concurrency (동시성)

2개 이상의 스레드 (혹은 프로세스)가 동시에 실행되는 상태이다.
1개의 CPU가 여러 스레드를 번갈아 실행할 때 나타난다.

parallelism (병렬성)

2개 이상의 스레드가 서로 다른 CPU 상에서 같은 시간에 동시에 실행되는 병렬성이다.

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Deadlock

💡 a set of processes is deadlocked when each process in the set is blocked awaiting an event (or a recource) that can only be triggered (released) by another blocked process in the set

Two general categories of resources

✔️ Reusable resource

• Can be safely used by only one process at a time and is not depleted by that use
• ex) processors, memorym I/O devices, files, database and semaphores

✔️ Consumable resource

• Can be created (produced) and destroyed (consumed)
• Typically, there is no limit on the number of consumable resources
• ex) interrupts, signals, messages, data in I/O buffers

Conditions or Deadlock

데드락이 발생하는 조건!

The first three conditions are necessary but not sufficient conditions for a deadlock.

The fourth condition is actually a consequence of the first three.

Given that the first three condition exist, a sequence of events may occur that lead to an unresolvable circular wait.

✔️ Mutual exclusion

• Only one process may use a resource at a time

✔️ Hold and wait

• A process may hold allocated resources when waiting for the other resources
  • Nothing or All

✔️ No preemption

• No resource can be forcibly removed from a process holding it 강제로 뺏을 수 없을 때

✔️ Circular wait

• A closed chain of processes exists such that each process holds at least one resource needed by the next process in the chain

Three Approaches for Deadlocks

데드락을 해결하기 위한 3가지 방안

✔️ Deadlock prevention

• 가장 보수적이고 비효율적인 방법
• Adopt a policy that eliminated one of the conditions 1 through 4

✔️ Deadlock avoidance

• 중도
• Make the appropriate choices dynamically based on the current state of resource allocation

✔️ Deadlock detection

• 내버려둬~ 주기적으로 감지하기
• Allow the deadlock to occur, attempt to detect the presence of deadlock, and recover if a deadlock is detected

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🔗 Reference

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