徹底搞懂 IO 底層原理

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"武俠小說裏有很多的“心法”和“招式”。計算機技術裏的“心法”和“招式”呢，我們可以簡稱爲“道”和“術”；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"“道” ","attrs":{}},{"type":"text","text":" 最基礎的計算機理論，隱藏於表象之下，非常抽象、晦澀難懂，需要用具象化的事物加以理解；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"“術” ","attrs":{}},{"type":"text","text":"具體的技藝，它有可能是一門語言，比如：python 出手見效快；","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們今天要給大家講的底層的IO就屬於“道”的範疇，看上去簡單，實則抽象。並且在它之上衍生出了語言層面用於實戰的技術，比如我們熟悉的java語言中的NIO或者像Netty這樣的框架。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"一、混亂的 IO 概念","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"IO是Input和Output的縮寫，即輸入和輸出。廣義上的圍繞計算機的輸入輸出有很多：鼠標、鍵盤、掃描儀等等。而我們今天要探討的是在計算機裏面，主要是作用在內存、網卡、硬盤等硬件設備上的輸入輸出操作。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"談起IO的模型，大多數人腦子裏肯定是一坨混亂的概念，“阻塞”、“非阻塞”，“同步”、“異步”有什麼區別？很多同學傻傻分不清，有嘗試去搜索相關資料去探究真相，結果又被淹沒在茫茫的概念之中。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這裏嘗試簡單地去解釋下爲啥會出現這種現象，其中一個很重要的原因就是大家看到的資料對概念的解釋都站在了不同的角度，有的站在了底層內核的視角，有的直接在java層面或者Netty框架層面給大家介紹API，所以給大家造成了一定程度的困擾。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以在開篇之前，還是要說下本文所站的視角，我們將會從底層內核的層面給大家講解下IO。因爲萬變不離其宗，只有瞭解了底層原理，不管語言層面如何花裏胡哨，我們都能以不變應萬變。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"二、用戶空間和內核空間","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了便於大家理解複雜的IO以及零拷貝相關的技術，我們還是得花點時間在回顧下操作系統相關的知識。這一節我們重點看下用戶空間和內核空間，基於此後面我們才能更好地聊聊多路複用和零拷貝。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/21/215778a93e00c1e250154d3acd83309c.webp","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"boxShadow"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"硬 件 層（Hardware） ","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"包括和我們熟知的和IO相關的CPU、內存、磁盤和網卡幾個硬件；","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"內核空間（Kernel Space） ","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"計算機開機後首先會運行內核程序，內核程序佔用的一塊私有的空間就是內核空間，並且可支持訪問CPU所有的指令集（ring0 - ring3）以及所有的內存空間、IO及硬件設備；","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"用戶空間（User Space）","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"每個普通的用戶進程都有一個單獨的用戶空間，用戶空間只能訪問受限的資源（CPU的“保護模式”）也就是說用戶空間是無法直接操作像內存、網卡和磁盤等硬件的；","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如上所述，那我們可能會有疑問，用戶空間的進程想要去訪問或操作磁盤和網卡該怎麼辦呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲此，操作系統在內核中開闢了一塊唯一且合法的調用入口“System Call Interface”，也就是我們常說的系統調用，系統調用爲上層用戶提供了一組能夠操作底層硬件的API。這樣一來，用戶進程就可以通過系統調用訪問到操作系統內核，進而就能夠間接地完成對底層硬件的操作。這個訪問的過程也即用戶態到內核態的切換。常見的系統調用有很多，比如：內存映射mmap()、文件操作類的open()、IO讀寫read()、write()等等。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"三、IO模型","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"1、 BIO（Blocking IO）","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們先看一下大家都熟悉的BIO模型的 Java 僞代碼：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"\nServerSocket serverSocket = new ServerSocket(8080); // step1: 創建一個ServerSocket，並監聽8080端口\nwhile(true) { // step2: 主線程進入死循環\n Socket socket = serverSocket.accept(); // step3: 線程阻塞，開啓監聽\n \n BufferedReader reader = new BufferedReader(nwe InputStreamReader(socket.getInputStream()));\n System.out.println(\"read data: \" + reader.readLine()); // step4: 數據讀取\n \n \n PrintWriter print = new PrintWriter(socket.getOutputStream(), true);\n print.println(\"write data\"); // step5: socket數據寫入\n}","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這段代碼可以簡單理解成一下幾個步驟：","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"創建ServerSocket，並監聽8080端口；","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"主線程進入死循環，用來阻塞監聽客戶端的連接，socket.accept()；","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"數據讀取，socket.read()；","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"寫入數據，socket.write()；","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"underline","attrs":{}},{"type":"strong","attrs":{}}],"text":"問題","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上三個步驟：accept(...)、read(...)、write(...)都會造成線程阻塞。上述這個代碼使用了單線程，會導致主線程會直接夯死在阻塞的地方。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"underline","attrs":{}},{"type":"strong","attrs":{}}],"text":"優化","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們要知道一點“","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"進程的阻塞是不會消耗CPU資源的","attrs":{}},{"type":"text","text":"”，所以在多核的環境下，我們可以創建多線程，把接收到的請求拋給多線程去處理，這樣就有效地利用了計算機的多核資源。甚至爲了避免創建大量的線程處理請求，我們還可以進一步做優化，創建一個線程池，利用池化技術，對暫時處理不了的請求做一個緩衝。