Grand Central Dispatch and multi-threading [iCONdev 2014]
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Grand Central Dispatch (GCD) is Apple's API for managing application threads and tasks. It provides queues for serially or concurrently executing tasks and handles lower-level threading details. GCD aims to simplify threading by focusing on tasks rather than threads directly. It allows specifying parallelization of tasks without worrying about threads. Key aspects include dispatch queues for serial or concurrent execution of blocks, dispatch groups for synchronization, and avoiding shared mutable state which can cause threading issues.
![iOS threading technologies
• Multiple processes – forking disabled, no XPC
• Low-level threads
• POSIX Threads (pthread)
• NSThread
• -[NSObject performSelectorInBackground:withObject:]
• Higher-level abstractions
• NSOperationQueue, NSOperation
• GCD](https://image.slidesharecdn.com/icondev-gcd-140326100357-phpapp01/85/Grand-Central-Dispatch-and-multi-threading-iCONdev-2014-11-320.jpg)
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Grand Central Dispatch and multi-threading [iCONdev 2014]
- 1. Grand Central Dispatch and multi-threading Kuba Brecka @kubabrecka
- 2. Terminology • Parallel • Multi-threaded • Concurrent • Simultaneous • Asynchronous
- 3. Why parallelize? • Responsiveness • “when I scroll, it’s smooth” • Performance • “it works fast” • Energy saving • “it doesn’t drain my battery” • Convenience • some things are parallel by nature, e.g. running two completely separate apps
- 4. How? • Multiple processes • XPC, fork • Multiple threads • POSIX Threads, NSThread • High-level thread abstraction • Operation queues, dispatch queues • GPGPU • Instruction-level parallelism • superscalar CPUs, pipelining, vector instructions • Multiple PCs • servers, clouds
- 5. Threads • What is a thread? • It’s an abstraction made by the OS • The CPU has no such concept • Represents a line of calculation • Has an ID, a stack, thread-local storage, priority, CPU registers • Shares memory and resources within a process • The OS scheduler runs/pauses threads • context switching
- 6. Issues with threading • Race conditions • the result depends on the timing of the scheduler • the behavior is non-deterministic • can result in almost anything • crash, wrong result, corrupted data • So, you have to use locks/mutexes/… • More issues: deadlocks, livelocks, starvation • Even the best guys have trouble with these • Security consequences, vulnerabilities
- 7. Know your enemy • The compiler • The CPU • The memory • Time • Your brain
- 8. The iPhone has matured iPhone 4 512 MB RAM A4 SoC (1 core) 800 MHz iPhone 4S 512 MB RAM A5 SoC (2 core) 800 MHz iPhone 5 1 GB RAM A6 SoC (2 core) 1300 MHz iPhone 5S 1 GB RAM A7 SoC (2 core) 1300 MHz
- 9. ARM has matured • Apple A5 (2011) • ARM Cortex-A9 MPCore • 2 cores • out-of-order execution • speculative execution • superscalar, pipelining (8 stages) • NEON 128-bit SIMD • Apple A7 (2013) • ARMv8-A “Cyclone” • 64-bit, 32 registers, per-core L1 cache
- 10. iOS has matured • The kernel knows a lot more about the system than the developer • GCD • Operation Queues • LLVM, compiler optimizations • GPU computations • Accelerate.framework
- 11. iOS threading technologies • Multiple processes – forking disabled, no XPC • Low-level threads • POSIX Threads (pthread) • NSThread • -[NSObject performSelectorInBackground:withObject:] • Higher-level abstractions • NSOperationQueue, NSOperation • GCD
- 12. Parallelizing tasks vs. algorithms • Task = a standalone unit of work • has some inputs, gives some outputs • “add a blur effect to these 1000 photos” • 1 photo = 1 task (independent) • “add a blur effect to this one 5000x5000px photo” • 1 task = ? • Some algorithms simply cannot be parallelized (you will not get any significant speedup)
- 13. What’s thread safety? • “Thread-safe object” • you can safely use the object from multiple threads at the same time • the internal state of the object will not get corrupted and it will behave correctly • When you don’t know if an object is thread-safe, you have to assume it isn’t • How do you make your object thread-safe? • immutability, locks, atomic reads/writes
- 14. Shared mutable state • Exclusive immutable object = no problem • Shared immutable object = no problem • Exclusive mutable object = no problem • Shared mutable object • root of all evil • you always want to minimize this
- 15. Global variables • “Global variables are bad” • Multi-threading is another very good reason not to use global variables / global state • Global variables are always shared • Watch out for “hidden” global state: • working directory, chdir() • environment variables, putenv()
- 16. Thread-safety vs. iOS • Terrible lack of proper documentation • Most of the low-level Obj-C runtime is thread-safe • memory management, ARC, weak references, … • Immutable objects (NSString, NSArray, …) are thread-safe • A few other classes are thread-safe • Usually it’s thread-safe to call class methods • google for “iOS thread safety” • https://developer.apple.com/library/ios/DOCUMENTATION/Cocoa/ Conceptual/Multithreading/ThreadSafetySummary/ ThreadSafetySummary.html
- 17. Grand Central Dispatch • Let’s not think about threads • Instead, let’s think about tasks • New concepts: • Tasks • Queues • Queue-specific data • Dispatch groups • Dispatch sources • Synchronization • Semaphores, barriers • C API (!) but has ARC and works with blocks
- 18. GCD queues • Main queue • there is just one, executed on the main thread • Concurrent queue • tasks run concurrently • 4 pre-made concurrent queues with different priorities • DISPATCH_QUEUE_PRIORITY_DEFAULT, _HIGH, _LOW, _BACKGROUND • you can make your own • Serial queue • only one task at a time, in order • you can make your own
- 19. GCD task API • Get/create a queue: • dispatch_get_global_queue • dispatch_get_main_queue • dispatch_queue_create • Submit task: • dispatch_sync • dispatch_async • dispatch_apply
- 20. GCD convenience API • dispatch_once • guarantees the code run only run once • use to implement a proper and fast singleton • dispatch_after • execute the task at a specific time
- 21. It’s not threads • GCD uses threads, but the threads are completely managed by GCD • You can’t assume your code will run on any specific thread • even two tasks from the same serial queue can run on different threads • Don’t use thread-local storage • Don’t use thread priorities
- 22. Demo
- 23. Solutions • Avoid shared mutable state • communicate by message passing • design your objects as immutable • avoid multithreading • Synchronization • You must always have “a plan” • if you can’t tell which code is supposed to run in which thread/queue, then nobody can help you • if you can’t tell which data can be accessed from which thread/queue, then nobody can help you
- 24. • Thank you. Kuba Brecka @kubabrecka iosak.cz