COMP 3000 2012 Week 10 Notes: Difference between revisions
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Concurrency | Concurrency | ||
*Examples: | |||
** multi threaded | |||
** multi processes | |||
** multi hosts | |||
**Kernel tasks/ handlers | |||
Mutual exclusion | Mutual exclusion | ||
*"Take turns" | |||
HOw? | HOw? | ||
* State "variable". Stores state saying who's turn is it? COudl be memory location, | * State "variable". Stores state saying who's turn is it? COudl be memory location, variable, file on disk | ||
variable, file on disk | |||
* These include locks, semaphores, mutexes, monitors | * These include locks, semaphores, mutexes, monitors | ||
* Variables: set a variable to true or false if a current facility is being used. | * 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: | * "TOCTTOU" Time to check to time to use: | ||
** if ( m < 1) then m = 1; | ** if ( m < 1) then m = 1; | ||
** Between the if statment and the assignment, the CPU may schedule anothe rprocedure that accesses | ** Between the if statment and the assignment, the CPU may schedule anothe rprocedure that accesses variable m. | ||
Concerning assignement: | Concerning assignement: | ||
| Line 30: | Line 26: | ||
Starvation | Starvation | ||
* There's an issue with writing and reading locks | * 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 | * If a process depends on a lock and that lock is never freed, that process will starve | ||
starve | |||
Deadlock | Deadlock | ||
Revision as of 17:25, 8 November 2012
Concurrency
- Examples:
- multi threaded
- multi processes
- multi hosts
- Kernel tasks/ handlers
Mutual exclusion
- "Take turns"
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
- "Death Pact"
- Mutually dependent on resources
- see wikipage
- happens when there are multiple locks
- 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
- Watchdog timer. "If you don't call me every 30 minutes, call the cops!!!"
- 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
- just a counting variable
Mutex
- semaphore with values of either 1 or 0
- mutual exclusion
- flock is a file based mutext