目录
简单版本1优化版本1优化版本2
对于QPS要求很高或者对性能有一定要求的服务器程序，同步写日志会对服务的关键性逻辑的快速执行和及时响应带来一定的性能损失，因为写日志时等待磁盘IO完成工作也需要一定时间。为了减少这种损失，一般采用异步写日志。
本质上仍然是一个生产者与消费者模型，产生日志的线程是生产者，将日志写入文件的线程是消费者。
如果有多个消费者线程，可能存在写日志的时间顺序错位，所以一般将日志消费者线程数量设置为1.
简单版本1
下面给出一个简单的版本：
#include <iostream>#include <thread>#include <mutex>#include <list>#include <string>#include <sstream>#include <vector>// 保护队列的mutexstd::mutex log_mutex;std::list<std::string> cached_logs;FILE* log_file = nullptr;bool init_log_file(){// 以追加的模式写入文件，如果文件不存在，则创建log_file = fopen("my.log","a+");return log_file != nullptr;}void uninit_log_file(){if (log_file != nullptr)fclose(log_file);}bool write_log_tofile(const std::string& line){if (log_file == nullptr)return false;if (fwrite((void *)line.c_str(), 1, line.length(), log_file) != line.length())return false;// 将日志flush到文件中fflush(log_file);return true;}void log_producer(){int index = 0;while (true) {++index;std::ostringstream os;os << "This is log, index :" << index << ", producer threadID:" << std::this_thread::get_id() << "\n";{std::lock_guard<std::mutex> lock(log_mutex);cached_logs.emplace_back(os.str());}// 生产出一个log之后，休眠100ms再生产std::chrono::milliseconds duration(100);std::this_thread::sleep_for(duration);}}void log_consumer(){std::string line;while (true) {{std::lock_guard<std::mutex> lock(log_mutex);if (!cached_logs.empty()) {line = cached_logs.front();cached_logs.pop_front();}}// 如果取出来的行为空，说明队列里面是空的，消费者休眠一会儿再去消费if (line.empty()) {std::chrono::milliseconds duration(1000);std::this_thread::sleep_for(duration);continue;}// 否则将line写入到log_file中write_log_tofile(line);line.clear();}}int main(int argc, char* argv[]){if (!init_log_file()) {std::cout << "init log file error." << std::endl;return -1;}std::thread log_producer1(log_producer);std::thread log_producer2(log_producer);std::thread log_producer3(log_producer);std::thread log_consumer1(log_consumer);std::thread log_consumer2(log_consumer);std::thread log_consumer3(log_consumer);log_producer1.join();log_producer2.join();log_producer3.join();log_consumer1.join();log_consumer2.join();log_consumer3.join();uninit_log_file();return 0;}
效果：
This is log, index :1, producer threadID:139910877185792This is log, index :1, producer threadID:139910868793088This is log, index :1, producer threadID:139910860400384This is log, index :2, producer threadID:139910877185792This is log, index :2, producer threadID:139910860400384This is log, index :3, producer threadID:139910877185792This is log, index :3, producer threadID:139910860400384This is log, index :2, producer threadID:139910868793088This is log, index :3, producer threadID:139910868793088This is log, index :4, producer threadID:139910877185792This is log, index :4, producer threadID:139910860400384This is log, index :4, producer threadID:139910868793088This is log, index :5, producer threadID:139910877185792This is log, index :5, producer threadID:139910860400384This is log, index :5, producer threadID:139910868793088This is log, index :6, producer threadID:139910860400384This is log, index :6, producer threadID:139910868793088This is log, index :7, producer threadID:139910877185792This is log, index :7, producer threadID:139910860400384This is log, index :7, producer threadID:139910868793088This is log, index :8, producer threadID:139910877185792This is log, index :8, producer threadID:139910860400384This is log, index :8, producer threadID:139910868793088This is log, index :9, producer threadID:139910877185792This is log, index :9, producer threadID:139910860400384This is log, index :9, producer threadID:139910868793088This is log, index :6, producer threadID:139910877185792This is log, index :10, producer threadID:139910860400384This is log, index :10, producer threadID:139910868793088This is log, index :10, producer threadID:139910877185792This is log, index :11, producer threadID:139910860400384This is log, index :11, producer threadID:139910868793088This is log, index :12, producer threadID:139910860400384This is log, index :12, producer threadID:139910868793088This is log, index :11, producer threadID:139910877185792This is log, index :12, producer threadID:139910877185792This is log, index :13, producer threadID:139910860400384This is log, index :13, producer threadID:139910868793088This is log, index :13, producer threadID:139910877185792This is log, index :14, producer threadID:139910868793088This is log, index :14, producer threadID:139910877185792This is log, index :15, producer threadID:139910868793088This is log, index :15, producer threadID:139910877185792This is log, index :14, producer threadID:139910860400384This is log, index :15, producer threadID:139910860400384This is log, index :16, producer threadID:139910877185792This is log, index :16, producer