1、前言

  最近项目中用到一个环形缓冲区(ring buffer),代码是由linux内核的kfifo改过来的。缓冲区在文件系统中经常用到,通过缓冲区缓解cpu读写内存和读写磁盘的速度。例如一个进程A产生数据发给另外一个进程B,进程B需要对进程A传的数据进行处理并写入文件,如果B没有处理完,则A要延迟发送。为了保证进程A减少等待时间,可以在A和B之间采用一个缓冲区,A每次将数据存放在缓冲区中,B每次冲缓冲区中取。这是典型的生产者和消费者模型,缓冲区中数据满足FIFO特性,因此可以采用队列进行实现。Linux内核的kfifo正好是一个环形队列,可以用来当作环形缓冲区。生产者与消费者使用缓冲区如下图所示:

  环形缓冲区的详细介绍及实现方法可以参考http://en.wikipedia.org/wiki/Circular_buffer,介绍的非常详细,列举了实现环形队列的几种方法。环形队列的不便之处在于如何判断队列是空还是满。维基百科上给三种实现方法。

2、linux 内核kfifo

  kfifo设计的非常巧妙,代码很精简,对于入队和出对处理的出人意料。首先看一下kfifo的数据结构:

struct kfifo {
unsigned char *buffer; /* the buffer holding the data */
unsigned int size; /* the size of the allocated buffer */
unsigned int in; /* data is added at offset (in % size) */
unsigned int out; /* data is extracted from off. (out % size) */
spinlock_t *lock; /* protects concurrent modifications */
};

kfifo提供的方法有:

 //根据给定buffer创建一个kfifo
struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,
gfp_t gfp_mask, spinlock_t *lock);
//给定size分配buffer和kfifo
struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask,
spinlock_t *lock);
//释放kfifo空间
void kfifo_free(struct kfifo *fifo)
//向kfifo中添加数据
unsigned int kfifo_put(struct kfifo *fifo,
const unsigned char *buffer, unsigned int len)
//从kfifo中取数据
unsigned int kfifo_put(struct kfifo *fifo,
const unsigned char *buffer, unsigned int len)
//获取kfifo中有数据的buffer大小
unsigned int kfifo_len(struct kfifo *fifo)

定义自旋锁的目的为了防止多进程/线程并发使用kfifo。因为in和out在每次get和out时,发生改变。初始化和创建kfifo的源代码如下:

 struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,
gfp_t gfp_mask, spinlock_t *lock)
{
struct kfifo *fifo;
/* size must be a power of 2 */
BUG_ON(!is_power_of_2(size));
fifo = kmalloc(sizeof(struct kfifo), gfp_mask);
if (!fifo)
return ERR_PTR(-ENOMEM);
fifo->buffer = buffer;
fifo->size = size;
fifo->in = fifo->out = ;
fifo->lock = lock; return fifo;
}
struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock)
{
unsigned char *buffer;
struct kfifo *ret;
if (!is_power_of_2(size)) {
BUG_ON(size > 0x80000000);
size = roundup_pow_of_two(size);
}
buffer = kmalloc(size, gfp_mask);
if (!buffer)
return ERR_PTR(-ENOMEM);
ret = kfifo_init(buffer, size, gfp_mask, lock); if (IS_ERR(ret))
kfree(buffer);
return ret;
}

  在kfifo_init和kfifo_calloc中,kfifo->size的值总是在调用者传进来的size参数的基础上向2的幂扩展,这是内核一贯的做法。这样的好处不言而喻--对kfifo->size取模运算可以转化为与运算,如:kfifo->in % kfifo->size 可以转化为 kfifo->in & (kfifo->size – 1)

kfifo的巧妙之处在于in和out定义为无符号类型,在put和get时,in和out都是增加,当达到最大值时,产生溢出,使得从0开始,进行循环使用。put和get代码如下所示:

 static inline unsigned int kfifo_put(struct kfifo *fifo,
const unsigned char *buffer, unsigned int len)
{
unsigned long flags;
unsigned int ret;
spin_lock_irqsave(fifo->lock, flags);
ret = __kfifo_put(fifo, buffer, len);
spin_unlock_irqrestore(fifo->lock, flags);
return ret;
} static inline unsigned int kfifo_get(struct kfifo *fifo,
unsigned char *buffer, unsigned int len)
{
unsigned long flags;
unsigned int ret;
spin_lock_irqsave(fifo->lock, flags);
ret = __kfifo_get(fifo, buffer, len);
//当fifo->in == fifo->out时,buufer为空
if (fifo->in == fifo->out)
fifo->in = fifo->out = ;
spin_unlock_irqrestore(fifo->lock, flags);
return ret;
} unsigned int __kfifo_put(struct kfifo *fifo,
const unsigned char *buffer, unsigned int len)
{
unsigned int l;
//buffer中空的长度
len = min(len, fifo->size - fifo->in + fifo->out);
/*
* Ensure that we sample the fifo->out index -before- we
* start putting bytes into the kfifo.
*/
smp_mb();
/* first put the data starting from fifo->in to buffer end */
l = min(len, fifo->size - (fifo->in & (fifo->size - )));
memcpy(fifo->buffer + (fifo->in & (fifo->size - )), buffer, l);
/* then put the rest (if any) at the beginning of the buffer */
memcpy(fifo->buffer, buffer + l, len - l); /*
* Ensure that we add the bytes to the kfifo -before-
* we update the fifo->in index.
*/
smp_wmb();
fifo->in += len; //每次累加,到达最大值后溢出,自动转为0
return len;
} unsigned int __kfifo_get(struct kfifo *fifo,
unsigned char *buffer, unsigned int len)
{
unsigned int l;
//有数据的缓冲区的长度
len = min(len, fifo->in - fifo->out);
/*
* Ensure that we sample the fifo->in index -before- we
* start removing bytes from the kfifo.
*/
smp_rmb();
/* first get the data from fifo->out until the end of the buffer */
l = min(len, fifo->size - (fifo->out & (fifo->size - )));
memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - )), l);
/* then get the rest (if any) from the beginning of the buffer */
memcpy(buffer + l, fifo->buffer, len - l);
/*
* Ensure that we remove the bytes from the kfifo -before-
* we update the fifo->out index.
*/
smp_mb();
fifo->out += len; //每次累加,到达最大值后溢出,自动转为0
return len;
}

  put和get在调用__put和__get过程都进行加锁,防止并发。从代码中可以看出put和get都调用两次memcpy,这针对的是边界条件。例如下图:蓝色表示空闲,红色表示占用。

(1)空的kfifo,

(2)put一个buffer后

(3)get一个buffer后

(4)当此时put的buffer长度超出in到末尾长度时,则将剩下的移到头部去

3、测试程序

仿照kfifo编写一个ring_buffer,现有线程互斥量进行并发控制。设计的ring_buffer如下所示:

 /**@brief 仿照linux kfifo写的ring buffer
*@atuher Anker date:2013-12-18
* ring_buffer.h
* */ #ifndef KFIFO_HEADER_H
#define KFIFO_HEADER_H #include <inttypes.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <assert.h> //判断x是否是2的次方
#define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))
//取a和b中最小值
#define min(a, b) (((a) < (b)) ? (a) : (b)) struct ring_buffer
{
void *buffer; //缓冲区
uint32_t size; //大小
uint32_t in; //入口位置
uint32_t out; //出口位置
pthread_mutex_t *f_lock; //互斥锁
};
//初始化缓冲区
struct ring_buffer* ring_buffer_init(void *buffer, uint32_t size, pthread_mutex_t *f_lock)
{
assert(buffer);
struct ring_buffer *ring_buf = NULL;
if (!is_power_of_2(size))
{
fprintf(stderr,"size must be power of 2.\n");
return ring_buf;
}
ring_buf = (struct ring_buffer *)malloc(sizeof(struct ring_buffer));
if (!ring_buf)
{
fprintf(stderr,"Failed to malloc memory,errno:%u,reason:%s",
errno, strerror(errno));
return ring_buf;
}
memset(ring_buf, , sizeof(struct ring_buffer));
ring_buf->buffer = buffer;
ring_buf->size = size;
ring_buf->in = ;
ring_buf->out = ;
ring_buf->f_lock = f_lock;
return ring_buf;
}
//释放缓冲区
void ring_buffer_free(struct ring_buffer *ring_buf)
{
if (ring_buf)
{
if (ring_buf->buffer)
{
free(ring_buf->buffer);
ring_buf->buffer = NULL;
}
free(ring_buf);
ring_buf = NULL;
}
} //缓冲区的长度
uint32_t __ring_buffer_len(const struct ring_buffer *ring_buf)
{
return (ring_buf->in - ring_buf->out);
} //从缓冲区中取数据
uint32_t __ring_buffer_get(struct ring_buffer *ring_buf, void * buffer, uint32_t size)
{
assert(ring_buf || buffer);
uint32_t len = ;
size = min(size, ring_buf->in - ring_buf->out);
/* first get the data from fifo->out until the end of the buffer */
len = min(size, ring_buf->size - (ring_buf->out & (ring_buf->size - )));
memcpy(buffer, ring_buf->buffer + (ring_buf->out & (ring_buf->size - )), len);
/* then get the rest (if any) from the beginning of the buffer */
memcpy(buffer + len, ring_buf->buffer, size - len);
ring_buf->out += size;
return size;
}
//向缓冲区中存放数据
uint32_t __ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
{
assert(ring_buf || buffer);
uint32_t len = ;
size = min(size, ring_buf->size - ring_buf->in + ring_buf->out);
/* first put the data starting from fifo->in to buffer end */
len = min(size, ring_buf->size - (ring_buf->in & (ring_buf->size - )));
memcpy(ring_buf->buffer + (ring_buf->in & (ring_buf->size - )), buffer, len);
/* then put the rest (if any) at the beginning of the buffer */
memcpy(ring_buf->buffer, buffer + len, size - len);
ring_buf->in += size;
return size;
} uint32_t ring_buffer_len(const struct ring_buffer *ring_buf)
{
uint32_t len = ;
pthread_mutex_lock(ring_buf->f_lock);
len = __ring_buffer_len(ring_buf);
pthread_mutex_unlock(ring_buf->f_lock);
return len;
} uint32_t ring_buffer_get(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
{
uint32_t ret;
pthread_mutex_lock(ring_buf->f_lock);
ret = __ring_buffer_get(ring_buf, buffer, size);
//buffer中没有数据
if (ring_buf->in == ring_buf->out)
ring_buf->in = ring_buf->out = ;
pthread_mutex_unlock(ring_buf->f_lock);
return ret;
} uint32_t ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
{
uint32_t ret;
pthread_mutex_lock(ring_buf->f_lock);
ret = __ring_buffer_put(ring_buf, buffer, size);
pthread_mutex_unlock(ring_buf->f_lock);
return ret;
}
#endif

