COMP 3000 2012 Week 10 Notes

Concurrency

Examples:
- multi threaded
- multi processes
- multi hosts
- Kernel tasks/ handlers

Mutual exclusion

HOw?

State "variable". Stores state saying who's turn is it? COudl be memory location, variable, file on disk
These include locks, semaphores, mutexes, monitors

Variables: set a variable to true or false if a current facility is being used. THis is a stupid method though, because two procedures may write to that variable at the same time
"TOCTTOU" Time to check to time to use:
- if ( m < 1) then m = 1;
- Between the if statment and the assignment, the CPU may schedule anothe rprocedure that accesses variable m.

Concerning assignement:

 * Each echo (line) is atomic.
 * Flock controls access to writing to race.txt
 * Flock tries to write to lock, but if it can't, it'll wait

Starvation

 * There's an issue with writing and reading locks
 * If a process depends on a lock and that lock is never freed, that process will starve

Deadlock

requirements for deadlock:
- mutual exclusion
- hold + wait.
- no preemption. "Sit there, waiting for that lock forever"
- circular wait "I'm waiting on you you and you're waiting on me"

How do deal with deadlock?
- 1. Ignore it! Ostrich algorithm
- 2. detection. Are we in deadlock?
  - Example: Timeout. If a process times out, then it's depending on resources that

aren't there.

- - Watchdog timer. "If you don't call me every 30 minutes, call the cops!!!"
    - check state every interval, react if something isn't right
- 3. Prevention. Make sure that one of the one of the requirements for deadlock is never satisfied
- 4. Avoidance. Requirements are satisfied... So watch the transactions to see if

anything fishy happens. If so, pull out. (banker's algorithm):w

Semaphores

Mutex

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