目录
关于Memcached:
Memcached分析
网络模型流程分析
网络模型数据结构
main启动入口
工作线程worker thread
主线程main thread
memcached是一款非常普及的服务器端缓存软件,memcached主要是基于Libevent库进行开发的。
如果你还不了解libevent相关知识,请先看我的libevent这篇文章《Linux c 开发 - libevent 》
memcached也是使用autotools的进行代码编译管理的,如果你还不了解autotools,你可以先看下文章:《Linux c 开发 - Autotools使用详细解读 》
memcached你可以去官网上获取源代码。官网地址
笔者这边分析的memcached版本是1.4.21版本,你也可以通过我的《环境安装系列 - Linux下 Memcache的安装和简单管理》去下载
Memcached主要是基于Libevent的事件库来实现网络线程模型的。我们先需要下载memcached的源码包,上面我们已经给出了源码包下载地址。
Memcached的网络线程模型主要涉及两个主要文件:memcached.c 和thread.c文件。
我们这边主要分析tcp的模型。memcached也支持udp。
流程
流程图
CQ_ITEM:主要用于存储用户socket连接的基本信息。
主线程会将用户的socket连接信息封装成CQ_ITEM,并放入工作线程的处理队列中。工作线程得到主线程的pipe通知后,就会将队列中的ITEM取出来,创建libevent的socket读事件。
/* An item in the connection queue. */
typedef struct conn_queue_item CQ_ITEM;
struct conn_queue_item {
int sfd; //socket的fd
enum conn_states init_state; //事件类型
int event_flags; //libevent的flags
int read_buffer_size; //读取的buffer的size
enum network_transport transport;
CQ_ITEM *next; //下一个item的地址
};
CQ:每个线程的处理队列结构。
/* A connection queue. */
typedef struct conn_queue CQ;
struct conn_queue {
CQ_ITEM *head;
CQ_ITEM *tail;
pthread_mutex_t lock;
};
LIBEVENT_THREAD:每个工作线程的数据结构。
每一个工作线程都有有这么一个自己的数据结构,主要存储线程信息、处理队列、pipe信息等。
typedef struct {
//线程ID
pthread_t thread_id; /* unique ID of this thread */
//线程的 event_base,每个线程都有自己的event_base
struct event_base *base; /* libevent handle this thread uses */
//异步event事件
struct event notify_event; /* listen event for notify pipe */
//管道接收端
int notify_receive_fd; /* receiving end of notify pipe */
//管道发送端
int notify_send_fd; /* sending end of notify pipe */
//线程状态
struct thread_stats stats; /* Stats generated by this thread */
//新连接队列结构
struct conn_queue *new_conn_queue; /* queue of new connections to handle */
cache_t *suffix_cache; /* suffix cache */
uint8_t item_lock_type; /* use fine-grained or global item lock */
} LIBEVENT_THREAD;
我们需要找到memcached.c中的main()方法。下面的代码中只列出了我们需要的重要部分。
int main (int argc, char **argv) {
//...省去一部分代码
/* initialize main thread libevent instance */
//初始化一个event_base
main_base = event_init();
/* initialize other stuff */
stats_init();
assoc_init(settings.hashpower_init);
conn_init();
slabs_init(settings.maxbytes, settings.factor, preallocate);
/*
* ignore SIGPIPE signals; we can use errno == EPIPE if we
* need that information
*/
if (sigignore(SIGPIPE) == -1) {
perror("failed to ignore SIGPIPE; sigaction");
exit(EX_OSERR);
}
/* start up worker threads if MT mode */
//这边非常重要,这个方法主要用来创建工作线程,默认会创建4个工作线程
thread_init(settings.num_threads, main_base);
if (start_assoc_maintenance_thread() == -1) {
exit(EXIT_FAILURE);
}
if (settings.slab_reassign &&
start_slab_maintenance_thread() == -1) {
exit(EXIT_FAILURE);
}
/* Run regardless of initializing it later */
init_lru_crawler();
/* initialise clock event */
clock_handler(0, 0, 0);
/* create unix mode sockets after dropping privileges */
if (settings.socketpath != NULL) {
errno = 0;
if (server_socket_unix(settings.socketpath,settings.access)) {
vperror("failed to listen on UNIX socket: %s", settings.socketpath);
