WWDC 2017

Table of Contents

Modernizing GCD Usage, How to stay on core - Wednesday

Modernizing GCD Usage, How to stay on core - Wednesday

Session video and resources: https://developer.apple.com/videos/play/wwdc2017/706/

Parallelism: Simultaneously execution of closely related computations

Leverage existing system frameworks
Express explicit parallelism with DispatchQueue.concurrentPerform

Concurrency: Composition of independently executed tasks

Use case: UI & Networking & Database at the same time
Repeatedly bouncing between contexts like network and database can become expensive. Reasons could be,
- Repeatedly waiting for exclusive access to contented resources
  - Fair Locks: Gives a chance to every thread to run in sequence
  - Unfair Locks: Reserves thread to finish
  - Lock ownership helps resolve priority inversion

Lock ownership

Single owners, no owners, multiple owners

Using GCD for Concurrency

Serial Dispatch Queue

Mutex
FIFO
Concurrent atomic enqueue

Target Queue Hierarchy

Serial queues and sources can form a tree

Qualitiy of Service (QoS) and Target Queue Hierarchy

priority inversion resolves waiting problems according to importance. E.g User initiated task will always come first eventhough it is queued afterwards

Event Handling on Many independent Queues

Introducing Unified Queue Identity

Unified queue identity can block certain threads for priority

Modernizing Existing Code

No mutation past activation
- Set the properties of inactive objects before activation
  - Source handlers
  - Target queues
Protecting the Target Queue Hierarchy
- Build your queue hierarchy from bottom to top
- Opt into "static queue hierarchy"

Summary

Not going off core is ever more important
Size your work appropriately
Choose good granularity of concurrency
Modernize your GCD usage