? ? ? ??AsyncLogging是muduo的日志，程序如果直接讓文件寫日志可能會發生阻塞，所以有了異步日志這一說。muduo前端設計了2個BufferPtr，分別是currentBuffer_和nextBuffer_，還有一個存放BufferPtr的vector(buffers_)。

多個前端線程往currentBuffer_寫數據，currentBuffer_寫滿了將其放入buffers_，通知后端線程讀。前端線程將currentBuffer_和nextBuffer_替換繼續寫currentBuffer_。
后端也有2個BufferPtr，分別為newBuffer1和newBuffer2，還有一個BufferVector(buffersToWrite)。后端線程在收到前端通知之后，利用buffersToWrite和buffers_進行交換，并且用newBuffer1和newBuffer2歸還給前端的currentBuffer_和nextBuffer_，然后把日志寫入文件。

AsyncLogging.h

// Use of this source code is governed by a BSD-style license // that can be found in the License file. // // Author: Shuo Chen (chenshuo at chenshuo dot com)#include "muduo/base/AsyncLogging.h" #include "muduo/base/LogFile.h" #include "muduo/base/Timestamp.h"#include <stdio.h>using namespace muduo;//異步日志 AsyncLogging::AsyncLogging(const string& basename,off_t rollSize,int flushInterval): flushInterval_(flushInterval),running_(false),basename_(basename),rollSize_(rollSize),// 預留的日志大小thread_(std::bind(&AsyncLogging::threadFunc, this), "Logging"),// 執行該異步日志記錄器的線程latch_(1),mutex_(),cond_(mutex_),currentBuffer_(new Buffer), //當前緩沖區nextBuffer_(new Buffer),//預備緩沖區buffers_()//緩沖區隊列 {currentBuffer_->bzero(); nextBuffer_->bzero();buffers_.reserve(16);//設置緩沖區隊列大小為16 }//所有LOG_*最終都會調用append函數 void AsyncLogging::append(const char* logline, int len) {muduo::MutexLockGuard lock(mutex_);if (currentBuffer_->avail() > len) // 如果當前buffer還有空間，就添加到當前日志{currentBuffer_->append(logline, len);//調用vector的append}else{//將使用完后的buffer添加到 buffer vector后buffers_.push_back(std::move(currentBuffer_));if (nextBuffer_) // 重新設置當前buffer{currentBuffer_ = std::move(nextBuffer_);}else{currentBuffer_.reset(new Buffer); // Rarely happens//如果前端寫入速度太快了，一下子把兩塊緩沖都用完了，那么只好分配一塊新的buffer,作當前緩沖，這是極少發生的情況}currentBuffer_->append(logline, len);// 通知日志線程，有數據可寫 // 也就是說，只有當緩沖區滿了之后才會將數據寫入日志文件中cond_.notify();} }void AsyncLogging::threadFunc() // 線程調用的函數，主要用于周期性的flush數據到日志文件中 {assert(running_ == true);latch_.countDown();LogFile output(basename_, rollSize_, false);BufferPtr newBuffer1(new Buffer); //這兩個是后臺線程的buffer BufferPtr newBuffer2(new