IO 多路复用是什么意思，一直没弄明白？
linux IO多路复用有epoll， poll, select，知道epoll性能比其他几者要好。也在网上查了一下这几者的区别，表示没有弄明白。IO多路复用是什么意思，在实际的应用中是指什么情况？以及IO多路复用在一些异步web server中的实际应用，就好比nginx是怎样使用epoll来实现异步IO处理的，一个请求到来了，nginx使用epoll接收请求的过程是怎样的？
按投票排序
这个还是很好说清楚的。
假设你是一个机场的空管， 你需要管理到你机场的所有的航线， 包括进港，出港， 有些航班需要放到停机坪等待，有些航班需要去登机口接乘客。
你会怎么做?
最简单的做法，就是你去招一大批空管员，然后每人盯一架飞机， 从进港，接客，排位，出港，航线监控，直至交接给下一个空港，全程监控。
那么问题就来了：
很快你就发现空管塔里面聚集起来一大票的空管员，交通稍微繁忙一点，新的空管员就已经挤不进来了。 空管员之间需要协调，屋子里面就1, 2个人的时候还好，几十号人以后 ，基本上就成菜市场了。空管员经常需要更新一些公用的东西，比如起飞显示屏，比如下一个小时后的出港排期，最后你会很惊奇的发现，每个人的时间最后都花在了抢这些资源上。 现实上我们的空管同时管几十架飞机稀松平常的事情， 他们怎么做的呢？
他们用这个东西
这个东西叫flight progress strip.
每一个块代表一个航班，不同的槽代表不同的状态，然后一个空管员可以管理一组这样的块（一组航班），而他的工作，就是在航班信息有新的更新的时候，把对应的块放到不同的槽子里面。这个东西叫flight progress strip.
每一个块代表一个航班，不同的槽代表不同的状态，然后一个空管员可以管理一组这样的块（一组航班），而他的工作，就是在航班信息有新的更新的时候，把对应的块放到不同的槽子里面。这个东西现在还没有淘汰哦，只是变成电子的了而已。。是不是觉得一下子效率高了很多，一个空管塔里可以调度的航线可以是前一种方法的几倍到几十倍。
如果你把每一个航线当成一个Sock(I/O 流),
空管当成你的服务端Sock管理代码的话.第一种方法就是最传统的多进程并发模型 (每进来一个新的I/O流会分配一个新的进程管理。)第二种方法就是I/O多路复用 (单个线程，通过记录跟踪每个I/O流(sock)的状态，来同时管理多个I/O流 。)其实“I/O多路复用”这个坑爹翻译可能是这个概念在中文里面如此难理解的原因。所谓的I/O多路复用在英文中其实叫 I/O multiplexing. 如果你搜索multiplexing啥意思，基本上都会出这个图：
于是大部分人都直接联想到"一根网线，多个sock复用" 这个概念，包括上面的几个回答， 其实不管你用多进程还是I/O多路复用， 网线都只有一根好伐。多个Sock复用一根网线这个功能是在内核＋驱动层实现的。
重要的事情再说一遍： I/O multiplexing 这里面的 multiplexing 指的其实是在单个线程通过记录跟踪每一个Sock(I/O流)的状态(对应空管塔里面的Fight progress strip槽)来同时管理多个I/O流. 发明它的原因，是尽量多的提高服务器的吞吐能力。 是不是听起来好拗口，看个图就懂了.在同一个线程里面， 通过拨开关的方式，来同时传输多个I/O流， (学过EE的人现在可以站出来义正严辞说这个叫“时分复用”了）。
什么，你还没有搞懂“一个请求到来了，nginx使用epoll接收请求的过程是怎样的”， 多看看这个图就了解了。提醒下，ngnix会有很多链接进来， epoll会把他们都监视起来，然后像拨开关一样，谁有数据就拨向谁，然后调用相应的代码处理。－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－了解这个基本的概念以后，其他的就很好解释了。
select, poll, epoll 都是I/O多路复用的具体的实现，之所以有这三个鬼存在，其实是他们出现是有先后顺序的。
I/O多路复用这个概念被提出来以后， select是第一个实现 (1983 左右在BSD里面实现的)。
select 被实现以后，很快就暴露出了很多问题。
select 会修改传入的参数数组，这个对于一个需要调用很多次的函数，是非常不友好的。 select 如果任何一个sock(I/O stream)出现了数据，select 仅仅会返回，但是并不会告诉你是那个sock上有数据，于是你只能自己一个一个的找，10几个sock可能还好，要是几万的sock每次都找一遍，这个无谓的开销就颇有海天盛筵的豪气了。select 只能监视1024个链接， 这个跟草榴没啥关系哦，linux 定义在头文件中的，参见FD_SETSIZE。select 不是线程安全的，如果你把一个sock加入到select, 然后突然另外一个线程发现，尼玛，这个sock不用，要收回。对不起，这个select 不支持的，如果你丧心病狂的竟然关掉这个sock, select的标准行为是。。呃。。不可预测的， 这个可是写在文档中的哦.
