What program apes are thinking about while waiting for the elevator?

Author | Cooper Song

Zebian | Elle

Produced | Life program (ID: coder_life)

Have waited for so long, how the elevator has not come yet? ? ? There must be an elevator scheduling problem! Let me give it to design an elevator scheduling algorithm.

Elevator dispatching and scheduling disk operating system are linked. I thought about three years ago elevator scheduling problem, when I had just moved into high-rise residential, but I was still solid enough professional knowledge, there is no in-depth study. Until now I met a disk scheduling algorithms operating system, I think the elevator scheduling algorithm. The same purpose, the same thing is nothing but scheduling. Disk scheduling, the head pointer is moved (relatively to say), and in elevator dispatching, the movement of the elevator.

Then lift the scheduling algorithm, what does? They are applicable to situations in which it?

First come first serve algorithm

Referred to as first come first serve algorithm is FCFS, it is First Come First Serve acronym. As the name suggests, it is the first to come in front of the elevator (or first press the elevator up and down buttons) first passenger elevator service experience.

For example, Lee uncle pressed upward in a floor button, after which the 15 sheets of Uncle floor button pressed down, after which Uncle Wang and 8th floor button is pressed down. Uncle Wang Zhang, who lives with an appointment to go grocery shopping together.

So this time, no matter which floor the elevator now will go to the first floor then Uncle Lee.

After Lee uncle entered the elevator, whether he was going to a few buildings (assuming that Uncle Lee to go to the 20th floor), destination (20th floor), will go to the 15th floor elevator access Uncle Zhang.

Zhang, who lives upstairs in the elevator 15, he was going to jolly, so he went to the first floor, he pressed into the elevator on the first floor of a button.

So began whistling call the elevator down, this time also on the 8th floor of Uncle Wang watched through the 8th floor elevator continues to run down, even ignoring him! ! !

Uncle Zhang successfully reached the first floor, then the elevator came to pick up on the 8th floor king uncle, Uncle Wang this take the elevator to the first floor and Zhang Grandpa and will.

This put Uncle Wang was furious, and my heart is not condemning the property stupid X, is scheduled to write the elevator cursing programmer silly X ......

Malpractice first come first serve algorithm were revealed in the above example, but it also has the advantage of it, which was simple, programmers save! Joke, advantage is relatively fair, passenger elevator service to get the order must first come first served, people will not be cut in line.

The shortest seek time first algorithm

最短寻道时间优先算法的简称是SSTF，是Shortest Seek Time First的缩写，顾名思义，就是距离当前电梯位置最近的乘客，会最先得到电梯服务。

大爷们是否能得到电梯的服务，与电梯当前的位置有关。

还是举上面那个例子，假如在大爷们来到电梯门口前电梯停在1楼。李大爷起初在1楼，无疑是距离电梯最近的，他先上电梯。李大爷来到20楼下了电梯。电梯此时在20楼，距离20楼最近的服务请求来自15楼的张大爷，于是电梯呼呼呼下行来到15楼接上张大爷，此时电梯在15楼，距离15楼最近的服务请求来自8楼的王大爷，这一次电梯没有无视王大爷，接上了王大爷后，王大爷和张大爷一起开开心心坐到1楼去菜市场买菜去了。王大爷和张大爷一边说着物业费没白交，一边夸着写电梯调度的小伙子技术高。

王大爷和张大爷开心了，可把住在30楼的钱大爷气坏了。原来在三位大爷按完按钮之后（电梯刚接上1楼的李大爷）就按了按钮，可是钱大爷看着电梯上行到15楼就改下行了......电梯到达15楼时，所有请求（包含服务请求和目的地到达请求）有这些：张大爷请求到1楼，8楼的王大爷请求上电梯，再就是30楼的钱大爷请求上电梯了。钱大爷距离电梯还差着15层楼呢，按照最短寻道时间优先算法电梯肯定要先去8楼接王大爷。接完王大爷电梯肯定离着目的地1楼最近，也不会上去接钱大爷。

按照这样想下去，如果此时3楼的赵大妈想下楼买菜，钱大爷还得眼睁睁看着电梯从1楼上行到3楼再改下行，估计要是真这样钱大爷连搬家的想法都有了......

最短寻道时间优先算法的弊端在上面这个例子中暴露无遗，那就是距离电梯较远的乘客，可能永远不会得到服务（如果电梯附近的楼层一直有服务请求）。

扫描算法

扫描算法的简称是SCAN，SCAN算法是电梯调度中使用最广泛的一种算法。SCAN算法与当前电梯移动的方向有关（上行/下行），当前移动方向上距离电梯最短的请求将最先得到服务。电梯调度与操作系统磁盘调度不同的是，磁盘调度仅仅是为了读写磁盘，并没有目的地这一说，而电梯调度是有目的地的。乘客进入电梯后按的楼层，就是目的地到达请求的楼层。