Linux 下优化多路复用机制（以 epoll 为例）
- 性能瓶颈：
- 大量连接时，每次调用
epoll_wait 可能会有较多的上下文切换开销。
- 如果对文件描述符的管理不合理，如频繁添加或删除文件描述符，会影响性能。
- 优化策略：
- 使用边缘触发（ET）模式：减少不必要的事件触发，只有在状态发生变化时才触发事件。
- 批量操作文件描述符：减少
epoll_ctl 的调用次数。
- C 语言代码示例：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <sys/epoll.h>
#define MAX_EVENTS 10
int main() {
int listen_fd, conn_fd;
struct sockaddr_in servaddr;
listen_fd = socket(AF_INET, SOCK_STREAM, 0);
if (listen_fd < 0) {
perror("socket creation failed");
exit(EXIT_FAILURE);
}
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(8080);
servaddr.sin_addr.s_addr = INADDR_ANY;
if (bind(listen_fd, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {
perror("bind failed");
close(listen_fd);
exit(EXIT_FAILURE);
}
if (listen(listen_fd, 10) < 0) {
perror("listen failed");
close(listen_fd);
exit(EXIT_FAILURE);
}
int epoll_fd = epoll_create1(0);
if (epoll_fd < 0) {
perror("epoll_create1 failed");
close(listen_fd);
exit(EXIT_FAILURE);
}
struct epoll_event ev;
ev.data.fd = listen_fd;
ev.events = EPOLLIN | EPOLLET;
if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, listen_fd, &ev) < 0) {
perror("epoll_ctl add listen_fd failed");
close(listen_fd);
close(epoll_fd);
exit(EXIT_FAILURE);
}
struct epoll_event events[MAX_EVENTS];
while (1) {
int num_events = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);
if (num_events < 0) {
perror("epoll_wait failed");
break;
}
for (int i = 0; i < num_events; ++i) {
if (events[i].data.fd == listen_fd) {
conn_fd = accept(listen_fd, NULL, NULL);
if (conn_fd < 0) {
perror("accept failed");
continue;
}
ev.data.fd = conn_fd;
ev.events = EPOLLIN | EPOLLET;
if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, conn_fd, &ev) < 0) {
perror("epoll_ctl add conn_fd failed");
close(conn_fd);
}
} else {
conn_fd = events[i].data.fd;
char buf[1024];
ssize_t n = read(conn_fd, buf, sizeof(buf));
if (n < 0) {
if (errno == EAGAIN || errno == EWOULDBLOCK) {
continue;
} else {
perror("read failed");
close(conn_fd);
epoll_ctl(epoll_fd, EPOLL_CTL_DEL, conn_fd, NULL);
}
} else if (n == 0) {
close(conn_fd);
epoll_ctl(epoll_fd, EPOLL_CTL_DEL, conn_fd, NULL);
} else {
// 处理读取到的数据
write(conn_fd, buf, n);
}
}
}
}
close(listen_fd);
close(epoll_fd);
return 0;
}
Windows 下优化多路复用机制（以 select 为例，因为 Windows 没有 epoll 和 poll）
- 性能瓶颈：
select 函数支持的文件描述符数量有限（FD_SETSIZE，默认 64）。- 每次调用
select 都需要将整个文件描述符集合从用户态拷贝到内核态，开销较大。
- 优化策略：
- 合理管理文件描述符集合：尽量减少文件描述符的数量，只将活跃的文件描述符加入集合。
- 减少
select 调用频率：可以结合定时器等机制，批量处理事件。
- C 语言代码示例：
#include <winsock2.h>
#include <stdio.h>
#include <stdlib.h>
#pragma comment(lib, "ws2_32.lib")
#define DEFAULT_PORT 8080
#define MAX_CLIENTS 64
int main() {
WSADATA wsaData;
if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
printf("WSAStartup failed: %d\n", WSAGetLastError());
return 1;
}
SOCKET listenSocket = socket(AF_INET, SOCK_STREAM, 0);
if (listenSocket == INVALID_SOCKET) {
printf("Socket creation failed: %d\n", WSAGetLastError());
WSACleanup();
return 1;
}
sockaddr_in serverAddr;
serverAddr.sin_family = AF_INET;
serverAddr.sin_port = htons(DEFAULT_PORT);
serverAddr.sin_addr.s_addr = INADDR_ANY;
if (bind(listenSocket, (sockaddr*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR) {
printf("Bind failed: %d\n", WSAGetLastError());
closesocket(listenSocket);
WSACleanup();
return 1;
}
if (listen(listenSocket, 5) == SOCKET_ERROR) {
printf("Listen failed: %d\n", WSAGetLastError());
closesocket(listenSocket);
WSACleanup();
return 1;
}
fd_set readFds, tmpFds;
FD_ZERO(&readFds);
FD_SET(listenSocket, &readFds);
SOCKET clientSockets[MAX_CLIENTS];
int clientCount = 0;
while (1) {
tmpFds = readFds;
int activity = select(0, &tmpFds, NULL, NULL, NULL);
if (activity == SOCKET_ERROR) {
printf("Select error: %d\n", WSAGetLastError());
break;
} else if (activity > 0) {
if (FD_ISSET(listenSocket, &tmpFds)) {
SOCKET clientSocket = accept(listenSocket, NULL, NULL);
if (clientSocket == INVALID_SOCKET) {
printf("Accept failed: %d\n", WSAGetLastError());
continue;
}
if (clientCount < MAX_CLIENTS) {
clientSockets[clientCount++] = clientSocket;
FD_SET(clientSocket, &readFds);
} else {
closesocket(clientSocket);
}
}
for (int i = 0; i < clientCount; ++i) {
if (FD_ISSET(clientSockets[i], &tmpFds)) {
char buffer[1024];
int bytesRead = recv(clientSockets[i], buffer, sizeof(buffer), 0);
if (bytesRead == SOCKET_ERROR) {
printf("Recv error: %d\n", WSAGetLastError());
FD_CLR(clientSockets[i], &readFds);
closesocket(clientSockets[i]);
for (int j = i; j < clientCount - 1; ++j) {
clientSockets[j] = clientSockets[j + 1];
}
clientCount--;
i--;
} else if (bytesRead == 0) {
FD_CLR(clientSockets[i], &readFds);
closesocket(clientSockets[i]);
for (int j = i; j < clientCount - 1; ++j) {
clientSockets[j] = clientSockets[j + 1];
}
clientCount--;
i--;
} else {
// 处理读取到的数据
send(clientSockets[i], buffer, bytesRead, 0);
}
}
}
}
}
for (int i = 0; i < clientCount; ++i) {
closesocket(clientSockets[i]);
}
closesocket(listenSocket);
WSACleanup();
return 0;
}
总结
- 在 Linux 下，epoll 的边缘触发模式和合理的文件描述符管理能有效提升性能，适合处理大量并发连接。
- 在 Windows 下，由于没有类似 epoll 的高效多路复用机制，需要在
select 的使用上更加谨慎，合理管理文件描述符集合和减少调用频率来优化性能。
