CS 452/652 Winter 2022 - Lecture 28

Priority Inversion

• lower-priority tasks indirectly keeps higher-priority task from running
• performance (latency) problem
• correctness problem (starvation), if system fully loaded
• examples: lower number = higher priority
• example scenario A
  • t1 sends to t10, then t5 preempts
  • t5 preempts work done for t1
  • ⇒ adjust server priority to current client's priority
• example scenario B
  • t10 sends to t1, then t5 wants to run
  • t1 runs on behalf of t10, so t5 should be able to preempt
  • ⇒ adjust server priority to current client's priority
• example scenario C
  • t10 runs on behalf of t8, t7 preempts, then t5 wants to send to t10
  • t7 keeps t10 from completing its computation, t5 blocked
  • ⇒ boost server priority to waiting client's priority
• also: priority-based Send queues?

Sensor Processing

• intuition: run sensor querying at highest possible priority
• caveat: track communication is half-duplex
• sensor reports → track commands
• ordering track commands vs. next sensor query?

Context-Switch Testing

• computation / loop with easily verifiable results
• high register coverage
• use subroutine, recursion (to a limit)
• trigger frequent timer interrupt
  • randomized timeout
  • interrupt handler (kernel): perform similar computation
  • clobber as much state as possible
• test with and without optimization

Kernel Testing

• repeat speed measurements as kernel testing method
  • should see similar measurements
• test reliability of train commands
  • speed variation and timing
  • change direction and verify using sensors
  • add lights on/off to increase command intensity

Train Control Robustness

• insert "Chaos Monkey" at track server
• fake or suppress reports/commands
  • sensor
  • turnout
  • speed