测试Lighttpd accept的惊群现象

lighttpd里面采用的是prefork的模型，在fork进程之前就已经创建好了listen socket
那么fork了进程池之后，所有进程都有一份自己独立的listen socket fd，

但实际上这个独立的fd 对应的确是一个文件表项，即实际上任然是一个共享的文件描述符

在阻塞模型中，各进程分别通过accept阻塞，等待连接到达，当一个连接到达时，所有的进程都会被唤醒，但只有其中一个进程可以成功accept该连接，其余的则继续投入睡眠，这就是所谓的惊群现象

lighttpd使用的是非阻塞IO复用模型，测试一下是否会有惊群现象呢？

先把结论给出：

1.比如有20个进程注册了listen socket的请求连接事件，当一个连接到达确实会有多个进程 被通知有事件要处理（但不是全部，大约只有5，6个进程）
2.被唤醒的这几个进程会调用accept函数,其中只有一个成功返回连接fd,其余进程均返回EAGAIN或者 EWOULDBLOCK错误（因为是非阻塞的）

测试方法，自己写了一个prefork进程 + epoll的非阻塞server，启动20个进程，client telnet，打印服务器日志

try to accept new connection,pid=29879
try to accept new connection,pid=29876
try to accept new connection,pid=29880

process 29879 accept connection

accept EAGAIN error pid=29876
try to accept new connection,pid=29875
accept EAGAIN error pid=29880
accept EAGAIN error pid=29875

四个进程被通知有事件处理，1个成功accept,3个返回EAGAIN

、
在lighttpd中，server当被通知有连接要处理时，server会通过循环执行
accept，直到返回错误，或者超过一个上限值

这样，当海量请求连接到达时，似乎惊群不会带来太多的性能损耗。


部分测试代码
base.h

enum conn_states {
    conn_listening,  /** the socket which listens for connections */
    conn_read,       /** reading in a command line */
    conn_write,      /** writing out a simple response */
    conn_nread,      /** reading in a fixed number of bytes */
    conn_swallow,    /** swallowing unnecessary bytes w/o storing */
    conn_closing,    /** closing this connection */
    conn_mwrite,     /** writing out many items sequentially */
};
typedef struct{
	int fd;	
 	int state;	
}conn;

server.c

#include<stdio.h>
#include<unistd.h>
#include<sys/types.h>
#include<stdlib.h>
#include<sys/socket.h>
#include<errno.h>
#include<fcntl.h>
#include<netinet/in.h>
#include<sys/resource.h>
#include "event.h"
#include "base.h"

//forward declaration

static fdevents *ev;
static conn **conns;
static int freetotal;
static int freecurr;

static int create_listen_fd(char *addr,int port);
static int conn_init();
static conn *get_conn_from_freelist();
static int add_conn_to_freelist(conn *c);
conn *conn_new(int fd,int state);

static int conn_init(){
	freetotal=200;
	freecurr=0;
	conns=(conn **)malloc(freetotal * sizeof(*conns));
	if(!conns){
		return -1;
	}
	return 0;
}
static conn *get_conn_from_freelist(){
	conn *con;
	if(freecurr > 0){
		con=conns[--freecurr];
		conns[freecurr]=NULL;
		return con;
	}
	return NULL;
}
static int add_conn_to_freelist(conn *c){
	if(freecurr<freetotal){
		conns[freecurr++]=c;
		return 0;
	}else{
		conn **new_conns=(conn **)realloc(conns,sizeof(*new_conns)*2*freetotal);
		if(new_conns){
			freetotal*=2;
			conns=new_conns;
			conns[freecurr++]=c;
			return 0;
		}		
	}
	return -1;
}
conn *conn_new(int fd,int state){
	conn *c;
	c=get_conn_from_freelist();
	if(!c){
		c=(conn *)malloc(sizeof(*c));		
	}
	
	c->fd=fd;
	c->state=state;

	return c;
	
