linux0.11源代码电梯算法解析

来源：互联网发布：顶点软件怎么样编辑：程序博客网时间：2024/05/21 10:56

在看linux0.11代码里面的电梯算法的时候，产生了一些疑惑，经过分析解决了，发现网上也有不少讨论，我自己的分析记录下来。

00017 int file_read(struct m_inode * inode, struct file * filp, char * buf, int count)00018 {00019     int left,chars,nr;00020     struct buffer_head * bh;00021 00022     if ((left=count)<=0)00023         return 0;00024     while (left) {00025         if ((nr = bmap(inode,(filp->f_pos)/BLOCK_SIZE))) {00026             if (!(bh=bread(inode->i_dev,nr)))00027                 break;00028         } else00029             bh = NULL;00030         nr = filp->f_pos % BLOCK_SIZE;00031         chars = MIN( BLOCK_SIZE-nr , left );00032         filp->f_pos += chars;00033         left -= chars;00034         if (bh) {00035             char * p = nr + bh->b_data;00036             while (chars-->0)00037                 put_fs_byte(*(p++),buf++);00038             brelse(bh);00039         } else {00040             while (chars-->0)00041                 put_fs_byte(0,buf++);00042         }00043     }00044     inode->i_atime = CURRENT_TIME;00045     return (count-left)?(count-left):-ERROR;00046 }

00270 struct buffer_head * bread(int dev,int block)00271 {00272     struct buffer_head * bh;00273 00274     if (!(bh=getblk(dev,block)))00275         panic("bread: getblk returned NULL\n");00276     if (bh->b_uptodate)00277         return bh;00278     ll_rw_block(READ,bh);00279     wait_on_buffer(bh);00280     if (bh->b_uptodate)00281         return bh;00282     brelse(bh);00283     return NULL;00284 }

00145 void ll_rw_block(int rw, struct buffer_head * bh)00146 {00147     unsigned int major;00148 00149     if ((major=MAJOR(bh->b_dev)) >= NR_BLK_DEV ||00150     !(blk_dev[major].request_fn)) {00151         printk("Trying to read nonexistent block-device\n\r");00152         return;00153     }00154     make_request(major,rw,bh);00155 }

00088 static void make_request(int major,int rw, struct buffer_head * bh)00089 {00090     struct request * req;00091     int rw_ahead;00092 00093 /* WRITEA/READA is special case - it is not really needed, so if the */00094 /* buffer is locked, we just forget about it, else it's a normal read */00095     if ((rw_ahead = (rw == READA || rw == WRITEA))) {00096         if (bh->b_lock)00097             return;00098         if (rw == READA)00099             rw = READ;00100         else00101             rw = WRITE;00102     }00103     if (rw!=READ && rw!=WRITE)00104         panic("Bad block dev command, must be R/W/RA/WA");00105     lock_buffer(bh);00106     if ((rw == WRITE && !bh->b_dirt) || (rw == READ && bh->b_uptodate)) {00107         unlock_buffer(bh);00108         return;00109     }00110 repeat:00111 /* we don't allow the write-requests to fill up the queue completely:00112  * we want some room for reads: they take precedence. The last third00113  * of the requests are only for reads.00114  */00115     if (rw == READ)00116         req = request+NR_REQUEST;00117     else00118         req = request+((NR_REQUEST*2)/3);00119 /* find an empty request */00120     while (--req >= request)00121         if (req->dev<0)00122             break;00123 /* if none found, sleep on new requests: check for rw_ahead */00124     if (req < request) {00125         if (rw_ahead) {00126             unlock_buffer(bh);00127             return;00128         }00129         sleep_on(&wait_for_request);00130         goto repeat;00131     }00132 /* fill up the request-info, and add it to the queue */00133     req->dev = bh->b_dev;00134     req->cmd = rw;00135     req->errors=0;00136     req->sector = bh->b_blocknr<<1;00137     req->nr_sectors = 2;00138     req->buffer = bh->b_data;00139     req->waiting = NULL;00140     req->bh = bh;00141     req->next = NULL;00142     add_request(major+blk_dev,req);00143 }

00040 #define IN_ORDER(s1,s2) \00041 ((s1)->cmd<(s2)->cmd || ((s1)->cmd==(s2)->cmd && \00042 ((s1)->dev < (s2)->dev || ((s1)->dev == (s2)->dev && \00043 (s1)->sector < (s2)->sector))))