threadID:139910860400384This is log, index :17, producer threadID:139910877185792This is log, index :17, producer threadID:139910860400384This is log, index :17, producer threadID:139910868793088This is log, index :16, producer threadID:139910868793088This is log, index :18, producer threadID:139910877185792This is log, index :19, producer threadID:139910860400384This is log, index :19, producer threadID:139910877185792This is log, index :19, producer threadID:139910868793088This is log, index :20, producer threadID:139910860400384This is log, index :18, producer threadID:139910860400384This is log, index :20, producer threadID:139910877185792This is log, index :18, producer threadID:139910868793088This is log, index :20, producer threadID:139910868793088This is log, index :21, producer threadID:139910868793088This is log, index :21, producer threadID:139910877185792This is log, index :21, producer threadID:139910860400384This is log, index :22, producer threadID:139910860400384This is log, index :22, producer threadID:139910877185792This is log, index :22, producer threadID:139910868793088This is log, index :23, producer threadID:139910860400384This is log, index :23, producer threadID:139910877185792This is log, index :23, producer threadID:139910868793088This is log, index :24, producer threadID:139910860400384This is log, index :24, producer threadID:139910868793088This is log, index :24, producer threadID:139910877185792This is log, index :25, producer threadID:139910860400384This is log, index :25, producer threadID:139910868793088This is log, index :25, producer threadID:139910877185792This is log, index :26, producer threadID:139910868793088This is log, index :26, producer threadID:139910860400384This is log, index :26, producer threadID:139910877185792This is log, index :27, producer threadID:139910868793088This is log, index :27, producer threadID:139910860400384This is log, index :27, producer threadID:139910877185792This is log, index :28, producer threadID:139910868793088This is log, index :28, producer threadID:139910877185792This is log, index :28, producer threadID:139910860400384This is log, index :29, producer threadID:139910877185792This is log, index :29, producer threadID:139910868793088This is log, index :29, producer threadID:139910860400384This is log, index :30, producer threadID:139910877185792This is log, index :30, producer threadID:139910868793088This is log, index :30, producer threadID:139910860400384This is log, index :31, producer threadID:139910868793088This is log, index :31, producer threadID:139910877185792This is log, index :31, producer threadID:139910860400384This is log, index :32, producer threadID:139910877185792This is log, index :32, producer threadID:139910868793088This is log, index :32, producer threadID:139910860400384This is log, index :33, producer threadID:139910860400384This is log, index :33, producer threadID:139910868793088This is log, index :33, producer threadID:139910877185792This is log, index :34, producer threadID:139910860400384This is log, index :34, producer threadID:139910877185792This is log, index :34, producer threadID:139910868793088This is log, index :35, producer threadID:139910860400384This is log, index :35, producer threadID:139910868793088This is log, index :35, producer threadID:139910877185792This is log, index :36, producer threadID:139910877185792This is log, index :36, producer threadID:139910868793088This is log, index :37, producer threadID:139910860400384This is log, index :37, producer threadID:139910877185792This is log, index :37, producer threadID:139910868793088This is log, index :38, producer threadID:139910860400384This is log, index :38, producer threadID:139910877185792This is log, index :38, producer threadID:139910868793088This is log, index :39, producer threadID:139910860400384This is log, index :39, producer threadID:139910877185792This is log, index :39, producer threadID:139910868793088This is log, index :40, producer threadID:139910860400384This is log, index :40, producer threadID:139910877185792This is log, index :40, producer threadID:139910868793088This is log, index :36, producer threadID:139910860400384This is log, index :41, producer threadID:139910860400384This is log, index :41, producer threadID:139910877185792This is log, index :42, producer threadID:139910860400384This is log, index :42, producer threadID:139910877185792This is log, index :42, producer threadID:139910868793088This is log, index :43, producer threadID:139910860400384This is log, index :43, producer threadID:139910868793088This is log, index :43, producer threadID:139910877185792This is log, index :44, producer threadID:139910860400384This is log, index :44, producer threadID:139910868793088This is log, index :44, producer threadID:139910877185792This is log, index :45, producer threadID:139910860400384This is log, index :45, producer threadID:139910868793088This is log, index :45, producer threadID:139910877185792This is log, index :46, producer threadID:139910860400384This is log, index :46, producer threadID:139910868793088This is log, index :46, producer threadID:139910877185792This is log, index :47, producer threadID:139910860400384This is log, index :47, producer threadID:139910868793088This is log, index :47, producer threadID:139910877185792This is log, index :48, producer threadID:139910860400384This is log, index :48, producer threadID:139910868793088This is log, index :48, producer threadID:139910877185792This is log, index :49, producer threadID:139910860400384This is log, index :49, producer threadID:139910877185792This is log, index :49, producer threadID:139910868793088This is log, index :50, producer threadID:139910877185792This is log, index :50, producer threadID:139910860400384This is log, index :50, producer threadID:139910868793088This is log, index :41, producer threadID:139910868793088
优化版本1
上面的代码，在当前缓存队列没有日志记录的时候，消费日志线程会做无用功。
这里可以使用条件变量，如果当前队列中没有日志记录，就将日志消费者线程挂起；
当产生了新的日志后，signal条件变量，唤醒消费线程，将被日志从队列中取出，并写入文件。
下面是主要修改
#include <condition_variable>std::condition_variable log_cv;void log_producer(){int index = 0;while (true) {++index;std::ostringstream os;os << "This is log, index :" << index << ", producer threadID:" << std::this_thread::get_id() << "\n";{std::lock_guard<std::mutex> lock(log_mutex);cached_logs.emplace_back(os.str());log_cv.notify_one();}// 生产出一个log之后，休眠100ms再生产std::chrono::milliseconds duration(100);std::this_thread::sleep_for(duration);}}void log_consumer(){std::string line;while (true) {{std::unique_lock<std::mutex> lock(log_mutex);while (cached_logs.empty()) {// 无限等待log_cv.wait(lock);}line = cached_logs.front();cached_logs.pop_front();}// 如果取出来的行为空，说明队列里面是空的，消费者休眠一会儿再去消费if (line.empty()) {std::chrono::milliseconds duration(1000);std::this_thread::sleep_for(duration);continue;}// 否则将line写入到log_file中write_log_tofile(line);line.clear();}}
优化版本2
还可以使用信号量来设计异步日志系统。
信号量是带有资源计数的线程同步对象，每产生一条日志，就将信号量资源计数+1，日志消费线程默认等待这个信号量是否signal，如果signal，就唤醒一个日志消费线程，信号量计数自动-1。如果当前资源计数为0，则将消费者自动挂起。
C++现在还没有提供不同平台的信号量对象封装，这里以Linux系统为例：
明显能感觉运行相同时间，写入的日志更多了。。
#include <unistd.h>#include <iostream>#include <thread>#include <mutex>#include <list>#include <string>#include <sstream>#include <semaphore.h>// 保护队列的mutexpthread_mutex_t log_mutex = PTHREAD_MUTEX_INITIALIZER;sem_t log_semphore;std::list<std::string> cached_logs;FILE* log_file = nullptr;bool init(){pthread_mutex_init(&log_mutex, nullptr);// 初始信号量资源数量为0sem_init(&log_semphore, 0, 0);// 以追加的模式写入文件，如果文件不存在，则创建log_file = fopen("my.log","a++");return log_file != nullptr;}void uninit(){pthread_mutex_destroy(&log_mutex);sem_destroy(&log_semphore);if (log_file != nullptr)fclose(log_file);}bool write_log_tofile(const std::string& line){if (log_file == nullptr)return false;// 对于比较长的日志应该分段写入，因为单次写入可能只写入部分内容// 这里逻辑从简if (fwrite((void *)line.c_str(), 1, line.length(), log_file) != line.length())return false;// 将日志flush到文件中fflush(log_file);return true;}void* log_producer(void* arg){int index = 0;while (true) {++index;std::ostringstream os;os << "This is log, index :" << index << ", producer threadID:" << std::this_thread::get_id() << "\n";pthread_mutex_lock(&log_mutex);cached_logs.emplace_back(os.str());pthread_mutex_unlock(&log_mutex);sem_post(&log_semphore);usleep(1000);}}void* log_consumer(void* arg){std::string line;while (true) {// 无限等待sem_wait(&log_semphore);pthread_mutex_lock(&log_mutex);if (!cached_logs.empty()) {line = cached_logs.front();cached_logs.pop_front();}pthread_mutex_unlock(&log_mutex);// 如果取出来的行为空，说明队列里面是空的，消费者休眠一会儿再去消费if (line.empty()) {sleep(1);continue;}// 否则将line写入到log_file中write_log_tofile(line);line.clear();}}int main(int argc, char* argv[]){if (!init()) {std::cout << "init log file error." << std::endl;return -1;}// 创建三个生产日志线程pthread_t producer_thread_id[3];for (size_t i = 0; i < sizeof(producer_thread_id) / sizeof(producer_thread_id[0]); ++i) {pthread_create(&producer_thread_id[i], NULL, log_producer, NULL);}// 创建三个消费日志线程pthread_t consumer_thread_id[3];for (size_t i = 0; i < sizeof(consumer_thread_id) / sizeof(consumer_thread_id[0]); ++i) {pthread_create(&consumer_thread_id[i], NULL, log_consumer, NULL);}// 等待生产者线程退出for (size_t i = 0; i < sizeof(producer_thread_id) / sizeof(producer_thread_id[0]); ++i) {pthread_join(producer_thread_id[i], NULL);}// 等待消费者线程退出for (size_t i = 0; i < sizeof(consumer_thread_id) / sizeof(consumer_thread_id[0]); ++i) {pthread_join(consumer_thread_id[i], NULL);}uninit();return 0;}