采用多线程模拟生产者和消费者编写测试程序,如下所示:

 /**@brief ring buffer测试程序,创建两个线程,一个生产者,一个消费者。
* 生产者每隔1秒向buffer中投入数据,消费者每隔2秒去取数据。
*@atuher Anker date:2013-12-18
* */
#include "ring_buffer.h"
#include <pthread.h>
#include <time.h> #define BUFFER_SIZE 1024 * 1024 typedef struct student_info
{
uint64_t stu_id;
uint32_t age;
uint32_t score;
}student_info; void print_student_info(const student_info *stu_info)
{
assert(stu_info);
printf("id:%lu\t",stu_info->stu_id);
printf("age:%u\t",stu_info->age);
printf("score:%u\n",stu_info->score);
} student_info * get_student_info(time_t timer)
{
student_info *stu_info = (student_info *)malloc(sizeof(student_info));
if (!stu_info)
{
fprintf(stderr, "Failed to malloc memory.\n");
return NULL;
}
srand(timer);
stu_info->stu_id = + rand() % ;
stu_info->age = rand() % ;
stu_info->score = rand() % ;
print_student_info(stu_info);
return stu_info;
} void * consumer_proc(void *arg)
{
struct ring_buffer *ring_buf = (struct ring_buffer *)arg;
student_info stu_info;
while()
{
sleep();
printf("------------------------------------------\n");
printf("get a student info from ring buffer.\n");
ring_buffer_get(ring_buf, (void *)&stu_info, sizeof(student_info));
printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));
print_student_info(&stu_info);
printf("------------------------------------------\n");
}
return (void *)ring_buf;
} void * producer_proc(void *arg)
{
time_t cur_time;
struct ring_buffer *ring_buf = (struct ring_buffer *)arg;
while()
{
time(&cur_time);
srand(cur_time);
int seed = rand() % ;
printf("******************************************\n");
student_info *stu_info = get_student_info(cur_time + seed);
printf("put a student info to ring buffer.\n");
ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));
printf("******************************************\n");
sleep();
}
return (void *)ring_buf;
} int consumer_thread(void *arg)
{
int err;
pthread_t tid;
err = pthread_create(&tid, NULL, consumer_proc, arg);
if (err != )
{
fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",
errno, strerror(errno));
return -;
}
return tid;
}
int producer_thread(void *arg)
{
int err;
pthread_t tid;
err = pthread_create(&tid, NULL, producer_proc, arg);
if (err != )
{
fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",
errno, strerror(errno));
return -;
}
return tid;
} int main()
{
void * buffer = NULL;
uint32_t size = ;
struct ring_buffer *ring_buf = NULL;
pthread_t consume_pid, produce_pid; pthread_mutex_t *f_lock = (pthread_mutex_t *)malloc(sizeof(pthread_mutex_t));
if (pthread_mutex_init(f_lock, NULL) != )
{
fprintf(stderr, "Failed init mutex,errno:%u,reason:%s\n",
errno, strerror(errno));
return -;
}
buffer = (void *)malloc(BUFFER_SIZE);
if (!buffer)
{
fprintf(stderr, "Failed to malloc memory.\n");
return -;
}
size = BUFFER_SIZE;
ring_buf = ring_buffer_init(buffer, size, f_lock);
if (!ring_buf)
{
fprintf(stderr, "Failed to init ring buffer.\n");
return -;
}
#if 0
student_info *stu_info = get_student_info();
ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
stu_info = get_student_info();
ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
ring_buffer_get(ring_buf, (void *)stu_info, sizeof(student_info));
print_student_info(stu_info);
#endif
printf("multi thread test.......\n");
produce_pid = producer_thread((void*)ring_buf);
consume_pid = consumer_thread((void*)ring_buf);
pthread_join(produce_pid, NULL);
pthread_join(consume_pid, NULL);
ring_buffer_free(ring_buf);
free(f_lock);
return ;
}

测试结果如下所示:

4、参考资料

http://blog.csdn.net/linyt/article/details/5764312

http://en.wikipedia.org/wiki/Circular_buffer

http://yiphon.diandian.com/post/2011-09-10/4918347

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