iOS并发编程--8种加锁方式及比较

iOS开发中不可避免的会遇到加锁的情况，今天就来比较一下各种加锁方式。

目前我所知道的有如下几种加锁方式：

1. @synchronized 关键字加锁

2. NSLock 对象锁

3. NSCondition

4. NSConditionLock 条件锁

5. NSRecursiveLock 递归锁

6. pthread_mutex 互斥锁（C语言）

7. dispatch_semaphore 信号量实现加锁（GCD）

8. OSSpinLock

这里分别使用8种方式加锁解锁1千万次，执行方法如下：

#import <pthread.h>
#import <libkern/OSAtomic.h>

- (void)runLock{
     CFTimeInterval timeBefore;
    CFTimeInterval timeCurrent;
    NSUInteger i;
    NSUInteger count = 1000*10000;//执行一千万次

    //@synchronized
    id obj = [[NSObject alloc]init];;
    timeBefore = CFAbsoluteTimeGetCurrent();
    for(i=0; i<count; i++){
        @synchronized(obj){
        }
    }
    timeCurrent = CFAbsoluteTimeGetCurrent();
    printf("@synchronized used : %f\n", timeCurrent-timeBefore);

    //NSLock
    NSLock *lock = [[NSLock alloc]init];
    timeBefore = CFAbsoluteTimeGetCurrent();
    for(i=0; i<count; i++){
        [lock lock];
        [lock unlock];
    }
    timeCurrent = CFAbsoluteTimeGetCurrent();
    printf("NSLock used : %f\n", timeCurrent-timeBefore);

    //NSCondition
    NSCondition *condition = [[NSCondition alloc]init];
    timeBefore = CFAbsoluteTimeGetCurrent();
    for(i=0; i<count; i++){
        [condition lock];
        [condition unlock];
    }
    timeCurrent = CFAbsoluteTimeGetCurrent();
    printf("NSCondition used : %f\n", timeCurrent-timeBefore);

    //NSConditionLock
    NSConditionLock *conditionLock = [[NSConditionLock alloc]init];
    timeBefore = CFAbsoluteTimeGetCurrent();
    for(i=0; i<count; i++){
        [conditionLock lock];
        [conditionLock unlock];
    }
    timeCurrent = CFAbsoluteTimeGetCurrent();
    printf("NSConditionLock used : %f\n", timeCurrent-timeBefore);

    //NSRecursiveLock
    NSRecursiveLock *recursiveLock = [[NSRecursiveLock alloc]init];
    timeBefore = CFAbsoluteTimeGetCurrent();
    for(i=0; i<count; i++){
        [recursiveLock lock];
        [recursiveLock unlock];
    }
    timeCurrent = CFAbsoluteTimeGetCurrent();
    printf("NSRecursiveLock used : %f\n", timeCurrent-timeBefore);

    //pthread_mutex
    pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    timeBefore = CFAbsoluteTimeGetCurrent();
    for(i=0; i<count; i++){
        pthread_mutex_lock(&mutex);
        pthread_mutex_unlock(&mutex);
    }
    timeCurrent = CFAbsoluteTimeGetCurrent();
    printf("pthread_mutex used : %f\n", timeCurrent-timeBefore);

    //dispatch_semaphore
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
    timeBefore = CFAbsoluteTimeGetCurrent();
    for(i=0; i<count; i++){
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_signal(semaphore);
    }
    timeCurrent = CFAbsoluteTimeGetCurrent();
    printf("dispatch_semaphore used : %f\n", timeCurrent-timeBefore);

    //OSSpinLockLock
    OSSpinLock spinlock = OS_SPINLOCK_INIT;
    timeBefore = CFAbsoluteTimeGetCurrent();
    for(i=0; i<count; i++){
        OSSpinLockLock(&spinlock);
        OSSpinLockUnlock(&spinlock);
    }
    timeCurrent = CFAbsoluteTimeGetCurrent();
    printf("OSSpinLock used : %f\n", timeCurrent-timeBefore);
}

执行结果：

由图可见发现：

OSSpinLock的性能最好，GCD的dispatch_semaphore紧随其后；

NSConditionLock和@synchronized性能较差；

PS：

1. 需要注意的是这里仅仅是对各种锁直接Lock和Unlock的性能测试，其中部分锁的使用条件上还是有细微的差异的，比如NSLock之类的还有tryLock等方法用于加锁，不同对象锁的功能偏向不一样等等，有兴趣的可以逐个搜索再更深入的研究不同锁之间的区别。

2. 另外，一般来说客户端很少会有这么大量的加锁解锁操作，所以日常来说这些锁的性能都是可以满足使用需求的。

本文对于iOS中的锁也是抛砖引玉，欢迎各位留言讨论。