Linux System Programming 学习笔记(四) 高级I/O
1. Scatter/Gather I/O
a single system call to read or write data between single data stream and multiple buffers
This type of I/O is so named because the data is scattered into or gathered from the given vector of buffers
Scatter/Gather I/O 相比于 C标准I/O有如下优势：
(1)更自然的编码模式
如果数据是自然分隔的(预先定义的结构体字段)，那么 Scatter/Gather I/O 更直观
(2)效率高
A single vectored I/O operation can replace multiple linear I/O operations
(3)性能好
减少了系统调用数
(4)原子性
Scatter/Gather I/O 是原子的，而 multiple linear I/O operations 则有多个进程交替I/O的风险
#include <sys/uio.h> struct iovec { void *iov_base; /* pointer to start of buffer */ size_t iov_len; /* size of buffer in bytes */ };
Both functions always operate on the segments in order, starting with iov[0], then iov[1], and so on, through iov[count-1].
/* The readv() function reads count segments from the file descriptor fd into the buffers described by iov */ ssize_t readv (int fd, const struct iovec *iov, int count);
/* The writev() function writes at most count segments from the buffers described by iov into the file descriptor fd */ ssize_t writev (int fd, const struct iovec *iov, int count);
注意：在Scatter/Gather I/O操作过程中，内核必须分配内部数据结构来表示每个buffer分段，正常情况下，是根据分段数count进行动态内存分配的，
但是当分段数count较小时(一般<=8)，内核直接在内核栈上分配，这显然比在堆中动态分配要快
#include <stdio.h> #include <unistd.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <string.h> #include <sys/uio.h> int main(int argc, char* argv[]) { struct iovec iov[3]; char* buf[] = { "The term buccaneer comes from the word boucan.\n", "A boucan is a wooden frame used for cooking meat.\n", "Buccaneer is the West Indies name for a pirate.\n" }; int fd = open("wel.txt", O_WRONLY | O_CREAT | O_TRUNC); if (fd == -1) { fprintf(stderr, "open error\n"); return 1; } /* fill out three iovec structures */ for (int i = 0; i < 3; ++i) { iov[i].iov_base = buf[i]; iov[i].iov_len = strlen(buf[i]) + 1; } /* with a single call, write them out all */ ssize_t nwrite = writev(fd, iov, 3); if (nwrite == -1) { fprintf(stderr, "writev error\n"); return 1; } fprintf(stdout, "wrote %d bytes\n", nwrite); if (close(fd)) { fprintf(stdout, "close error\n"); return 1; } return 0; }
#include <stdio.h> #include <unistd.h> #include <string.h> #include <fcntl.h> #include <sys/uio.h> #include <sys/stat.h> int main(int argc, char* argv[]) { char foo[48], bar[51], baz[49]; struct iovec iov[3]; int fd = open("wel.txt", O_RDONLY); if (fd == -1) { fprintf(stderr, "open error\n"); return 1; } /* set up our iovec structures */ iov[0].iov_base = foo; iov[0].iov_len = sizeof(foo); iov[1].iov_base = bar; iov[1].iov_len = sizeof(bar); iov[2].iov_base = baz; iov[2].iov_len = sizeof(baz); /* read into the structures with a single call */ ssize_t nread = readv(fd, iov, 3); if (nread == -1) { fprintf(stderr, "readv error\n"); return 1; } for (int i = 0; i < 3; ++i) { fprintf(stdout, "%d: %s", i, (char*)iov[i].iov_base); } if (close(fd)) { fprintf(stderr, "close error\n"); return 1; } return 0; }
writev的简单实现：
#include <unistd.h> #include <sys/uio.h> ssize_t my_writev(int fd, const struct iovec* iov, int count) { ssize_t ret = 0; for (int i = 0; i < count; ++i) { ssize_t nr = write(fd, iov[i].iov_base, iov[i].iov_len); if (nr == -1) { if (errno == EINTR) continue; ret -= 1; break; } ret += nr; } return nr; }
