1. [iOS
multithreading:GCD];

2. •Multithreading in iOS
•Grand Central Dispatch
•Queues
•Example

3. Multithreading in iOS

4. •Pthreads : C-based interface for creating
and manipulating threads
•NSThread : Cocoa threads
•Grand Central Dispatch : a
lightweight multithreading engine developed
by Apple Inc
•NSOperation : wrapper for task

5. Pthreads
NSThreads
GCD
NSOperation
complexity
low
high
abstraction level
low
high

6. Grand Central Dispatch

7. What is GCD?
• A lightweight multithreading engine
• Uses a thread pool
• Automatically optimizes threading
• Scheduling of tasks
• Uses look-free exclusion rather than mutual
exclusion

8. • define the tasks
• block or function
• add them to an appropriate dispatch queue
dispatch_async(queue, ^{
some_async_work();
});

9. thread
thread
thread
Queue based
task task task task task
queue
task task task task task
queue
task
task
task

10. Queue types
• Serial Queues
• only one task running at a time
• user queue or main queue
• Concurrent Queue
• tasks started in order but run concurrently
• 3 priority levels: HIGHT, DEFAULT, LOW

11. Frequently Used APIs
• dispatch_async(queue, block)
• dispatch_once(queue, block)
• dispatch_apply(iterations, queue, block)
• dispatch_group_async(group, queue, block)

12. Example

13. Palindromic words
• palindromic words - 回文
• is a word that reads the same forward as it does
backward :“radar”
• problem
• count the palindromic words in a dictionary
radar
wrong
way
yaw
result = 3

14. for (size_t index = 0; index < Words.count; index++) {
NSString *word = [Words objectAtIndex:index];
NSString *reverseString = [self reverseString:word];
if([word caseInsensitiveCompare:reverseString] == NSOrderedSame) {
self.palindromicCount++;
} else {
for (size_t i = index + 1; i < Words.count - 1; i++) {
NSString *secondWord = [Words objectAtIndex:i];
if([reverseString caseInsensitiveCompare:secondWord] == NSOrderedSame) {
self.palindromicCount += 2;
break;
}
}
}
};
dispatch_apply(Words.count, queue, ^(size_t index) {
NSString *word = [Words objectAtIndex:index];
NSString *reverseString = [self reverseString:word];
if([word caseInsensitiveCompare:reverseString] == NSOrderedSame) {
self.palindromicCount++;
} else {
for (size_t i = index + 1; i < Words.count - 1; i++) {
NSString *secondWord = [Words objectAtIndex:i];
if([reverseString caseInsensitiveCompare:secondWord] == NSOrderedSame) {
self.palindromicCount += 2;
break;
}
}
}
});

15. for (size_t index = 0; index < Words.count; index++) {
NSString *word = [Words objectAtIndex:index];
NSString *reverseString = [self reverseString:word];
if([word caseInsensitiveCompare:reverseString] == NSOrderedSame) {
self.palindromicCount++;
} else {
for (size_t i = index + 1; i < Words.count - 1; i++) {
NSString *secondWord = [Words objectAtIndex:i];
if([reverseString caseInsensitiveCompare:secondWord] == NSOrderedSame) {
self.palindromicCount += 2;
break;
}
}
}
};
dispatch_apply(Words.count, queue, ^(size_t index) {
NSString *word = [Words objectAtIndex:index];
NSString *reverseString = [self reverseString:word];
if([word caseInsensitiveCompare:reverseString] == NSOrderedSame) {
self.palindromicCount++;
} else {
for (size_t i = index + 1; i < Words.count - 1; i++) {
NSString *secondWord = [Words objectAtIndex:i];
if([reverseString caseInsensitiveCompare:secondWord] == NSOrderedSame) {
self.palindromicCount += 2;
break;
}
}
}
});

16. Results
• iPhone 5 : dual-core A6 chip
• dictionary : 25000 words
1.86 times faster
Sequential GCD
274s 147s