“If a file descriptor being monitored by select() is closed in another thread, the result is
unspecified”
霸不霸气于是14年以后(1997年）一帮人又实现了poll,
poll 修复了select的很多问题，比如
poll 去掉了1024个链接的限制，于是要多少链接呢， 主人你开心就好。 poll 从设计上来说，不再修改传入数组，不过这个要看你的平台了，所以行走江湖，还是小心为妙。其实拖14年那么久也不是效率问题， 而是那个时代的硬件实在太弱，一台服务器处理1千多个链接简直就是神一样的存在了，select很长段时间已经满足需求。
但是poll仍然不是线程安全的， 这就意味着，不管服务器有多强悍，你也只能在一个线程里面处理一组I/O流。你当然可以那多进程来配合了，不过然后你就有了多进程的各种问题。于是5年以后, 在2002, 大神 Davide Libenzi 实现了epoll.
epoll 可以说是I/O 多路复用最新的一个实现，epoll 修复了poll 和select绝大部分问题, 比如：
epoll 现在是线程安全的。 epoll 现在不仅告诉你sock组里面数据，还会告诉你具体哪个sock有数据，你不用自己去找了。 epoll 当年的patch，现在还在，下面链接可以看得到：贴一张霸气的图，看看当年神一样的性能（测试代码都是死链了， 如果有人可以刨坟找出来，可以研究下细节怎么测的).
横轴Dead connections 就是链接数的意思，叫这个名字只是它的测试工具叫deadcon. 纵轴是每秒处理请求的数量，你可以看到，epoll每秒处理请求的数量基本不会随着链接变多而下降的。poll 和/dev/poll 就很惨了。可是epoll 有个致命的缺点。。只有linux支持。比如BSD上面对应的实现是kqueue。
其实有些国内知名厂商把epoll从安卓里面裁掉这种脑残的事情我会主动告诉你嘛。什么，你说没人用安卓做服务器，尼玛你是看不起p2p软件了啦。 而ngnix 的设计原则里面， 它会使用目标平台上面最高效的I/O多路复用模型咯，所以才会有这个设置。一般情况下，如果可能的话，尽量都用epoll/kqueue吧。详细的在这里:PS: 上面所有这些比较分析，都建立在大并发下面，如果你的并发数太少，用哪个，其实都没有区别。 如果像是在欧朋数据中心里面的转码服务器那种动不动就是几万几十万的并发，不用epoll我可以直接去撞墙了。
多路网络连接复用一个io线程。
多路复用，一条路可以很多人同时走的路
简单的说下自己的理解io分为磁盘io和网络io，这里说的是网络io。我们知道计算机之间传输数据是流传输。一台计算机网络io只会有一个。这里说单进程在最基本的c/s demo中，send/recv就是在一条io通道收发数据，这就是基本的网络io，但是这种操作是不能“填满”io的，也就是说大部分io资源你没有用，仅仅有一个io操作，当然你可以开多进程或多线程，代价可想而知此时出现了io多路复用，自己的话翻译一下，复用网络io从而有多个io操作能在网络io中执行。linux下网络io使用socket套接字来通信，普通io模型只能监听一个socket，而io多路复用可同时监听多个socketio多路复用避免阻塞在io上，原本为多进程或多线程来接收多个连接的消息变为单进程或单线程保存多个socket的状态后轮询处理实现io多路复用需要函数来支持，就是你说的linux下的select/poll，epoll以及win下kqueue至于它们有什么区别，网上有很多了。