Buffer);newBuffer1->bzero();newBuffer2->bzero();BufferVector buffersToWrite; //用來和前臺線程的buffers_進行swapbuffersToWrite.reserve(16);while (running_){assert(newBuffer1 && newBuffer1->length() == 0);assert(newBuffer2 && newBuffer2->length() == 0);assert(buffersToWrite.empty());{muduo::MutexLockGuard lock(mutex_);//如果buffer為空，那么表示沒有數據需要寫入文件，那么就等待指定的時間（注意這里沒有用倒數計數器）if (buffers_.empty()) // unusual usage!{cond_.waitForSeconds(flushInterval_);}//無論cond是因何而醒來，都要將currentBuffer_放到buffers_中。 //如果是因為時間到而醒，那么currentBuffer_還沒滿，此時也要將之寫入LogFile中。 //如果已經有一個前臺buffer滿了，那么在前臺線程中就已經把一個前臺buffer放到buffers_中 //了。此時，還是需要把currentBuffer_放到buffers_中（注意，前后放置是不同的buffer， //因為在前臺線程中，currentBuffer_已經被換成nextBuffer_指向的buffer了）buffers_.push_back(std::move(currentBuffer_)); //currentBuffer_是當前緩沖區/*---歸還一個buffer---*/ // 將新的buffer轉成當前緩沖區currentBuffer_ = std::move(newBuffer1);//使用新的未使用的 buffersToWrite 交換 buffers_，將buffers_中的數據在異步線程中寫入LogFile中buffersToWrite.swap(buffers_);//內部指針交換，而非復制if (!nextBuffer_){nextBuffer_ = std::move(newBuffer2);/*-----假如需要，歸還第二個----*/}}assert(!buffersToWrite.empty());// 如果將要寫入文件的buffer列表中buffer的個數大于25，那么將多余數據刪除 // 消息堆積//前端陷入死循環，拼命發送日志消息，超過后端的處理能力//這是典型的生產速度超過消費速度，會造成數據在內存中的堆積//嚴重時引發性能問題(可用內存不足),//或程序崩潰(分配內存失敗)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)));// 丟掉多余日志，以騰出內存，僅保留兩塊緩沖區buffersToWrite.erase(buffersToWrite.begin()+2, buffersToWrite.end());}// 將buffersToWrite的數據寫入到日志中for (const auto& buffer : buffersToWrite){// FIXME: use unbuffered stdio FILE ? or use ::writev ?output.append(buffer->data(), buffer->length());}// 重新調整buffersToWrite的大小 if (buffersToWrite.size() > 2){// drop non-bzero-ed buffers, avoid trashingbuffersToWrite.resize(2);}if (!newBuffer1){assert(!buffersToWrite.empty());// 從buffersToWrite中彈出一個作為newBuffer1newBuffer1 = std::move(buffersToWrite.back());buffersToWrite.pop_back();newBuffer1->reset();// 清理newBuffer1}//前臺buffer是由newBuffer1 2 歸還的?，F在把buffersToWrite的buffer歸還給后臺bufferif (!newBuffer2){assert(!buffersToWrite.empty());newBuffer2 = std::move(buffersToWrite.back());buffersToWrite.pop_back();newBuffer2->reset();}buffersToWrite.clear();output.flush();}output.flush(); }

