对付声明为atomic并且又本技艺动实现getter可能setter的属性,也可以用barrier来改造:
- @property (atomic, copy) NSString *someString;
-
- - (NSString *)someString {
- __block NSString *tempString;
- dispatch_sync(_syncQueue, ^{
- tempString = _someString;
- });
- return tempString;
- }
-
- - (void)setSomeString :(NSString *)someString {
- dispatch_barrier_async(_syncQueue, ^{
- _someString = someString
- ...
- }
- }
在做到atomic的同时,getter之间还可以并发执行,比直接把setter和getter都放到串行行列可能加平凡锁要更高效。
读者写者锁能晋升几多服从?
行使读者写者锁必然比全部读写都加锁以及行使串行行列要快,可是到底能快几多呢?Dmytro Anokhin在[3]中做了尝试比拟,测出了别离行使NSLock、GCD barrier和pthread_rwlock时获取锁所必要的均匀时刻,尝试样本数在100到1000之间,去掉最高和最低的10%,功效如下列图表所示:
3 writers / 10 readers
1 writer / 10 readers
5 writers / 5 readers
10 writers / 1 reader
说明可知:
- 行使读者写者锁(GCD barrier、pthread_rwlock),对比纯真行使平凡锁(NSLock),服从有明显晋升;
- 读者数目越多,写者数目越少,行使读者写者锁的服从上风越明明;
- 行使GCD barrier和行使pthread_rwlock的服从差别不大。
因为pthread_rwlock不易行使且轻易堕落,并且GCD barrier和pthread_rwlock比拟机能相等,提议行使GCD barrier来办理iOS开拓中碰着的读者写者题目。其它,行使GCD尚有个隐藏上风:GCD面向行列而非线程,dispatch至某一行列的使命,也许在任一线程上执行,这些对开拓者是透明的,这样计划的甜头显而易见,GCD可以按照现实环境从本身打点的线程池中挑选出开销最小的线程来执利用命,最洪流平减小context切换次数。
何时行使读者写者锁
必要留意的是,并非全部的多线程读写场景都必然是读者写者题目,行使时要留意分辨。譬喻以下YYCache的代码:
- //读cache
- - (id)objectForKey:(id)key {
- if (!key) return nil;
- pthread_mutex_lock(&_lock);
- _YYLinkedMapNode *node = CFDictionaryGetValue(_lru->_dic, (__bridge const void *)(key));
- if (node) {
- node->_time = CACurrentMediaTime();
- [_lru bringNodeToHead:node];
- }
- pthread_mutex_unlock(&_lock);
- return node ? node->_value : nil;
- }
- //写cache
- - (void)setObject:(id)object forKey:(id)key withCost:(NSUInteger)cost {
- if (!key) return;
- if (!object) {
- [self removeObjectForKey:key];
- return;
- }
- pthread_mutex_lock(&_lock);
- _YYLinkedMapNode *node = CFDictionaryGetValue(_lru->_dic, (__bridge const void *)(key));
- NSTimeInterval now = CACurrentMediaTime();
- if (node) {
- _lru->_totalCost -= node->_cost;
- _lru->_totalCost += cost;
- node->_cost = cost;
- node->_time = now;
- node->_value = object;
- [_lru bringNodeToHead:node];
- } else {
- node = [_YYLinkedMapNode new];
- node->_cost = cost;
- node->_time = now;
- node->_key = key;
- node->_value = object;
- [_lru insertNodeAtHead:node];
- }
- if (_lru->_totalCost > _costLimit) {
- dispatch_async(_queue, ^{
- [self trimToCost:_costLimit];
- });
- }
- if (_lru->_totalCount > _countLimit) {
- _YYLinkedMapNode *node = [_lru removeTailNode];
- if (_lru->_releaseAsynchronously) {
- dispatch_queue_t queue = _lru->_releaseOnMainThread ? dispatch_get_main_queue() : YYMemoryCacheGetReleaseQueue();
- dispatch_async(queue, ^{
- [node class]; //hold and release in queue
- });
- } else if (_lru->_releaseOnMainThread && !pthread_main_np()) {
- dispatch_async(dispatch_get_main_queue(), ^{
- [node class]; //hold and release in queue
- });
- }
- }
- pthread_mutex_unlock(&_lock);
- }
(编辑:河北网)
【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容!
|