00064 static void add_request(struct blk_dev_struct * dev, struct request * req)00065 {00066     struct request * tmp;00067 00068     req->next = NULL;00069     cli();00070     if (req->bh)00071         req->bh->b_dirt = 0;00072     if (!(tmp = dev->current_request)) {00073         dev->current_request = req;00074         sti();00075         (dev->request_fn)();00076         return;00077     }00078     for ( ; tmp->next ; tmp=tmp->next)00079         if ((IN_ORDER(tmp,req) || 00080             !IN_ORDER(tmp,tmp->next)) &&00081             IN_ORDER(req,tmp->next))00082             break;00083     req->next=tmp->next;00084     tmp->next=req;00085     sti();00086 }

以上代码列出了从file_read函数到add_request的过程，其中add_request使用了电梯算法来添加请求到等待队列。这里关键是分析电梯算法按一个什么样的逻辑插入到等待队列当中。

#define IN_ORDER(s1,s2) \((s1)->cmd<(s2)->cmd || (s1)->cmd==(s2)->cmd && \((s1)->dev < (s2)->dev || ((s1)->dev == (s2)->dev && \(s1)->sector < (s2)->sector)))

先来了解一下request项的关键结构：

struct request {int dev;/* -1 if no request */int cmd;/* READ or WRITE */int errors;unsigned long sector;unsigned long nr_sectors;char * buffer;struct task_struct * waiting;struct buffer_head * bh;struct request * next;};

dev是设备号，根据linux0.11代码里面涉及的每个存储设备编了号；cmd是指这个请求的命令，也就是读或写，其中定义READ=0,WRITE=1；sector是扇区号；buffer是读或写的缓冲区；waiting这个task_struct结构是当前进程等待队列指针；next是指向下一请求的指针。另外的errors，nr_sectors，bh我们目前不关注。

用结构更清晰的代码开始分析IN_ORDER的原理：

bool inorder(request &s1,request &s2){if (s1.cmd<s2.cmd){return true;//only when s1.cmd = READ; s2.cmd = WRITE;}else if ( s1.cmd == s2.cmd ){if (s1.dev < s2.dev){return true;// when (s1.cmd==s2.cmd) && (s1.dev<s2.dev)}else if ( s1.dev == s2.dev ){if (s1.sector<s2.sector){return true;// when when (s1.cmd==s2.cmd) && (s1.sector<s2.sector)}return false;// when when (s1.cmd==s2.cmd) && (s1.sector>=s2.sector)}return false;// when (s1.cmd==s2.cmd) && (s1.dev>s2.dev)}return false;// when s1.cmd>s2.cmd}

从上面的每个函数返回地方的注释看出返回时的比较状态。用自然语言描述一下：IN_ORDER返回s1与s2的优先级比较结果，s1比s2高的话，返回true；s1不高于s2的话，返回false；

IN_ORDER的内部比较思想是：先比较操作类型，读操作优先于写操作；如果操作类型相同，则比较设备号，设备号小的设备优先于设备号大的；如果设备号也相同，则比较扇区号，先处理扇区号小的扇区，意思就是从磁头从里向外读写了；如果扇区号也相同，那么返回false，也就是前面的s1优先级低于后面的s2。

然后我们就可以来分析插入的代码了：

if (!(tmp = dev->current_request)) {dev->current_request = req;sti();(dev->request_fn)();return;}

上面代码的意思是：如果设备的当前处理请求项为空，那么把需要处理的请求项设置为当前处理项，当立即调用处理请求函数dev->request_fn()。

for ( ; tmp->next ; tmp=tmp->next)if ((IN_ORDER(tmp,req) ||    !IN_ORDER(tmp,tmp->next)) &&    IN_ORDER(req,tmp->next))break;req->next=tmp->next;tmp->next=req;

上面的代码是在设备的当前请求项tmp不为空（也就是设备在忙）的情况下使用。我们可以dev->current_request理解为等待队列的一个头节点；这个头指点指向一个等待队列；我们需要把请求按一定的方法(也就是IN_ORDER算法)插入到等待队列中。