如果楼主看不懂它们的区别，建议查查资料或者看看apue或unp的相关章节弥补一下不理解的部分。简单说epoll和select/poll最大区别是1.epoll内部使用了mmap共享了用户和内核的部分空间，避免了数据的来回拷贝2.epoll基于事件驱动，epoll_ctl注册事件并注册callback回调函数，epoll_wait只返回发生的事件避免了像select和poll对事件的整个轮寻操作。nginx中使用了epoll，是基于事件驱动模型的，由一个或多个事件收集器来收集或者分发事件，epoll就属于事件驱动模型的事件收集器，将注册过的事件中发生的事件收集起来，master进程负责管理worker进程
Multiplexing：one to many你用手机上网，但是一次只能连一个基站，这是one to one。手机基站，同时要处理一堆手机的上网需求，这是one to many，就需要multiplexing技术，也就是我们常常听到的什么CDMA啦，TDMA，OFDMA啦之类的。对于Linux网络接口来说，你只有一个网卡，却需要同时处理N个链接，这里就需要Multiplexing；上升到软件层面，也就是题主关心的几个系统调用，就是IO（网络socket包括在内）需要one to many的时候提供一种multiplexing机制。
为啥重来没人邀请我答题呢...是我太水还是太傲娇了 &-_&-在阻塞的IO中(为了保证能够得到或发送数据),当一个从一个文件描述符读数据,然后将数据写到另一个文件描述符时,可以用一个循环完成:while ((n = read(fd_read, buf, SIZE)) & 0)
if ( write(fd_write, buf, SIZE) != n)
echo("write error");
这种阻塞式的IO,在一对一的时候,基本还是胜任的.但是如果变成两对文件,仍然使用这种方式那么就有可能长时间阻塞在一个描述符上,其他描述符得不到及时的处理.为了处理这种情况,可以使用非阻塞IO轮询,只不过会浪费CPU.也可以使用异步IO,基于SIGIO进行处理,但是局限很多.为此系统中衍生出了另一种技术,IO多路转接.IO多路转接技术的内容就是:线构造一张或多张描述符的表,然后调用一个函数,知道有表中的描述符准备好可以进行IO操作时,该函数才返回,在返回时告诉进程哪一个描述符已经准备好了,可以进行IO操作,对该描述符进行IO操作就不会阻塞.select和pselect的区别是:select会改变参数中的timeout参数,以此可以计算还有多久超时,pselect不会更新timeout参数.当然pselect还有个sigmask,在使用上:ready = pselect(nfds, &readfds, &writefds, &exceptfds,
timeout, &sigmask);
相当于:ready = pselect(nfds, &readfds, &writefds, &exceptfds,
timeout, &sigmask);
poll和ppoll类似于select和pselect的关系.epoll相当于一个针对大量文件描述符的版本
diy快递公司的地址是x省x市x地区x街道x门牌（快递公司名看作域名，地址看作ip，经纬度看作mac地址）。快递公司分为几个工作区，其中80工作区（端口）负责处理订单业务。来自各地（客户端）的快递车（链接请求）经过大门（上面的地址），进入80工作区快递公司为80工作区安排了8组处理小队（worker进程），快递车进入80工作区后，80工作区分配一个待处理业务号（描述符），工作区将待处理业务号广播后，休闲处理小队开始处理对应业务io多路可以看作工作区的多个快递车，或者多个业务订单号，复用就是这些快递车复用同一个快递公司地址和80工作区diy boss设计了三套方案实现有限处理小队对大量快递业务的处理
