muduo的高性能异步日志

1、一个日志库大体可分为前端（frontend）与后端（backend）。前端是供应用程序使用的接口（API），并生成日志信息；后端则是负责将日志信息写到目的地。每个线程都有自己的前端，而整个程序共用一个后端。对于生产者（前端）而言，要尽量做到低延迟、低CPU开销、无阻塞；对消费者（后端）而言，要尽量做到足够大的吞吐量，并占用较少的资源。

    对C++程序而言，最好整个程序（包括主程序和程序库）都使用相同的日志库，程序有一个整体的日志输出，而不要每个组件都有各自的日志输出。从这个意义上讲，日志库是个singleton。

    异步日志：每个【进程】最好只写一个日志文件，这样分析日志文件更容易，不必再多个文件中跳来跳去。实现方法是，用一个背景线程负责收集日志消息，并写入日志文件，其他业务线程只管往这个“日志线程”发送日志消息，这称为“异步日志”。

    muduo日志库采用的是双缓冲技术，基本思路是准备两块缓冲：A与B，前端负责往buffer A中填数据（日志消息），后端负责将buffer B中的数据写入文件。当buffer A写满之后，交换A与B，让后端将buffer A中的数据写入文件，而前端负责往buffer B中填入新的日志文件。如此往复。用两个buffer的好处是在新建日志消息的时候不必等待磁盘文件操作，也避免每条消息都触发（唤醒）了后端日志线程。换言之，前端不是将一条条消息分别传送给后端，而是将多个日志消息拼成一个大的buffer传送给后端，相当于批处理，减少了线程唤醒的频率，降低了开销。另外，为了及时将消息写入文件，即使前端的buffer A未写满，日志库也会每三秒执行一次上述交换写入操作。

【.h】

#ifndef MUDUO_BASE_ASYNCLOGGING_H
#define MUDUO_BASE_ASYNCLOGGING_H

#include <muduo/base/BlockingQueue.h>
#include <muduo/base/BoundedBlockingQueue.h>
#include <muduo/base/CountDownLatch.h>
#include <muduo/base/Mutex.h>
#include <muduo/base/Thread.h>
#include <muduo/base/LogStream.h>

#include <atomic>
#include <vector>

namespace muduo
{

class AsyncLogging : noncopyable
{
 public:

  AsyncLogging(const string& basename,
               off_t rollSize,
               int flushInterval = 3);

  ~AsyncLogging()
  {
    if (running_)
    {
      stop();
    }
  }

  void append(const char* logline, int len);

  void start()
  {
    running_ = true;
    thread_.start();
    latch_.wait();
  }

  void stop()
  {
    running_ = false;
    cond_.notify();
    thread_.join();
  }

 private:

  void threadFunc();

  typedef muduo::detail::FixedBuffer<muduo::detail::kLargeBuffer> Buffer;
  typedef std::vector<std::unique_ptr<Buffer>> BufferVector;
  typedef BufferVector::value_type BufferPtr;

  const int flushInterval_;
  std::atomic<bool> running_;
  string basename_;
  off_t rollSize_;
  muduo::Thread thread_;
  muduo::CountDownLatch latch_;
  muduo::MutexLock mutex_;
  muduo::Condition cond_;
  BufferPtr currentBuffer_;
  BufferPtr nextBuffer_;
  BufferVector buffers_;
};

}
#endif  // MUDUO_BASE_ASYNCLOGGING_H

【.cpp】

#include <muduo/base/AsyncLogging.h>
#include <muduo/base/LogFile.h>
#include <muduo/base/Timestamp.h>

#include <stdio.h>

using namespace muduo;

AsyncLogging::AsyncLogging(const string& basename,
                           off_t rollSize,
                           int flushInterval)
  : flushInterval_(flushInterval),
    running_(false),
    basename_(basename),
    rollSize_(rollSize),
    thread_(std::bind(&AsyncLogging::threadFunc, this), "Logging"),
    latch_(1),
    mutex_(),
    cond_(mutex_),
    currentBuffer_(new Buffer),
    nextBuffer_(new Buffer),
    buffers_()
{
  currentBuffer_->bzero();
  nextBuffer_->bzero();
  buffers_.reserve(16);
}

void AsyncLogging::append(const char* logline, int len)
{
  muduo::MutexLockGuard lock(mutex_);
  if (currentBuffer_->avail() > len)
  {
    currentBuffer_->append(logline, len);
  }
  else
  {
    buffers_.push_back(std::move(currentBuffer_));

    if (nextBuffer_)
    {
      currentBuffer_ = std::move(nextBuffer_);
    }
    else
    {
      currentBuffer_.reset(new Buffer); // Rarely happens
    }
    currentBuffer_->append(logline, len);
    cond_.notify();
  }
}

void AsyncLogging::threadFunc()
{
  assert(running_ == true);
  latch_.countDown();
  LogFile output(basename_, rollSize_, false);
  BufferPtr newBuffer1(new Buffer);
  BufferPtr newBuffer2(new Buffer);
  newBuffer1->bzero();
  newBuffer2->bzero();
  BufferVector buffersToWrite;
  buffersToWrite.reserve(16);
  while (running_)
  {
    assert(newBuffer1 && newBuffer1->length() == 0);
    assert(newBuffer2 && newBuffer2->length() == 0);
    assert(buffersToWrite.empty());

    {
      muduo::MutexLockGuard lock(mutex_);
      if (buffers_.empty())  // unusual usage!
      {
        cond_.waitForSeconds(flushInterval_);
      }
      buffers_.push_back(std::move(currentBuffer_));
      currentBuffer_ = std::move(newBuffer1);
      buffersToWrite.swap(buffers_);
      if (!nextBuffer_)
      {
        nextBuffer_ = std::move(newBuffer2);
      }
    }

    assert(!buffersToWrite.empty());

    if (buffersToWrite.size() > 25)
    {
      char buf[256];
      snprintf(buf, sizeof buf, "Dropped log messages at %s, %zd larger buffers\n",
               Timestamp::now().toFormattedString().c_str(),
               buffersToWrite.size()-2);
      fputs(buf, stderr);
      output.append(buf, static_cast<int>(strlen(buf)));
      buffersToWrite.erase(buffersToWrite.begin()+2, buffersToWrite.end());
    }

    for (size_t i = 0; i < buffersToWrite.size(); ++i)
    {
      // FIXME: use unbuffered stdio FILE ? or use ::writev ?
      output.append(buffersToWrite[i]->data(), buffersToWrite[i]->length());
    }

    if (buffersToWrite.size() > 2)
    {
      // drop non-bzero-ed buffers, avoid trashing
      buffersToWrite.resize(2);
    }

    if (!newBuffer1)
    {
      assert(!buffersToWrite.empty());
      newBuffer1 = std::move(buffersToWrite.back());
      buffersToWrite.pop_back();
      newBuffer1->reset();
    }

    if (!newBuffer2)
    {
      assert(!buffersToWrite.empty());
      newBuffer2 = std::move(buffersToWrite.back());
      buffersToWrite.pop_back();
      newBuffer2->reset();
    }

    buffersToWrite.clear();
    output.flush();
  }
  output.flush();
}

在 陈硕所著《Linux多线程服务端编程中》，第117~119页，阐述了异步日志前端与后端交互的四种情形。在此不再详述。