}
static int create_listen_fd(char *addr,int port){
	int fd,val,flags;
	struct sockaddr_in sockaddr;
	fd=socket(AF_INET,SOCK_STREAM,0);
	if(fd==-1){
		fprintf(stderr,"socket()\n");
		return -1;
	}
	val=1;
	if(setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,&val,sizeof(val))<0){
		fprintf(stderr,"reuseaddr\n");
		return -1;
	}
	if((flags=fcntl(fd,F_GETFL,0)<0) || fcntl(fd,F_SETFL,flags | O_NONBLOCK) < 0){
		fprintf(stderr,"nonblocking\n");
		return -1;
	}

	bzero(&sockaddr,sizeof(sockaddr));
	sockaddr.sin_family=AF_INET;
	sockaddr.sin_port=htons(port);
	inet_pton(AF_INET,addr,&sockaddr.sin_addr);

	if(bind(fd,(struct sockaddr *)&sockaddr,sizeof(sockaddr))<0){
		fprintf(stderr,"bind error %s",strerror(errno));
		return -1;
	}
	if(listen(fd,2048)<0){
		fprintf(stderr,"listen %s",strerror(errno));
		return -1;
	}
	return fd;
}
void event_handler(int fd,void *ctx, int revents){
	struct sockaddr_in addr;
	socklen_t sock_len;
	int done=0,connfd;
	conn *c;
	c=(conn *)ctx;
	while(!done){
		switch(c->state){
			case conn_listening:
				printf("try to accept new connection,pid=%d\n",getpid());
				sock_len=sizeof(addr);
				connfd=accept(fd,(struct sockaddr *)&addr,&sock_len);
				if(connfd>0){
					printf("process %d accept connection\n",getpid());
					c = conn_new(connfd,conn_read);
					fdevent_register(ev,connfd,event_handler,c);
					fdevent_event_add(ev,connfd,FDEVENT_IN);
				}else{
						if(errno== EAGAIN || errno == EWOULDBLOCK){
							printf("accept EAGAIN error pid=%d\n",getpid());
						}
						if(errno==EINTR){
							printf("accept EINTR error pid=%d\n",getpid());
						}
						if(errno==ECONNABORTED){ /* this is a FreeBSD thingy */
							printf("accept EABORTED error pid=%d\n",getpid());
						}
						if(errno==EMFILE){
							printf("accept EMFILE error pid=%d\n",getpid());
						}
				}
				done=1;
				break;
			case conn_read:
				printf("on read");
				break;
		}
	}
}
int main(int argc,char **argv){
	int fd,o;
	char *listen_addr;
	int port,num_childs,max_fds;
	struct rlimit rlim;
	conn *c;
	port=0;
	num_childs=5;
	while(-1!=(o=getopt(argc,argv,"l:p:f:h"))){
		switch(o){
			case 'l':
				listen_addr=strdup(optarg);
				break;
			case 'p':
				port=atoi(optarg);
				break;
			case 'f':
				num_childs=atoi(optarg);
				break;
			case 'h':
				printf("Usage -l listen addr\n");
				printf("Usage -p listen port \n");
				printf("Usage -f fork num\n");
				exit(1);
		}
	}
	if(!listen_addr){
		listen_addr=strdup("127.0.0.1");
	}
	if(!port){
		printf("port is unknown\n");
		exit(1);
	}
	if(0 != getrlimit(RLIMIT_NOFILE,&rlim)){
		fprintf(stderr,"getrlimit failed.reason %s\n",strerror(errno));
		exit(1);
	}

	max_fds=rlim.rlim_cur;

	//create listen socket
	if(-1==(fd=create_listen_fd(listen_addr,port))){
		fprintf(stderr,"create listen fd failed\n");
		exit(1);
	}
	//prefork child
	if(num_childs > 0){
		int child=0;
		while(!child){
			if(num_childs >0){
				switch(fork()){
					case -1:
						return -1;
					case 0:
						child=1;
						break;
					default:
						num_childs--;
						break;
				}
			}else{
				int status;
				if(-1 !=wait(&status)){
					num_childs++;
				}else{
					//ignore
				}
			}
		}
	}
	//child process event
	conn_init();
	c=conn_new(fd,conn_listening);
	ev=fdevent_init(max_fds);
	if(!ev){
		fprintf(stderr,"fdevent_init()\n");
		exit(1);
	}	
	fdevent_register(ev,fd,event_handler,c);
	fdevent_event_add(ev,fd,FDEVENT_IN);
	fdevent_poll(ev,1000);
}