要弄清问题 先要知道问题的出现原因 原因:
由于进程的执行过程是线性的(也就是顺序执
行),当我们调用低速系统I/O(read,write,
accept等等),进程可能阻塞,此时进程就阻塞
在这个调用上,不能执行其他操作.阻塞很正
常. 接下来考虑这么一个问题:
一个服务器进程和一个客户端进程通信,服
务器端read(sockfd1,bud,bufsize),此时客户
端进程没有发送数据,那么read(阻塞调用)将
阻塞直到客户端调用write(sockfd,but,size)
发来数据. 在一个客户和服务器通信时这没
什么问题,当多个客户与服务器通信时,若服
务器阻塞于其中一个客户sockfd1,当另一个
客户的数据到达套接字sockfd2时,服务器不
能处理,仍然阻塞在read(sockfd1,...)上;此时
问题就出现了,不能及时处理另一个客户的
服务,咋么办?I/O多路复用来解决!I/O多路复用:
继续上面的问题,有多个客户连接,
sockfd1,sockfd2,sockfd3..sockfdn
同时监听这n个客户,当其中有一个发来消息
时就从select的阻塞中返回,然后就调用read
读取收到消息的sockfd,然后又循环回select
这样就不会因为阻塞在其中一个上而不能处
理另一个客户的消息
硬答一下：首先，IO多路复用技术：通过该技术，系统内核缓冲I/O数据，当某个I/O准备好后，系统通知应用程序该I/O可读或可写，这样应用程序可以马上完成相应的I/O操作，而不需要等待系统完成相应I/O操作，从而应用程序不必因等待I/O操作而阻塞。（随便网上查都有233）据上面的描述，很清楚地知道IO多路复用就是字面上的复用IO。至于select,poll,epoll都是支持IO多路复用的系统调用。至于他们的区别有很多书或者博客描述。具体答一下Nginx处理请求。Nginx有几种模型。有点复杂。具体就从监听后分配到某个worker进程中。然后调用epoll那肯定是多个用户和一个进程保持TCP连接啦。首先是系统提前申请一个epoll对象。把套接字添加到epoll中，异步就在于有另外的系统进程帮你处理。处理好之后内核会通知你已经处理好。然后继续操作或者返回。（这个过程自己调调API写写就懂）嗯。先这样了。答不下去了。题主可以修改问题。可以吸引大牛们来答
这些名词比较绕口，理解涵义就好。一个epoll场景：一个酒吧服务员（一个线程），前面趴了一群醉汉，突然一个吼一声“倒酒”（事件），你小跑过去给他倒一杯，然后随他去吧，突然又一个要倒酒，你又过去倒上，就这样一个服务员服务好多人，有时没人喝酒，服务员处于空闲状态，可以干点别的玩玩手机。至于epoll与select，poll的区别在于后两者的场景中醉汉不说话，你要挨个问要不要酒，没时间玩手机了。io多路复用大概就是指这几个醉汉共用一个服务员。逻辑推理能力与阅读理解能力有无关系?为什么?_百度作业帮
逻辑推理能力与阅读理解能力有无关系?为什么?
逻辑推理能力与阅读理解能力有无关系?为什么?
逻辑推理能力与阅读理解能力有莫大关系.语言和逻辑虽然是两种不同的知识体系,但是它们之间存在着相互依存、互为表里、彼此对应、密不可分的广泛联系.如果把两者按照其对应关系结合起来学习,对于提高人们的思维能力及语言文字阅读、表达能力很有好处.如果能把两者结合起来学习,寓逻辑知识于语言之中,并通过语言训练发展思维能力,一定可以使思维更为严谨,语言文字表达更为准确畅达.
