Preventing Interrupt Overload Presented by Jiyong Park Seoul National University, Korea John Regehr, Usit Duogsaa, School of Computing, University of Utah

2 Summary  Interrupt overload  Too many interrupts that exceed predefined rate.  Solutions for preventing interrupt overload  Strict software scheduler  Bursty software scheduler  Hardware interrupt scheduler interrupt rate processor load for interrupt handling predefined maximum rate

3 Motivation  Interrupt overload can be occur often in embedded systems  Disconnected device (Apollo 11 in 1969)  Unexpected situation (Robot going on downhill)  Malicious peer (too many small packets)

4 Interrupt Handling Model Device pending bit (size 1 queue) enable bit global enable bit CPU interrupt interrupt handler

5 Solution 1: Strict Software Scheduler  Interrupt can not be processed faster than a predefined rate.  Modification to interrupt prologue time interrupt predefined rate pendingdiscard interrupt discard timer interrupt enable = 0 set one-shot timer enable = 1

6 Solution 2: Bursty Software Scheduler  Allows maximum N interrupts within T interval.  Interrupt handler prologue  counter ++;  if counter >= N then enable = off  Timer with period T  counter = 0; enable = on time T N = 3

7 Solution 3: Hardware Scheduler  Implemented on FPGA. Device CPU count = 0 ? down counter count true false reset pending interrupt

8 Evaluation  Overhead of each mechanism  On 4MHz Atmel AVR processor  Overhead: 2 ~ 10%

9 Evaluation  Interrupt handler works 250 cycles

10 Related Work  Receive live-lock, Lazy Receiver Processing  Switch to polling when overload  Early demultiplexing  Network centric  Concentrate on maximizing throughput (no consideration to timely execution of application)  Interrupt Thread  Just delays interrupt overload. Does not prevent it.  Big overhead on very small embedded systems

11 Strong Points  Simple but very effective solution  Modeling the interrupt handler as periodic task (C, T)  The urgent need for preventing interrupt overload is well explained using various examples.  The interrupt scheduler is extended to support multiple interrupt sources.  Hardware solution requires only small amount of transistors (600).

12 Weak Points  Not very much