高併發高性能服務器是如何實現的？

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"服務器是如何並行處理成千上萬個用戶請求呢？這裏面涉及到哪些技術呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://www.bilibili.com/video/BV1nt4y167K2","title":null},"content":[{"type":"text","text":"高併發場景下的單機服務模型","attrs":{}}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://www.bilibili.com/video/BV195411E7HV","title":null},"content":[{"type":"text","text":"高併發的TCP長連接","attrs":{}}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"多進程","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":"歷史上最早出現也是最簡單的一種並處處理多個請求的方法就是利用多進程。","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":"比如在Linux世界中，我們可以使用fork、exec等方法創建多個進程，我們可以在父進程中接收用戶的鏈接請求，然後創建子進程去處理用戶請求，就像這樣：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/83/8341a8aed82dab16066b0240bf255b6a.png","alt":"在這裏插入圖片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"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","text":"這種方法的優點就在於：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"編程簡單，非常容易理解","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由於各個進程的地址空間是相互隔離的，因此一個進程崩潰後並不會影響其它進程","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"充分利用多核資源","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","text":"多進程並行處理的優點和明顯，但是缺點同樣明顯：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"各個進程地址空間相互隔離，這一優點也會變成缺點，那就是進程間要想通信就會變得比較困難，你需要藉助進程間通信(IPC，interprocesscommunications)機制，想一想你現在知道哪些進程間通信機制，然後讓你用代碼實現呢？顯然，進程間通信編程相對複雜，而且性能也是一大問題。","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"創建進程開銷是比線程要大的，頻繁的創建銷燬進程無疑會加重系統負擔。","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"多線程","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":"由於線程共享進程地址空間，因此線程間通信天然不需要藉助任何通信機制，直接讀取內存就好了。","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":"線程創建銷燬的開銷也變小了，要知道線程就像寄居蟹一樣，房子(地址空間)都是進程的，自己只是一個租客，因此非常的輕量級，創建銷燬的開銷也非常小。","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":"我們可以爲每個請求創建一個線程，即使一個線程因執行I/O操作——比如讀取數據庫等——被阻塞暫停運行也不會影響到其它線程，就像這樣：","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/dd/dd8825c737b5b7a802907ca28c5acd63.png","alt":"在這裏插入圖片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"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":"但線程就是完美的、包治百病的嗎，顯然，計算機世界從來沒有那麼簡單。","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":"由於線程共享進程地址空間，這在爲線程間通信帶來便利的同時也帶來了無盡的麻煩。","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":"正是由於線程間共享地址空間，因此一個線程崩潰會導致整個進程崩潰退出，同時線程間通信簡直太簡單了，簡單到線程間通信只需要直接讀取內存就可以了，也簡單到出現問題也極其容易，死鎖、線程間的同步互斥、等等，這些極容易產生bug，無數程序員寶貴的時間就有相當一部分用來解決多線程帶來的無盡問題。","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":"雖然線程也有缺點，但是相比多進程來說，線程更有優勢，但想單純的利用多線程就能解決高併發問題也是不切實際的。","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":"因爲雖然線程創建開銷相比進程小，但依然也是有開銷的，對於動輒數萬數十萬的鏈接的高併發服務器來說，創建數萬個線程會有性能問題，這包括內存佔用、線程間切換，也就是調度的開銷。","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":"因此，我們需要進一步思考。","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":"link","attrs":{"href":"https://jq.qq.com/?_wv=1027&k=FqyVXbez","title":null},"content":[{"type":"text","text":"Linux、C/C++技術交流羣:【960994558】","attrs":{}}]},{"type":"text","text":"整理了一些個人覺得比較好的學習書籍、大廠面試題、和技術教學視頻資料共享在裏面（包括C/C++，Linux，Nginx，ZeroMQ，MySQL，Redis，fastdfs，MongoDB，ZK，流媒體，CDN，P2P，K8S，Docker，TCP/IP，協程，DPDK等等.），有需要的可以自行添加哦！~","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/5e/5ead9d9097a3510c30fcbefc68ee8949.png","alt":"在這裏插入圖片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Event Loop：事件驅動","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":"到目前爲止，我們提到“並行”二字就會想到進程、線程。但是，並行編程只能依賴這兩項技術嗎，並不是這樣的。","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":"還有另一項並行技術廣泛應用在GUI編程以及服務器編程中，這就是近幾年非常流行的事件驅動編程，event-based concurrency。","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":"大家不要覺得這是一項很難懂的技術，實際上事件驅動編程原理上非常簡單。","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":"這一技術需要兩種原料：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"event","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"處理event的函數，這一函數通常被稱爲event handler","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","text":"剩下的就簡單了：","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":"你只需要安靜的等待event到來就好，當event到來之後，檢查一下event的類型，並根據該類型找到對應的event處理函數，也就是event handler，然後直接調用該event handler就好了。","attrs":{}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/31/31445ec4946d10c14458e3562d98c866.png","alt":"在這裏插入圖片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"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","text":"以上就是事件驅動編程的全部內容。","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":"從上面的討論可以看到，我們需要不斷的接收event然後處理event，因此我們需要一個循環(用while或者for循環都可以)，這個循環被稱爲Event loop。","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":"使用僞代碼表示就是這樣：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"while(true) {\n event = getEvent();\n handler(event);\n}\n1234","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":"Event loop中要做的事情其實是非常簡單的，只需要等待event的帶來，然後調用相應的event處理函數即可。","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":"注意，這段代碼只需要運行在一個線程或者進程中，只需要這一個event loop就可以同時處理多個用戶請求。","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":"有的同學可以依然不明白爲什麼這樣一個event loop可以同時處理多個請求呢？","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":"原因很簡單，對於web服務器來說，處理一個用戶請求時大部分時間其實都用在了I/O操作上，像數據庫讀寫、文件讀寫、網絡讀寫等。