“不积小流无以成江海”，阅读理解就是“不积小流”，逻辑推理就是“成江海”。个人浅见，不值一哂。
嗯……是不是在网上跟人争论，然后被人家说逻辑混乱、看不明白别人说什么？如果不是就算了，当我胡说。个人之见，与其讨论这两者有无关系，不如说通过阅读理解能力就能直接看出一个人的逻辑水平，因为文字表达本身就是一种逻辑表示。如果是作业题，直接搜“逻辑 阅读理解”就会得到许多相关文章，深浅均有，相信你会整理出答案的。...其他类似试题
【高一英语】阅读理解The
Opening of the Book Nook
Saturday, October 4, 10a.m. to 10 p.m
You will not want to miss the opening of
your new neighbourhood bookstore! Located at 2289 Main Street, the Book Nook is within
walking distance of schools and many homes e and check out
the Book Nook on Saturday!
Activities
will include:
● Live music by local musicians
● One Book-of Cthe- Month Club membership
giveaway(赠送)
The Book Nook has three floors with books of
all kinds―any kind you could want. If we do not have the book you are looking
for, we can specially order it for you. You will have it in your hands within
Reading Nooks
We are proud of our children’s reading area on the first floor, as
well as our teenagers’ nook on the second floor. Come for the activities and
stay a while! Settle in one of these in take a seat with a
good book and a free cup of hot chocolate.You will discover the perfect way to
spend a few hours.
Book Events
The Book Nook will be featuring monthly book signings by different
authors, giving you a chance to meet and speak with well-known writers. Don’t
miss the experinec of hearing these authors read aloud their own books!
The Book-of-the- Month Club
Our Book-of Cthe-Month Club will feature 12 books each year. As a
member, you will be able to select one new book each month. The membership fee
is only $10.00 per month. That is a great price for 12 books each year!
So please join us on Saturday and learn about all that the Book Nook
has to offer. You can come anytime between 10 a.m and 10 p.m―our activities
last all day long!
25. What can we
learn about the Book Nook
A. It is a well-located bookstore.
B. Any interesting books can be found right
C. It is open from 10 a. m to 10 p.m every
D. The third floor is specially designed for
26. What can
people do in the Book Nook on October 4
A. They can have a taste of different kinds
of chocolate.
B. They can read aloud together with
well-known writers.
C. They can enjoy live music performed by
well-known writers.
D. They can become a member of the Book-of-the-Month
27. As a member of
the Book-of- the- Month Club, you ________.
A. can buy any books in the Book Nook at a
B. may borrow as many as 12 books every week
C. need to pay 120 dollars every year
D. should be over the age of 12
【高一英语】阅读理解On
April 22, 2012, I jumped from an airplane 13, 000 feet up. What was the
purpose It was just to overcome my fear of heights.
years ago, when I went to France,
I couldn’t go up the Eiffel
Tower because I was
afraid. It seemed so high. I wanted to overcome my fear, so I could dare to go
skydiving（跳伞）.
At first, I didn’t think I could, but I finally made it.
think that I could have jumped out of that plane without the encouragement of
my wonderful friends and classmates―Yodel and Tatiana! It
was a pity that we couldn’t jump together. However, as we prepared to jump,
there was something special among us-- a special bond.
a reservation（预定）to
go skydiving one week before the day of my jump. I felt nervous during the
whole week, but mysteriously. When it was the moment to jump from
the airplane, I didn’t feel any fear… I just jumped! That was wonderful!
Wonderful!
was a whole new world and the beautiful sky. I had never seen such a beautiful
world! First, I felt an awful acceleration（加速） and then I felt like I was lying in the
air…like a bird! I think it was an illusion, but it was wonderful!
oncoming wind blew me strongly. It was amazing! After about 50 seconds free
fall, the parachute opened. When I landed on the ground, I felt a lack of
oxygen. It was difficult to breathe in during the freefall.
so glad that I survived and was able to overcome my fear! Everything went well!
If possible, I want to jump again. My first jump is an outstanding memory for
What would be best title of the passage
Overcoming fear of heights.