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"2、“C10K”問題","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"“C10K”即“client 10k”用來指代數量龐大的客戶端；","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"BIO看上去非常的簡單，事實上採用“BIO+線程池”來處理少量的併發請求還是比較合適的，也是最優的。但是面臨數量龐大的客戶端和請求，這時候使用多線程的弊端就逐漸凸顯出來了：","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"嚴重依賴線程，線程還是比較耗系統資源的（一個線程大約佔用1M的空間）；","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"頻繁地創建和銷燬代價很大，因爲涉及到複雜的系統調用；","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"線程間上下文切換的成本很高，因爲發生線程切換前，需要保留上一個任務的狀態，以便切回來的時候，可以再次加載這個任務的狀態。如果線程數量龐大，會造成線程做上下文切換的時間甚至大於線程執行的時間，CPU負載變高；","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"3、NIO非阻塞模型","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面開始真正走向Java NIO或者Netty框架所描述的“","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"非阻塞","attrs":{}},{"type":"text","text":"”，NIO叫Non-Blocking IO或者New IO，由於BIO可能會引入的大量線程，所以可以簡單地理解NIO處理問題的方式是通過單線程或者少量線程達到處理大量客戶端請求的目的。爲了達成這個目的，首先要做的就是把阻塞的過程非阻塞化。要想做到非阻塞，那必須得要有內核的支持，同時需要對用戶空間的進程暴露系統調用函數。所以，這裏的“非阻塞”可以理解成系統調用API級別的，而真正底層的IO操作都是阻塞的，我們後面會慢慢介紹。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"事實上，內核已經對“非阻塞”做好了支持，舉個我們剛剛說的的accept()方法阻塞的例子（Tips：java中的accept方法對應的系統調用函數也叫accept），看下官方文檔對其非阻塞部分的描述。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/aa/aa7f0f467a2449669f1d75ed22b85281.webp","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"boxShadow"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"官方文檔對accetp()系統調用的描述是通過把\"","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"flags","attrs":{}},{"type":"text","text":"\"參數設成\"","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"SOCK_NONBLOCK","attrs":{}},{"type":"text","text":"\"就可以達到非阻塞的目的，非阻塞之後線程會一直處理輪詢調用，這時候可以通過每次返回特殊的異常碼“","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"EAGAIN","attrs":{}},{"type":"text","text":"”或\"","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"EWOULDBLOCK","attrs":{}},{"type":"text","text":"\"告訴主程序還沒有連接到達可以繼續輪詢。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們可以很容易想象程序非阻塞之後的一個大致過程。所以，非阻塞模式有個最大的特點就是：","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"用戶進程需要不斷去主動詢問內核數據準備好了沒有！","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面我們通過一段僞代碼，看下這個調用過程：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"\n// 循環遍歷\nwhile(1) {\n // 遍歷fd集合\n for (fdx in range(fd1, fdn)) {\n // 如果fdx有數據\n if (null != fdx.data) {\n // 進行讀取和處理\n read(fdx)&handle(fdx);\n }\n }\n}","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這種調用方式也暴露出非阻塞模式的最大的弊端，就是需要讓用戶進程不斷切換到內核態，對連接狀態或讀寫數據做輪詢。有沒有一種方式來簡化用戶空間for循環輪詢的過程呢？那就是我們下面要重點介紹的","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"IO多路複用模型","attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"4、IO多路複用模型","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"非阻塞模型會讓用戶進程一直輪詢調用系統函數，頻繁地做內核態切換。想要做優化其實也比較簡單，我們假想個業務場景，A業務系統會調用B的基礎服務查詢單個用戶的信息。隨着業務的發展，A的邏輯變複雜了，需要查100個用戶的信息。很明顯，A希望B提供一個批量查詢的接口，用集合作爲入參，一次性把數據傳遞過去就省去了頻繁的系統間調用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"多路複用實際也差不多就是這個實現思路，只不過入參這個“集合”需要你註冊/填寫感興趣的事件，讀fd、寫fd或者連接狀態的fd等，然後交給內核幫你進行處理。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那我們就具體來看看多路複用裏面大家都可能聽過的幾個系統調用","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":" - select()、poll()、epoll()。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"4.1 ","attrs":{}},{"type":"text","marks":[{"type":"underline","attrs":{}}],"text":"select()","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"select() ","attrs":{}},{"type":"text","text":"構造函數信息如下所示：","attrs":{}}]},{"type":"codeblock","attrs":{"lang":"java"},"content":[{"type":"text","text":"\n/**\n * select()系統調用\n *\n * 參數列表：\n * nfds - 值爲最大的文件描述符+1\n * *readfds - 用戶檢查可讀性\n * *writefds - 用戶檢查可寫性\n * *exceptfds - 用於檢查外帶數據\n * *timeout - 超時時間的結構體指針\n */\nint select（int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout）;","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"官方文檔對","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"select()","attrs":{}},{"type":"text","text":"的描述：","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"DESCRIPTION","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"select() and pselect() allow a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become \"ready\" for some class of I/O operation (e.g.,input possible). A file descriptor is considered ready if it is possible to perform the corresponding I/O operation (e.g., read(2)) without 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