當一個請求到來，簡單處理之後可能就需要查詢數據庫等I/O操作，我們知道I/O是非常慢的，當發起I/O後我們大可以不用等待該I/O操作完成就可以繼續處理接下來的用戶請求。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e6/e62d2c0655d35ef2781dd7de3d2772cd.png","alt":"在這裏插入圖片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"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":"現在你應該明白了吧，雖然上一個用戶請求還沒有處理完我們其實就可以處理下一個用戶請求了，這就是並行，這種並行就可以用事件驅動編程來處理。","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":"這就好比餐廳服務員一樣，一個服務員不可能一直等這上一個顧客下單、上菜、喫飯、買單之後才接待下一個顧客，服務員是怎麼做的呢？當一個顧客下完單後直接處理下一個顧客，當顧客喫完飯後會自己回來買單結賬的。","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":"看到了吧，同樣是一個服務員也可以同時處理多個顧客，這個服務員就相當於這裏的Event loop，即使這個event loop只運行在一個線程(進程)中也可以同時處理多個用戶請求。","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":"相信你已經對事件驅動編程有一個清晰的認知了，那麼接下來的問題就是事件驅動、事件驅動，那麼這個事件也就是event該怎麼獲取呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"事件來源：IO多路複用","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":"IO多路複用技術正是用來解決這一問題的，通過IO多路複用技術，我們一次可以監控多個文件描述，當某個文件(socket)可讀或者可寫的時候我們就能得到通知啦。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣IO多路複用技術就成了event loop的發動機，源源不斷的給我們提供各種event，這樣關於event來源就解決了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/00/008e52f5b7aba2806a22c59299fc2069.png","alt":"在這裏插入圖片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"問題：阻塞式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":"現在，我們可以使用一個線程(進程)就能基於事件驅動進行並行編程，再也沒有了多線程中讓人惱火的各種鎖、同步互斥、死鎖等問題了。 但是，計算機科學中從來沒有出現過一種能解決所有問題的技術，現在沒有，在可預期的將來也不會有。","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":"程序員最常用的這種IO方式被稱爲阻塞式IO，也就是說，當我們進行IO操作，比如讀取文件時，如果文件沒有讀取完成，那麼我們的程序(線程)會被阻塞而暫停執行，這在多線程中不是問題，因爲操作系統還可以調度其它線程。","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":"但是在單線程的event loop中是有問題的，原因就在於當我們在event loop中執行阻塞式IO操作時整個線程(event loop)會被暫停運行，這時操作系統將沒有其它線程可以調用，因爲系統中只有一個event loop在處理用戶請求，這樣當event loop線程被阻塞暫停運行時所有用戶請求都沒有辦法被處理，你能想象當服務器在處理其它用戶請求讀取數據庫導致你的請求被暫停嗎？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/29/29acc02de57d72573a2f4f039cf8500b.png","alt":"在這裏插入圖片描述","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"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","text":"因此，在基於事件驅動編程時有一條注意事項，那就是不允許發起阻塞式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":"有的同學可能會問，如果不能發起阻塞式IO的話，那麼該怎樣進行IO操作呢？","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":"有阻塞式IO，就有非阻塞式IO。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"非阻塞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":"爲克服阻塞式IO所帶來的問題，現代操作系統開始提供一種新的發起IO請求的方法，這種方法就是異步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":"異步IO時，假設調用aio_read函數(具體的異步IO API請參考具體的操作系統平臺)，也就是異步讀取，當我們調用該函數後可以立即返回，並繼續其它事情，雖然此時該文件可能還沒有被讀取，這樣就不會阻塞調用線程了。此外，操作系統還會提供其它方法供調用線程來檢測IO操作是否完成。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"就這樣，在操作系統的幫助下IO的阻塞調用問題也解決了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"基於事件編程的難點","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":"雖然有異步IO來解決event loop可能被阻塞的問題，但是基於事件編程依然是困難的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先，我們提到，event loop是運行在一個線程中的，顯然一個線程是沒有辦法充分利用多核資源的，有的同學可能會說那就創建多個event loop實例不就可以了，這樣就有多個event loop線程了，但是這樣一來多線程問題又會出現。","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":"異步編程需要結合回調函數，這種編程方式需要把處理邏輯分爲兩部分，一部分調用方自己處理，另一部分在回調函數中處理，這一編程方式的改變加重了程序員在理解上的負擔，基於事件編程的項目後期會很難擴展以及維護。","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":"那麼有沒有更好的方法呢？","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":"要找到更好的方法，我們需要解決問題的本質，那麼這個本質問題是什麼呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"更好的方法","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":"爲什麼我們要使用異步這種難以理解的方式編程呢？","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":"是因爲阻塞式編程雖然容易理解但會導致線程被阻塞而暫停運行。","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":"雖然基於事件編程有這樣那樣的缺點，但是在當今的高性能高併發服務器上基於事件編程方式依然非常流行，但已經不是純粹的基於單一線程的事件驅動了，而是event loop + multi thread + user level thread。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"總結","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":"高併發技術從最開始的多進程一路演進到當前的事件驅動，計算機技術就像生物一樣也在不斷演變進化，但不管怎樣，瞭解歷史才能更深刻的理解當下。希望這篇文章能對大家理解高併發服務器有所幫助。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}