《Unix/Linux系统编程》第十周学习笔记

《Unix/Linux系统编程》第十周学习笔记

块设备I/O和缓冲区管理

解释块设备I/O的原理和I/O缓冲的优点

I/O缓冲区:内核中的一系列NBUF缓冲区用作缓冲区缓存。每个缓冲区用一个结构体表示

typdef struct buf{
    struct buf *next_free;	//freelist pointer
    struct buf *next_dev；	//dev_list pointer
    int dev,blk；	        //assigned disk block；
    int opcode；	        //READ|WRITE
    int dirty;	                //buffer data modified
    int async;	                //ASYNC write flag
    int valid;	                //buffer data valid
    int busy;	                //buffer is in use
    int wanted；		//some process needs this buffer
    struct, semaphore lock=l ；	//buffer locking semaphore； value=L
    struct semaphore iodone=0；	//for process to wait for I/O completion；
    char buf[BLKSIZE];	        //block data area
} BUFFER;

BUFFER buf[NBUF], *freelist; // NBUF buffers and free buffer list

介绍Unix的缓冲区管理算法

I/O缓冲区：内核中的一系列NBUF 缓冲区用作缓冲区缓存。每个缓冲区用一个结构体表示。

typdef struct buf[
struct buf*next__free;// freelist pointer
struct buf *next__dev;// dev_list pointer int dev.,blk;
// assigmed disk block;int opcode;
// READ|wRITE int dirty;
// buffer data modified
int async;
// ASYNC write flag int valid;
//buffer data valid int buay;
// buffer is in use int wanted;
// some process needs this buffer struct semaphore lock=1; /
// buffer locking semaphore; value=1
struct semaphore iodone=0;// for process to wait for I/0 completion;// block data area char buf[BLKSIZE];)
} BUFFER;
BUFFER buf[NBUF],*freelist;// NBUF buffers and free buffer list

利用信号量设计新的缓冲区管理算法，以提高I/O缓冲区的缓存效率和性能

信号量的主要优点是：
  (1)计数信号量可用来表示可用资源的数量，例如：空闲缓冲区的数量。
  (2)当多个进程等待一个资源时，信号量上的V操作只会释放一个等待进程，该进程不必重试，因为它保证拥有资源。
使用信号量的缓冲区管理算法
  1.保证数据一致性；
  2.良好的缓存效果；
  3.高效率：没有重试循环，没有不必要的进程“唤醒”
  4.无死锁和饥饿。

介绍简单的PV算法及其特点

PV算法

BUFFER *getb1k(dev,blk)：
while(1){
(1). P(free);
//get a free buffer first
if (bp in dev_1ist){
(2). if (bp not BUSY){
remove bp from freelist;P(bp);
// lock bp but does not wait
(3).return bp;
// bp in cache but BUSY V(free);
// give up the free buffer
(4).P(bp);
// wait in bp queue
return bp;v
// bp not in cache,try to create a bp=(dev,blk)
(5).bp = frist buffer taken out of freelist;P(bp);
// lock bp,no wait
(6).if(bp dirty){
awzite(bp);
// write bp out ASYNC,no wait
continue;
// continue from (1)
(7).reassign bp to(dev,blk);1/ mark bp data invalid,not dir return bp;-
// end of while(1)；
brelse(BUFFER *bp)，
{
(8).iF (bp queue has waiter)( V(bp); return; ]
(9).if(bp dirty && free queue has waiter){ awrite(bp);zeturn;}(10).enter bp into(tail of) freelist;V(bp);V(free);
}

实践内容

生产者消费者进程冲突问题

#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#define N 100
#define true 1
#define producerNum  10
#define consumerNum  5
#define sleepTime 1000

typedef int semaphore;
typedef int item;
item buffer[N] = {0};
int in = 0;
int out = 0;
int proCount = 0;
semaphore mutex = 1, empty = N, full = 0, proCmutex = 1;

void * producer(void * a){
    while(true){
        while(proCmutex <= 0);
        proCmutex--;
        proCount++;
        printf("produce a product: ID %d, buffer location:%d\n",proCount,in);
        proCmutex++;

        while(empty <= 0){
            printf("buffer is full\n");
        }
        empty--;

        while(mutex <= 0);
        mutex--;

        buffer[in] = proCount;
        in = (in + 1) % N;

        mutex++;
        full++;
        sleep(sleepTime);
    }
}

void * consumer(void *b){
    while(true){
        while(full <= 0){
            printf("buffer is empty\n");
        }
        full--;

        while(mutex <= 0);
        mutex--;

        int nextc = buffer[out];
        buffer[out] = 0;//消费完将缓冲区设置为0

        out = (out + 1) % N;

        mutex++;
        empty++;

        printf("produce a product: ID %d, buffer location:%d\n", nextc,out);
        sleep(sleepTime);
    }
}

int main()
{
    pthread_t threadPool[producerNum+consumerNum];
    int i;
    for(i = 0; i < producerNum; i++){
        pthread_t temp;
        if(pthread_create(&temp, NULL, producer, NULL) == -1){
            printf("ERROR, fail to create producer%d\n", i);
            exit(1);
        }
        threadPool[i] = temp;
    }//创建生产者进程放入线程池

    for(i = 0; i < consumerNum; i++){
        pthread_t temp;
        if(pthread_create(&temp, NULL, consumer, NULL) == -1){
            printf("ERROR, fail to create consumer%d\n", i);
            exit(1);
        }
        threadPool[i+producerNum] = temp;
    }//创建消费者进程放入线程池

    void * result;
    for(i = 0; i < producerNum+consumerNum; i++){
        if(pthread_join(threadPool[i], &result) == -1){
            printf("fail to recollect\n");
            exit(1);
        }
    }//运行线程池
    return 0;
}