B. Not Daring to Go up the Eiffel Tower.[来源：17教育网]
Jumping from a plane
D. A special bond with friends
By saying “I finally made it”, the author means _________.
gave up going skydiving in the end
B. at last he succeeded in skydiving
made up his mind in the end
D. he finally made a reservation
The author felt it a pity that ____.
couldn’t encourage his friends
B. he couldn’t jump with his friends
didn’t climb the Eiffel
he had made the reservation earlier
Why did the author feel a lack of oxygen when he was landing on the ground
Because he was falling for about a minute.
Because it was his first time to do so.
Because people breathe in less air when falling freely.
Because the wind was blowing him strongly.
【高一英语】阅读理解Being a good friend isn’t always
easy, but taking the time to develop a lasting friendship is worth the effort. As
the years pass, you will realize that each friendship you keep is
priceless. 36 To be a good friend and deepen a friendship, just follow these
steps.[来源：17教育网]
Keep your promises.
Don’t ever make
a promise that you can’t keep. If you say you’ll hang out with a friend and an
unexpected situation arises, explain the situation. Give your friend a gift and
tell him or her you are sorry. Nobody is perfect, and it’s okay if you break a
promise once in a blue moon. 37
Apologize when you’ve made a mistake.
38 Though your friend won’t be happy that you made a mistake, he or she will be very pleased
that you admit it instead of just pretending that nothing is wrong.
To be truly
supportive, you will have to be able to watch out for your friend when he/she
is having a tough time. If you sense that your friend is getting into some
trouble, help him or her away from the situation by not being afraid to speak
up about it.
Give your friend some space when he or she needs it.
Part of being
supportive means supporting the fact that your friend won’t always want to
spend time with you. Learn to step back and give your friend space. 40 Don’t be jealous (嫉妒的) if your friend has lots of other friends.
A. However, don’t make it a regular thing.
B. Help your friend deal with his or her
struggles.
C. Learn to forgive your friend and move
D. Of course, to have a good friend, you
must be one.
E. If you want your friend to trust you,
you can’t act like you’re perfect.
F. Understand if your friend wants to be alone or to hang out with other people,
G. Take the time to truly understand your
friend when he or she is talking to you.
【高一英语】阅读理解For many years, I strongly
believed that my suffering was because of my size. I imagined that when the weight disappeared, it
would take old wounds, hurts and rejections with it.
Many people with
weight problems also mistakenly believe that changing our bodies will fix everything. Perhaps our mistake
is believing that being thin equals being loved, being special, and being cared
for. We fantasize(幻想) about what it will be like when we reach the long-awaited goal. We
work very hard to realize this dream. Then, at last, we find ourselves there.
But we often
gain back what we have lost. Even so, we continue to believe that next time it
will be different. Next time, we will keep it off. Next time, being thin will
finally bring us happiness, self-worth, and, of course, love.
It took me a
long time to realize that there was something more for me to learn about
beauty. Different cultures have different standards (标准) of beauty. In Samoa a woman is
not considered attractive unless she weighs more than 200 pounds. More
importantly, if it’s happiness that we want, why not put our energy there
rather than on the size of our body Why not look inside Many of us struggle
hard to change our body, but in vain. We have to find a way to live comfortably
inside our body and make friends with and care for ourselves. When we change
our attitudes towards ourselves, the whole world changes.
32. The passage tries to stress the
importance of ____________.
A. body size
different beauty standards
C. culture difference
attitudes towards life
33. What does the underlined word “everything” in paragraph 2 mean
A. The whole world
All the problems
C. All the dreams
D. All the truth
34. What can be inferred about the author
A. The author is a Samoan.
B. The author probably got wounded in wars or
accidents.
C. The author succeeded in losing weight.
D. The author has been troubled by her/his
35. Which of the following statements is
happiness is determined by a person’s weight.
B. Not all
countries take being thin as the standard of beauty.
C. It’s entirely
wrong to care about whether a person is fat or not.
D. Once a person
loses weight, he/she won’t gain back the weight easily.
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