In fact, all I/O inside the Linux kernel is vectored; read() and write() are implemented as vectored I/O with a vector of only one segment
2. epoll
poll 和 select 每次调用都需要监测整个文件描述符集，当描述符集增大时，每次调用对描述符集全局扫描就成了性能瓶颈
(1) creating a new epoll instance
/* A successful call to epoll_create1() instantiates a new epoll instance and returns a file descriptor associated with the instance */ #include <sys/epoll.h> int epoll_create(int size);
parameter size used to provide a hint about the number of file descriptors to be watched;
nowadays the kernel dynamically sizes the required data structures and this parameter just needs to be greater than zero
(2) controling epoll
/* The epoll_ctl() system call can be used to add file descriptors to and remove file descriptors from a given epoll context */ #include <sys/epoll.h> int epoll_ctl(int epfd, int op, int fd, struct epoll_event* event); struct epoll_event { __u32 events; /* events */ union { void* ptr; int fd; __u32 u32; __u64 u64; } data; };
a. op parameter
EPOLL_CTL_ADD // Add a monitor on the file associated with the file descriptor fd to the epoll instance associated with epfd EPOLL_CTL_DEL // Remove a monitor on the file associated with the file descriptor fd from the epoll instance associated with epfd EPOLL_CTL_MOD // Modify an existing monitor of fd with the updated events specified by event
b. event parameter
EPOLLET // Enables edge-triggered behavior for the monitor of the file ,The default behavior is level-triggered EPOLLIN // The file is available to be read from without blocking EPOLLOUT // The file is available to be written to without blocking
对于结构体struct epoll_event 里的data成员，通常做法是将data联合体里的fd设置为第二个参数fd，即 event.data.fd = fd
To add a new watch on the file associated with fd to the epoll instance epfd :
#include <sys/epoll.h> struct epoll_event event; event.data.fd = fd; event.events = EPOLLIN | EPOLLOUT int ret = epll_ctl(epfd, EPOLL_CTL_ADD, fd, &event); if (ret) { fprintf(stderr, "epll_ctl error\n"); }
To modify an existing event on the file associated with fd on the epoll instance epfd :
#include <sys/epoll.h> struct epoll_event event; event.data.fd = fd; event.events = EPOLLIN; int ret = epoll_ctl(epfd, EPOLL_CTL_MOD, fd, &event); if (ret) { fprintf(stderr, "epoll_ctl error\n"); }
To remove an existing event on the file associated with fd from the epoll instance epfd :
#include <sys/epoll.h> struct epoll_event event; int ret = epoll_ctl(epfd, EPOLL_CTL_DEL, fd, &event); if (ret) { fprintf(stderr, "epoll_ctl error\n"); }
(3) waiting for events with epoll
#include <sys/epoll.h> int epoll_wait(int epfd, struct epoll_event* events, int maxevents, int timeout);
The return value is the number of events, or −1 on error
#include <sys/epoll.h> #define MAX_EVENTS 64 struct epoll_event* events = malloc(sizeof(struct epoll_event) * MAX_EVENTS); if (events == NULL) { fprintf(stdout, "malloc error\n"); return 1; } int nready = epoll_wait(epfd, events, MAX_EVENTS, -1); if (nready < 0) { fprintf(stderr, "epoll_wait error\n"); free(events); return 1; } for (int i = 0; i < nready; ++i) { fprintf(stdout, "event=%ld on fd=%d\n", events[i].events, events[i].data.fd); /* we now can operate on events[i].data.fd without blocking */ } free(events);
(4) Level-Triggered vs. Edge-Triggered
考虑一个生产者和一个消费者通过Unix 管道通信：
a. 生产者向管道写 1KB数据
b. 消费者在管道上执行 epoll_wait, 等待管道中数据可读
如果是 水平触发，epoll_wait 调用会立即返回，表明管道已经 读操作就绪
