Dispatch Queue (Concurrency Control)

Grand Central Dispatch (GCD): low-level API for managing concurrent operations. (Schedules threads in shared pool)

= GCD is built on top of threads and manages a shared thread pool + decides which thread to execute them on.

& GCD decides how much parallelism it requires based on the system and available system resources.

* Doesn't work well in Simulators (CIDetector-x)

* Attempting to synchronously execute a work item on the main queue(UI-Update)=Serial results in Deadlock.

 

직렬 (Serial Queue-FIFO) / *동시 (Concurrent Queue)

 

 

 

* 동시성 제어 (Concurrency Control) - 프로세스/쓰레드

- parallelism (병렬성 - 실제 CPU 코어에 영향 받음): 실제 동시 작업 (Execution) - 물리적 동시성

- Time Slicing (시간 분배하면서, 문맥 교환): 동시 작업처럼 보이도록 Context Switching (Structure) - 논리적 동시성

 

 

 

 

※ 헷갈리기 쉬운, Seperate Concept

 

# 동시성/병렬성 - 테스크 스케줄링

# 동기/비동기 - 정해진 테스크 큐 내에서, 실행 방식 설정 (해당 테스크 응답전까지<await>, 다른 테스크 처리 가능)

 

- CPU Core (Single/Multi-Process)

ㄴ In a process, Multi-Thread (Managing Queue<Concurrent/Serial> - Add+Task / Remove-Task)

# GCD에서 제공하는 Queue Type 

- Main Queue: Serial Queue, Operated in Main Thread (UI-Flow_Life Cycle). 

- Global Queue: Priority (high, default, low, background) Concurrent Queue 

+ QoS Options 설정 가능 - User-interactive, User-initiated, Utility, Background

- Custom Queue: Serial or Concurrent Custom Queue

(But, The requests in these queues actually end up in one of the global queues.)

 

+ Call Functions in Queue <Synchronous/Asynchronous>

 

⇨ 즉, 총 4 종류: DispatchQueue - Concurrent/Serial + Async/Sync

 

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To be continued..

+ Await (요청/응답 대기)

+ iOS_Life Cycle & ARC

 

[References]

https://www.kodeco.com/28540615-grand-central-dispatch-tutorial-for-swift-5-part-1-2

https://developer.apple.com/documentation/DISPATCH

https://docs.swift.org/swift-book/documentation/the-swift-programming-language/concurrency