25
R24
R20-R23
R16-R19
R8-R15
R2-R7
R1
R0
Shared registers
User registers
Interrupt registers
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Lightweight Interrupt Handler

• Software Simulated register windows means less
  registers to work with gt more register spills
• Want a handler that is Lightweight
• Consumes less register (reduce register spills)
• Lower execution time (reduce lost interrupts)

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Lightweight IH (implementation)

• The IH does not fire off an event, but rather
  posts a thread that does.
• We can vary the amount of filtering where this is
  done.

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Lazy Interrupt Handler

• Most of the time the CPU is in sleep mode (I.e.,
  has no thread running).
• In those cases, saving register is unnecessary.
• Set aside a register for sleep mode, and check it
  during interrupts.

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Evaluation environment

• Simulated CPU, with Radio, LED, and Photo sensor.
  
• Benchmarks
• Single processor simulation
• Network simulation
• Measure
• Number of cycles in sleep / active mode
• Energy consumption
• Number of cycles with interrupts disabled
• Number of lost interrupts

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Evaluation

• Register window
• Radio interrupt is now 113 cycles (down from 165)
• But improvement is unstable because asymmetry of
  register set increases register pressure

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Evaluation (cont)

• Lightweight Handler
• Reduces execution time of Timer Interrupt handler
  by 57 Radio Interrupt handler by 33. But added
  thread posting/scheduling overhead for event
  handler.
• Bad for radio interrupt due to real time
  constrains and the overhead.
• Benchmark performance

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Evaluation (cont)

• Lazy Interrupt handler
• Reduces a typical radio interrupt from 165 cycles
  to 114 cycles
• On the bench mark