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| #define _GNU_SOURCE
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/stat.h>
#include <time.h>
/**
* 智能预读器
*/
typedef struct {
int fd;
off_t file_size;
off_t last_read_pos;
size_t read_pattern[10]; // 记录最近10次读取的大小
int pattern_index;
int pattern_count;
} smart_readaheater_t;
/**
* 初始化智能预读器
*/
int smart_readaheater_init(smart_readaheater_t *sr, const char *filename) {
sr->fd = open(filename, O_RDONLY);
if (sr->fd == -1) {
perror("打开文件失败");
return -1;
}
struct stat sb;
if (fstat(sr->fd, &sb) == -1) {
perror("获取文件状态失败");
close(sr->fd);
return -1;
}
sr->file_size = sb.st_size;
sr->last_read_pos = 0;
sr->pattern_index = 0;
sr->pattern_count = 0;
memset(sr->read_pattern, 0, sizeof(sr->read_pattern));
printf("智能预读器初始化完成\n");
printf("文件大小: %.2f MB\n", sr->file_size / (1024.0 * 1024.0));
return 0;
}
/**
* 分析读取模式
*/
size_t analyze_read_pattern(smart_readaheater_t *sr) {
if (sr->pattern_count < 3) {
return 1024 * 1024; // 默认1MB
}
// 计算平均读取大小
size_t total = 0;
int count = (sr->pattern_count < 10) ? sr->pattern_count : 10;
for (int i = 0; i < count; i++) {
total += sr->read_pattern[i];
}
return total / count;
}
/**
* 智能预读
*/
int smart_readahead(smart_readaheater_t *sr, off_t pos, size_t size) {
// 记录本次读取模式
sr->read_pattern[sr->pattern_index] = size;
sr->pattern_index = (sr->pattern_index + 1) % 10;
if (sr->pattern_count < 10) {
sr->pattern_count++;
}
// 分析读取模式
size_t predicted_size = analyze_read_pattern(sr);
// 预测下一个读取位置
off_t next_pos = pos + size;
// 如果下一个位置有效，则进行预读
if (next_pos < sr->file_size) {
size_t readahead_size = predicted_size * 2; // 预读两倍大小
if (next_pos + readahead_size > sr->file_size) {
readahead_size = sr->file_size - next_pos;
}
if (readahead(sr->fd, next_pos, readahead_size) == 0) {
printf("智能预读: 位置 %ld, 大小 %zu\n", next_pos, readahead_size);
return 0;
}
}
return -1;
}
/**
* 读取数据并触发智能预读
*/
ssize_t smart_read(smart_readaheater_t *sr, void *buf, size_t count, off_t offset) {
ssize_t bytes_read = pread(sr->fd, buf, count, offset);
if (bytes_read > 0) {
smart_readahead(sr, offset, bytes_read);
sr->last_read_pos = offset + bytes_read;
}
return bytes_read;
}
/**
* 演示智能预读策略
*/
int demo_smart_readahead() {
const char *filename = "smart_test.dat";
const size_t file_size = 50 * 1024 * 1024; // 50MB
smart_readaheater_t sr;
printf("=== 智能预读策略示例 ===\n");
// 创建测试文件
int fd = open(filename, O_CREAT | O_WRONLY | O_TRUNC, 0644);
if (fd == -1) {
perror("创建测试文件失败");
return -1;
}
char *buffer = malloc(1024 * 1024);
if (!buffer) {
perror("分配缓冲区失败");
close(fd);
return -1;
}
// 填充测试数据
for (size_t i = 0; i < 1024 * 1024; i++) {
buffer[i] = 'A' + (i % 26);
}
// 写入文件
size_t written = 0;
while (written < file_size) {
size_t to_write = (file_size - written < 1024 * 1024) ?
file_size - written : 1024 * 1024;
write(fd, buffer, to_write);
written += to_write;
}
free(buffer);
close(fd);
printf("创建了 %.2f MB 的测试文件\n", file_size / (1024.0 * 1024.0));
// 初始化智能预读器
if (smart_readaheater_init(&sr, filename) != 0) {
unlink(filename);
return -1;
}
// 模拟不同模式的读取
printf("\n开始智能预读测试:\n");
char *read_buffer = malloc(2 * 1024 * 1024); // 2MB缓冲区
if (!read_buffer) {
perror("分配读取缓冲区失败");
close(sr.fd);
unlink(filename);
return -1;
}
// 模拟顺序读取
printf("1. 顺序读取模式:\n");
for (off_t pos = 0; pos < 20 * 1024 * 1024; pos += 512 * 1024) {
ssize_t bytes_read = smart_read(&sr, read_buffer, 512 * 1024, pos);
if (bytes_read > 0) {
printf(" 读取位置 %ld, 大小 %zd\n", pos, bytes_read);
}
}
// 模拟随机读取
printf("\n2. 随机读取模式:\n");
srand(time(NULL));
for (int i = 0; i < 5; i++) {
off_t pos = (rand() % (int)(file_size - 1024 * 1024));
size_t size = 256 * 1024 + (rand() % (768 * 1024));
ssize_t bytes_read = smart_read(&sr, read_buffer, size, pos);
if (bytes_read > 0) {
printf(" 随机读取位置 %ld, 大小 %zd\n", pos, bytes_read);
}
}
free(read_buffer);
close(sr.fd);
unlink(filename);
printf("\n智能预读策略特点:\n");
printf(" - 学习读取模式\n");
printf(" - 动态调整预读大小\n");
printf(" - 适应不同的访问模式\n");
return 0;
}
int main() {
return demo_smart_readahead();
}
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