如果是 边缘触发，epoll_wait 调用不会立即返回，直到 步骤a 发生，也就是说，即使管道在eoll_wait调用的时候是可读的，该调用也不会立即返回直至数据已经写到管道。
select和epoll的默认工作方式是 水平触发，边缘触发通常是应用于 non-blocking I/O, 并且必须小心检查EAGAIN
3. Mapping Files into Memory
/* A call to mmap() asks the kernel to map len bytes of the object represented by the file descriptor fd,
starting at offset bytes into the file, into memory
*/ #include <sys/mman.h> void* mmap(void* addr, size_t len, int prot, int flags, int fd, off_t offset);
void* ptr = mmap(0, len, PROT_READ, MAP_SHARED, fd, 0);
参数addr和offset必须是内存页大小的整数倍
如果文件大小不是内存页大小的整数倍，那么就存在填补为0的空洞，读这段区域将返回0，写这段区域将不会影响到被映射的文件内容
Because addr and offset are usually 0, this requirement is not overly difficult to meet
int munmap (void *addr, size_t len);
munmap() removes any mappings that contain pages located anywhere in the process address space starting at addr,
which must be page-aligned, and continuing for len bytes
#include <stdio.h> #include <unistd.h> #include <sys/types.h> #include <sys/stat.h> #include <sys/mman.h> #include <fcntl.h> int main(int argc, char* argv[]) { if (argc < 2) { fprintf(stderr, "usage:%s <file>\n", argv[0]); return 1; } int fd = open(argv[1], O_RDONLY); if (fd == -1) { fprintf(stderr, "open error\n"); return 1; } struct stat sbuf; if (fsat(fd, &sbuf) == -1) { fprintf(stderr, "fstat error\n"); return 1; } if (!S_ISREG(sbuf.st_mode)) { fprintf(stderr, "%s is not a file\n", argv[1]); return 1; } void* ptr = mmap(0, sbuf.st_size, PROT_READ, MAP_SHARED, fd, 0); if (ptr == MAP_FAILED) { fprintf(stderr, "mmap error\n"); return 1; } if (close(fd)) { fprintf(stderr, "close error\n"); return 1; } for (int i = 0; i < sbuf.st_size; ++i) { fputc(ptr[i], stdout); } if (munmap(ptr, sbuf.st_size) == -1) { fprintf(stderr, "munmap error\n"); return 1; } return 0; }
mmap的优点：
(1) 读写内存映射文件 可以避免 read 和 write 系统调用发生的拷贝
(2) 除了潜在的内存页错误，读写内存映射文件不会有 read 和 write 系统调用时的上下文切换开销
(3) 当多个进程映射相同内存时，内存数据可以在多个进程之间共享
(4) 内存映射定位时只需要平常的指针操作，而不需要 lseek 系统调用
mmap的缺点：
(1) 内存映射必须是页大小的整数倍，通常在 调用返回的内存页 和 被映射的实际文件之间存在 剩余空间，例如内存页大小是4KB，那么映射7Byte文件将浪费4089Byte内存
(2) 内存映射必须适合进程的地址空间，32位地址空间，如果有大量的内存映射，将产生碎片，将很难寻找到连续的大区域
(3) 创建和维护 内存映射和内核相关数据结构 有一定开销
文件和映射内存之间的同步：
#include <sys/mman.h> int msync (void *addr, size_t len, int flags);
msync() flushes back to disk any changes made to a file mapped via mmap(), synchronizing the mapped file with the mapping
Without invocation of msync(), there is no guarantee that a dirty mapping will be written back to disk until the file is unmapped
4. 同步 异步
5. I/O调度和I/O性能
a single disk seek can average over 8 milliseconds，25 million times longer than a single processor cycle!
I/O schedulers perform two basic operations: merging and sorting：
Merging is the process of taking two or more adjacent I/O requests and combining them into a single request
假设一个请求读磁盘块5，另一个请求读磁盘块6和7，调度器可以合并成单一请求读磁盘块5到7，这将减少一半的I/O操作数
Sorting is the process of arranging pending I/O requests in ascending block order (尽可能保证顺序读写，而不是随机读写)
If an I/O scheduler always sorted new requests by the order of insertion，it would be possible to starve requests to
far-off blocks indefinitely (I/O调度算法，避免饥饿现象)
使用电梯调度算法避免某个I/O请求长时间得不到响应
Linus Elevator I/O scheduler The Deadline I/O Scheduler The Anticipatory I/O Scheduler The CFQ I/O Scheduler The Noop I/O Scheduler
Sorting by inode number is the most commonly used method for scheduling I/O requests in user space
