多线程（四）

作者: dandelionYD | 来源:发表于2019-04-12 10:00 被阅读0次

上篇多线程（三）我们看了多线程的安全隐患 以及各种锁的简单使用，接下来我们来看看锁的比较、以及自旋锁、互斥锁比较 和读写安全方案等
代码详见 gitHub_Demo

各种锁类别

锁名字	锁类别	特点
OSSpinLock	自旋锁	等待锁的线程会处于忙等（busy-wait）状态，一直占用CPU资源
os_unfair_lock	互斥锁	等待锁的线程会处于休眠状态，并非忙等
pthread_mutex	互斥锁	等待锁的线程会处于休眠状态
pthread_mutex – 递归	递归锁	需要初始化锁的属性(attr)
pthread_mutex –条件	条件锁	需要设置等待条件（condition）
NSLock	互斥锁	是对mutex锁的简单的封装
NSRecursiveLock	递归锁	是对mutex递归锁简单的封装
NSCondition	条件锁	是对mutex和cond的封装
NSConditionLock	条件锁	是对NSCondition更进一步的封装（可以设置具体的条件值）
SerialQueue	GCD	可以实现线程同步
dispatch_semaphore	GCD	信号量
@synchronized	互斥锁	是对mutex递归锁的封装
pthread_rwlock	读写锁	多读单写
dispatch_barrier_async	GCD	异步栅栏调用
...	...	...

iOS线程同步方案性能比较

Multithreading_33.png

自旋锁、互斥锁比较

Multithreading_34.png

atomic

Multithreading_35.png

nonatomic和atomic
atom：原子，不可再分割的单
atomic：原子性
 
给属性加上atomic修饰，可以保证属性的setter和getter都是原子性操作，也就是保证setter和gette内部是线程同步的
 
// 加锁
int a = 10;
int b = 20;
int c = a + b;
// 解锁

如：
- (void)setName:(NSString *)name{
    // 加锁
    _name = name;
    // 解锁
}

- (NSString *)name{
       //加锁
    return _name;
      // 解锁
}

atomic所说的线程安全只是保证了getter和setter存取方法的线程安全，并不能保证整个对象是线程安全的

#import "ViewController_18.h"
@interface ViewController_18 ()
@property (atomic,assign)NSInteger age;
@end

@implementation ViewController_18
- (void)viewDidLoad {
    [super viewDidLoad];
    //线程A
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        for(int i=0;i<20;i++){
            self.age = self.age+1;
            NSLog(@"--1---age = %ld---%@",(long)self.age,[NSThread currentThread]);
        }
    });
    //线程B
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        for(int i=0;i<20;i++){
            self.age = self.age+1;
            NSLog(@"--2---age = %ld---%@",(long)self.age,[NSThread currentThread]);
        }
    });
    sleep(5);
    NSLog(@"lastAge = %ld---%@",(long)self.age,[NSThread currentThread]);
}
@end

打印：
Multithreading[6412:538075] --2---age = 2---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 1---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538074] --1---age = 3---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 4---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538075] --2---age = 5---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 5---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 6---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 7---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538074] --1---age = 9---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 8---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 10---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 11---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 12---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 13---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 14---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 15---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 16---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 17---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 18---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538074] --1---age = 20---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 19---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 21---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 22---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 23---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 24---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 25---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 26---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 27---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 28---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 29---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 30---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 31---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 32---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 33---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 34---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 35---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 36---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538074] --1---age = 37---<NSThread: 0x600003e7f440>{number = 3, name = (null)}
Multithreading[6412:538075] --2---age = 38---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538075] --2---age = 39---<NSThread: 0x600003e70240>{number = 4, name = (null)}
Multithreading[6412:538035] lastAge = 39---<NSThread: 0x600003efed00>{number = 1, name = main}

发现：有2个5

Multithreading_36.png

iOS中的读写安全方案

Multithreading_37.png

pthread_rwlock

Multithreading_38.png

#import "ViewController_19.h"
#import <pthread.h>

@interface ViewController_19 ()
@property (assign, nonatomic) pthread_rwlock_t lock;
@property (nonatomic,assign)int  age;
@end

@implementation ViewController_19
- (void)viewDidLoad {
    [super viewDidLoad];
    
    // 初始化锁
    pthread_rwlock_init(&_lock, NULL);
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    for (int i = 0; i < 100; i++) {
        dispatch_async(queue, ^{
            [self read];
        });
        dispatch_async(queue, ^{
            [self write];
        });
    }
    
    sleep(5);
    NSLog(@"%@--age=%d",[NSThread currentThread],self.age);
}

- (void)read {
    pthread_rwlock_rdlock(&_lock);
    NSLog(@"%s--%@--age=%d", __func__,[NSThread currentThread],self.age);
    pthread_rwlock_unlock(&_lock);
}

- (void)write{
    pthread_rwlock_wrlock(&_lock);
    self.age+=1;
    NSLog(@"%s--%@--age=%d", __func__,[NSThread currentThread],self.age);
    pthread_rwlock_unlock(&_lock);
}

- (void)dealloc{
    pthread_rwlock_destroy(&_lock);
}
@end

dispatch_barrier_async

Multithreading_39.png

#import "ViewController_20.h"

@interface ViewController_20 ()
@property (nonatomic,strong)dispatch_queue_t  queue;
@end

@implementation ViewController_20

- (void)viewDidLoad {
    [super viewDidLoad];
    self.queue = dispatch_queue_create("rw_queue", DISPATCH_QUEUE_CONCURRENT);
    for (int i = 0; i < 10; i++) {
        [self read];
        [self read];
        [self read];
        [self write];
    }
}

- (void)read {
    dispatch_async(self.queue, ^{
        sleep(1);
        NSLog(@"read");
    });
}

- (void)write{
    dispatch_barrier_async(self.queue, ^{
        sleep(1);
        NSLog(@"write");
    });
}
@end

Multithreading_40.png

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