AsyncLogging.cc

// Use of this source code is governed by a BSD-style license // that can be found in the License file. // // Author: Shuo Chen (chenshuo at chenshuo dot com)#include "muduo/base/AsyncLogging.h" #include "muduo/base/LogFile.h" #include "muduo/base/Timestamp.h"#include <stdio.h>using namespace muduo;//異步日志 AsyncLogging::AsyncLogging(const string& basename,off_t rollSize,int flushInterval): flushInterval_(flushInterval),running_(false),basename_(basename),rollSize_(rollSize),// 預留的日志大小thread_(std::bind(&AsyncLogging::threadFunc, this), "Logging"),// 執行該異步日志記錄器的線程latch_(1),mutex_(),cond_(mutex_),currentBuffer_(new Buffer), //當前緩沖區nextBuffer_(new Buffer),//預備緩沖區buffers_()//緩沖區隊列 {currentBuffer_->bzero(); nextBuffer_->bzero();buffers_.reserve(16);//設置緩沖區隊列大小為16 }//所有LOG_*最終都會調用append函數 void AsyncLogging::append(const char* logline, int len) {muduo::MutexLockGuard lock(mutex_);if (currentBuffer_->avail() > len) // 如果當前buffer還有空間，就添加到當前日志{currentBuffer_->append(logline, len);//調用vector的append}else{//將使用完后的buffer添加到 buffer vector后buffers_.push_back(std::move(currentBuffer_));if (nextBuffer_) // 重新設置當前buffer{currentBuffer_ = std::move(nextBuffer_);}else{currentBuffer_.reset(new Buffer); // Rarely happens//如果前端寫入速度太快了，一下子把兩塊緩沖都用完了，那么只好分配一塊新的buffer,作當前緩沖，這是極少發生的情況}currentBuffer_->append(logline, len);// 通知日志線程，有數據可寫 // 也就是說，只有當緩沖區滿了之后才會將數據寫入日志文件中cond_.notify();} }void AsyncLogging::threadFunc() // 線程調用的函數，主要用于周期性的flush數據到日志文件中 {assert(running_ == true);latch_.countDown();LogFile output(basename_, rollSize_, false);BufferPtr newBuffer1(new Buffer); //這兩個是后臺線程的buffer BufferPtr newBuffer2(new Buffer);newBuffer1->bzero();newBuffer2->bzero();BufferVector buffersToWrite; //用來和前臺線程的buffers_進行swapbuffersToWrite.reserve(16);while (running_){assert(newBuffer1 && newBuffer1->length() == 0);assert(newBuffer2 && newBuffer2->length() == 0);assert(buffersToWrite.empty());{muduo::MutexLockGuard lock(mutex_);//如果buffer為空，那么表示沒有數據需要寫入文件，那么就等待指定的時間（注意這里沒有用倒數計數器）if (buffers_.empty()) // unusual usage!{cond_.waitForSeconds(flushInterval_);}//無論cond是因何而醒來，都要將currentBuffer_放到buffers_中。 //如果是因為時間到而醒，那么currentBuffer_還沒滿，此時也要將之寫入LogFile中。 //如果已經有一個前臺buffer滿了，那么在前臺線程中就已經把一個前臺buffer放到buffers_中 //了。此時，還是需要把currentBuffer_放到buffers_中（注意，前后放置是不同的buffer， //因為在前臺線程中，currentBuffer_已經被換成nextBuffer_指向的buffer了）buffers_.push_back(std::move(currentBuffer_)); //currentBuffer_是當前緩沖區/*---歸還一個buffer---*/ // 將新的buffer轉成當前緩沖區currentBuffer_ = std::move(newBuffer1);//使用新的未使用的 buffersToWrite 交換 buffers_，將buffers_中的數據在異步線程中寫入LogFile中buffersToWrite.swap(buffers_);//內部指針交換，而非復制if (!nextBuffer_){nextBuffer_ = std::move(newBuffer2);/*-----假如需要，歸還第二個----*/}}assert(!buffersToWrite.empty());// 如果將要寫入文件的buffer列表中buffer的個數大于25，那么將多余數據刪除 // 消息堆積//前端陷入死循環，拼命發送日志消息，超過后端的處理能力//這是典型的生產速度超過消費速度，會造成數據在內存中的堆積//嚴重時引發性能問題(可用內存不足),//或程序崩潰(分配內存失敗)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)));// 丟掉多余日志，以騰出內存，僅保留兩塊緩沖區buffersToWrite.erase(buffersToWrite.begin()+2, buffersToWrite.end());}// 將buffersToWrite的數據寫入到日志中for (const auto& buffer : buffersToWrite){// FIXME: use unbuffered stdio FILE ? or use ::writev ?output.append(buffer->data(), buffer->length());}// 重新調整buffersToWrite的大小 if (buffersToWrite.size() > 2){// drop non-bzero-ed buffers, avoid trashingbuffersToWrite.resize(2);}if (!newBuffer1){assert(!buffersToWrite.empty());// 從buffersToWrite中彈出一個作為newBuffer1newBuffer1 = std::move(buffersToWrite.back());buffersToWrite.pop_back();newBuffer1->reset();// 清理newBuffer1}//前臺buffer是由newBuffer1 2 歸還的。現在把buffersToWrite的buffer歸還給后臺bufferif (!newBuffer2){assert(!buffersToWrite.empty());newBuffer2 = std::move(buffersToWrite.back());buffersToWrite.pop_back();newBuffer2->reset();}buffersToWrite.clear();output.flush();}output.flush(); }

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