☕️【Java 技術之旅】360度全方位的教你認識網絡IO模型

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"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"指在不能立刻得到結果之前，該函數不會阻塞當前線程，而會立刻返回去完成其他任務。","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":"總結來說，是否是阻塞還是非阻塞，關注的是接口調用（發出請求）後等待數據返回時的狀態。被掛起無法執行其他操作的則是阻塞型的，可以被立即「抽離」去完成其他「任務」的則是非阻塞型的。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"阻塞和同步的討論","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"有人也許會把阻塞調用和同步調用等同起來，實際上它們是不同的。","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"對於同步調用來說，很多時候當前線程可能還是激活的，只是從邏輯上當前函數沒有返回而已，此時，這個線程可能也會處理其他的消息。還有一點，在這裏先擴展下：","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":1,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"如果這個線程在等待當前函數返回時，仍在執行其他消息處理，那這種情況就叫做同步非阻塞","attrs":{}},{"type":"text","text":"；","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":1,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"如果這個線程在等待當前函數返回時，沒有執行其他消息處理，而是處於掛起等待狀態，那這種情況就叫做同步阻塞","attrs":{}},{"type":"text","text":"；","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":"所以同步的實現方式會有兩種：同步阻塞、同步非阻塞；同理，異步也會有兩種實現：異步阻塞、異步非阻塞；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":2,"normalizeStart":2},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"對於阻塞調用來說，則當前線程就會被掛起等待當前函數返回；","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":"雖然表面上看非阻塞的方式可以明顯的提高CPU的利用率，但是也帶了另外一種後果就是系統的線程切換增加。增加的CPU執行時間能不能補償系統的切換成本需要好好評估","attrs":{}},{"type":"text","text":"。","attrs":{}}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"同步異步/阻塞非阻塞","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在說明synchronous IO和asynchronous IO的區別之前，需要先給出兩者的定義。Stevens給出的定義（其實是POSIX的定義）是這樣子的：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":" A synchronous I/O operation causes the requesting process to be blocked until that I/O operation completes; \n An asynchronous I/O operation does not cause the requesting process to be blocked;\n","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":"兩者的區別就在於synchronous IO做”IO operation”的時候會將process阻塞。按照這個定義，之前所述的blocking IO，non-blocking IO，IO multiplexing都屬於synchronous IO。","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"有人可能會說，non-blocking IO並沒有被block啊。這裏有個非常“狡猾”的地方，定義中所指的”IO operation”是指真實的IO操作，就是例子中的recvfrom這個system call","attrs":{}},{"type":"text","text":"。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"non-blocking IO在執行recvfrom這個system call的時候，如果kernel的數據沒有準備好，這時候不會block進程","attrs":{}},{"type":"text","text":"。","attrs":{}}]}]}],"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":"但是，當kernel中數據準備好的時候，recvfrom會將數據從kernel拷貝到用戶內存中，這個時候進程是被block了，在這段時間內，進程是被block的。","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"而asynchronous IO則不一樣，當進程發起IO 操作之後，就直接返回再也不理睬了，直到kernel發送一個信號，告訴進程說IO完成。在這整個過程中，進程完全沒有被block","attrs":{}},{"type":"text","text":"。","attrs":{}}]}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"同步異步/阻塞非阻塞","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"\"同步異步\"和\"阻塞非阻塞\"是兩個不同範圍的概念","attrs":{}},{"type":"text","text":"。","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"synchronous / asynchronous is to describe the relation between two modules.\nblocking / non-blocking is to describe the situation of one module.\n\nAn example:\n\"I\": a\n\"bookstore\" : b\n\na asks b: do you have a book named \"c++ primer\"?\n\nblocking: before b answers a, a keeps waiting there for the answer. Now a (one module) is blocking. a and b are two threads or two processes or one thread or one process? we DON'T know.\n\nnon-blocking: before b answers a, a just leaves there and every two minutes, a comes here for looking for the answer. Here a (one module) is non-blocking. a and b are two threads or two processes or one process? we DON'T know. BUT we are sure that a and b couldn't be one thread.\n\nsynchronous: before b answers a, a keeps waiting there for the answer. It means that a can't continue until b finishes its job. Now we say: a and b (two modules) is synchronous. a and b are two threads or two processes or one thread or one process? we DON'T know.\n\nasynchronous: before b answers a, a leaves there and a can do other jobs. When b gets the answer, b will call a: hey! I have it! Then a will come to b to get the book when a is free. Now we say: a and b (two modules) is asynchronous. a and b are two threads or two processes or one process? we DON'T know. BUT we are sure that a and b couldn't be one thread.\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"同步異步/阻塞非阻塞","attrs":{}}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"同步和異步是一個非常廣的概念，它們的重點在於多個任務和事件發生時，一個事件的發生或執行是否會導致整個流程的暫時等待","attrs":{}},{"type":"text","text":"。","attrs":{}}]}],"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":"阻塞和非阻塞的區別關鍵在於當發出請求一個操作時，如果條件不滿足，是會一直等待還是返回一個標誌信息","attrs":{}},{"type":"text","text":"。","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":"在討論IO（硬盤、網絡、外設）時，一個完整的IO讀請求操作包括兩個階段：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"1）查看數據是否就緒；","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2）進行數據拷貝（內核將數據拷貝到用戶線程）","attrs":{}}]}]}],"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":"阻塞（blocking 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IO）的區別就在於第一個階段，如果數據沒有就緒，在查看數據是否就緒的過程中是一直等待，還是直接返回一個標誌信息。","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":"在《Unix網絡編程》一書中對同步IO和異步IO的定義是這樣的：","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":"事實上，同步IO和異步IO模型是針對用戶線程和內核的交互來說的：","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":"同步IO：當用戶發出IO請求操作之後，如果數據沒有就緒，需要通過用戶線程或者內核不斷地去輪詢數據是否就緒，當數據就緒時，再將數據從內核拷貝到用戶線程","attrs":{}},{"type":"text","text":"；","attrs":{}}]}],"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":"異步IO：只有IO請求操作的發出是由用戶線程來進行的，IO操作的兩個階段都是由內核自動完成，然後發送通知告知用戶線程IO操作已經完成。也就是說在異步IO中，不會對用戶線程產生任何阻塞","attrs":{}},{"type":"text","text":"。","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":"這是同步IO和異步IO關鍵區別所在，同步IO和異步IO的關鍵區別反映在數據拷貝階段是由用戶線程完成還是內核完成。","attrs":{}},{"type":"text","text":" 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