Operating Systems Quiz

Operating Systems

• Operating systems (OS) focus on managing processes and threads.
• Multiprogramming manages multiple processes on a single processor.
• Multiprocessing manages multiple processes on multiple processors.
• Distributed processing manages multiple processes on multiple distributed systems.

Concurrency Terminologies

• Atomic operations: Execute as a group, no other process can see intermediate states or interrupt, effectively isolating it from concurrent processes.
• Critical Section: Section of code requiring access to shared resources, only one process can execute it at a time.
• Race Condition: Multiple threads/processes read and write shared data, final result depends on execution timing.
• Starvation: Runnable process is overlooked indefinitely by the scheduler, even though it's ready to proceed.

Process Interactions

• Competition: Processes unaware of each other, can be batch jobs, interactive sessions, or a mix. Potential problems: mutual exclusion, deadlock, starvation.
• Cooperation by Sharing: Processes share access to objects but may not be aware of each other. Potential problems: mutual exclusion, deadlock, starvation, data coherence.
• Cooperation by Communication: Processes communicate by process identification and work jointly. Potential problems: deadlock, starvation.

Mutual Exclusion

• Requires the following:
  • Only one process at a time in its critical section.
  • Halting in non-critical section does not interfere with other processes.
  • No deadlock or starvation.
  • Processes waiting to enter critical sections must be allowed without delay.
  • No assumptions on process speed or processor count.
  • Processes remain in critical section for a finite time.

Concurrency Solutions

• Semaphore: Control access to shared resources, signaling available resources.
• Mutex (Mutual Exclusion): Provides exclusive access to a resource, only the one who locks it can unlock, only the holder can operate.
• Condition Variable: A data type blocking a process/thread until a specific condition is met.
• Monitor: Encapsulates variables, access procedures, and initialization code, controlling access and ensuring only one process is active at a time.

Deadlock

• Occurs when:
  • Mutual exclusion: resource held by one process, others wait.
  • Hold and wait: process holds resource while waiting for another.
  • No preemption: resources cannot be forcibly removed.
  • Circular wait: processes waiting for each other in a cycle.

Deadlock Recovery

• Common Methods:
  • Aborting all deadlocked processes: most common.
  • Rolling back processes to checkpoints: requires rollback and restart mechanisms, but deadlock may recur.
  • Successively aborting processes until deadlock is gone: requires repeated detection.
  • Successively preempting resources until deadlock is gone: requires rolling back processes.

Deadlock Avoidance

• Deny resource requests if allocation might lead to deadlock:
  • Process initiation denial: don't start a process if its demands might lead to a deadlock.
  • Resource allocation denial: don't grant incremental requests if they might lead to a deadlock.

Deadlock Prevention

• Restrict system behavior:
  • No preemption: prevent by requiring processes to release held resources if denied requests, letting go of resources to allow other processes, or requesting additional resources.
  • Circular wait: prevent by defining a linear ordering of resource types.

Deadlock Avoidance Restrictions

• Maximum resource requirement for each process must be known in advance.