首先，req是待插入节点，我们以tmp为哨兵节点，IN_ORDER(tmp,req)，是比较哨兵节点是否比待插入节点优先级高。

我们作一个约定，a比b优先级高的话使用a>b的记号，否则是a<=b。注意=的临界状态，不少人因为临界状态理解不好影响整个插入算法的理解。
把上面的代码做一个伪代码来理解：

for( ; tmp->next; tmp=tmp->next)

{

if ( tmp>req && req>tmp->next) break;//也就是哨兵节点比待插入节点优先级要高，并且待插入节点的优先级要比哨兵后的一个节点要高，那么应该插入到tmp与tmp->next之间。

else if ( tmp <= tmp->next && req>tmp->next ) break;//也就是哨兵节点不比它后面的节点优先级高——我们知道这是一个已经排序的队列，tmp优先级肯定大于或等于tmp->next，这里出现tmp <= tmp->nex成立的唯一情况就是tmp=tmp->next，也就是两个节点的优先级一样高。那么这个时候req>tmp->next也一定不会成立，就会继续下一个循环；等等！我们漏掉了一个情况——当tmp是current_request的时候，有可能出现tmp<=tmp->next的情况，此时如果req>tmp->next也成立，那么就会插入current_request后面，也就是tmp的后面！这种情况必须要理解！

else continue;//如果不适合插入tmp后面，继续下一个循环；

}

所以上面的总体意思就是：优先级从大到小排列，同一优先级的话，插入这一系统相同优先级的节点的最后面。可以说的细一点，先处理读请求，再处理写请求；同一读或写请求先处理设备号小的设备请求，再处理设备号大的设备请求，同一读或写请求，同一设备，按先里面的扇区再到外面的扇区的顺序处理。

为了更有说服力，以下我按电梯算法编写了代码，可以修改注掉的inorder部分再做测试。

#include <stdio.h>#include <stdlib.h>#define READ 0#define WRITE 1struct request {int dev;/* -1 if no request */int cmd;/* READ or WRITE */int errors;unsigned long sector;unsigned long nr_sectors;char * buffer;//struct task_struct * waiting;//struct buffer_head * bh;struct request * next;};#define IN_ORDER(s1,s2) \((s1)->cmd<(s2)->cmd || (s1)->cmd==(s2)->cmd && \((s1)->dev < (s2)->dev || ((s1)->dev == (s2)->dev && \(s1)->sector < (s2)->sector)))// 作为解析，以明白的分支结构重写一个内容一样的inorder函数bool inorder(struct request *s1,struct request *s2){if (s1->cmd<s2->cmd){return true;//only when s1->cmd = READ; s2->cmd = WRITE;}else if ( s1->cmd == s2->cmd ){if (s1->dev < s2->dev){return true;// when (s1->cmd==s2->cmd) && (s1->dev<s2->dev)}else if ( s1->dev == s2->dev ){if (s1->sector<s2->sector){return true;// when when (s1->cmd==s2->cmd) && (s1->sector<s2->sector)}return false;// when when (s1->cmd==s2->cmd) && (s1->sector>=s1->sector)}return false;// when (s1->cmd==s2->cmd) && (s1->dev>s2->dev)}return false;// when s1->cmd>s2->cmd}void AddRequest(struct request * &head,struct request *req){if (!head){head = req;head->next = 0;return ;}struct request * tmp = head;for (;tmp->next;tmp = tmp->next){// 使用inorder和宏IN_ORDER是一样的结果//if ( ( inorder(tmp,req)||//!inorder(tmp,tmp->next)) &&//inorder(req,tmp->next))if ( ( IN_ORDER(tmp,req)||!IN_ORDER(tmp,tmp->next)) &&IN_ORDER(req,tmp->next)){break;}}req->next = tmp->next;tmp->next = req;return;}void PrintQueen(struct request * n){while (n){printf("(%d,%d,%d),",n->cmd,n->dev,n->sector);n = n->next;}printf("\n");}int main(int argc,char ** argv){struct request s1;struct request * pHead = 0;for (int i = 0;i<100;i++){struct request *req = new struct request;req->cmd = rand()%2;req->dev =  rand()%10;req->sector =  rand()%100;AddRequest(pHead,req);PrintQueen(pHead);}return 0;}

大家看结果，红色的是新插入的节点，和我上述的结果是一致的。

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