exit(EX_OSERR);
}
}
/* create the listening socket, bind it, and init */
if (settings.socketpath == NULL) {
const char *portnumber_filename = getenv("MEMCACHED_PORT_FILENAME");
char temp_portnumber_filename[PATH_MAX];
FILE *portnumber_file = NULL;
if (portnumber_filename != NULL) {
snprintf(temp_portnumber_filename,
sizeof(temp_portnumber_filename),
"%s.lck", portnumber_filename);
portnumber_file = fopen(temp_portnumber_filename, "a");
if (portnumber_file == NULL) {
fprintf(stderr, "Failed to open \"%s\": %s\n",
temp_portnumber_filename, strerror(errno));
}
}
errno = 0;
//这边的server_sockets方法主要是socket的bind、listen、accept等操作
//主线程主要用于接收客户端的socket连接,并且将连接交给工作线程接管。
if (settings.port && server_sockets(settings.port, tcp_transport,
portnumber_file)) {
vperror("failed to listen on TCP port %d", settings.port);
exit(EX_OSERR);
}
}
/* enter the event loop */
//这边开始进行主线程的事件循环
if (event_base_loop(main_base, 0) != 0) {
retval = EXIT_FAILURE;
}
//...省去一部分代码
}
主线程中主要是通过thread_init方法去创建N个工作线程:
thread_init(settings.num_threads, main_base);
通过server_sockets方法去创建socket server:
errno = 0;
if (settings.port && server_sockets(settings.port, tcp_transport,
portnumber_file)) {
vperror("failed to listen on TCP port %d", settings.port);
exit(EX_OSERR);
}
我们在thread.c文件中找到thread_init这个方法:
void thread_init(int nthreads, struct event_base *main_base) {
//...省了一部分代码
//这边通过循环的方式创建nthreads个线程
//nthreads应该是可以设置的
for (i = 0; i < nthreads; i++) {
int fds[2];
//这边会创建pipe,主要用于主线程和工作线程之间的通信
if (pipe(fds)) {
perror("Can't create notify pipe");
exit(1);
}
//threads是工作线程的基本结构:LIBEVENT_THREAD
//将pipe接收端和写入端都放到工作线程的结构体中
threads[i].notify_receive_fd = fds[0]; //接收端
threads[i].notify_send_fd = fds[1]; //写入端
//这个方法非常重要,主要是创建每个线程自己的libevent的event_base
setup_thread(&threads[i]);
/* Reserve three fds for the libevent base, and two for the pipe */
stats.reserved_fds += 5;
}
/* Create threads after we've done all the libevent setup. */
//这里是循环创建线程
//线程创建的回调函数是worker_libevent
for (i = 0; i < nthreads; i++) {
create_worker(worker_libevent, &threads[i]);
}
/* Wait for all the threads to set themselves up before returning. */
pthread_mutex_lock(&init_lock);
wait_for_thread_registration(nthreads);
pthread_mutex_unlock(&init_lock);
}
setup_thread方法:
/*
* Set up a thread's information.
*/
static void setup_thread(LIBEVENT_THREAD *me) {
//创建一个event_base
//根据libevent的使用文档,我们可以知道一般情况下每个独立的线程都应该有自己独立的event_base
me->base = event_init();
if (! me->base) {
fprintf(stderr, "Can't allocate event base\n");
exit(1);
}
/* Listen for notifications from other threads */
//这边非常重要,这边主要创建pipe的读事件EV_READ的监听
//当pipe中有写入事件的时候,libevent就会回调thread_libevent_process方法
event_set(&me->notify_event, me->notify_receive_fd,
EV_READ | EV_PERSIST, thread_libevent_process, me);
event_base_set(me->base, &me->notify_event);
//添加事件操作
if (event_add(&me->notify_event, 0) == -1) {
fprintf(stderr, "Can't monitor libevent notify pipe\n");
exit(1);
}
//初始化一个工作队列
me->new_conn_queue = malloc(sizeof(struct conn_queue));
if (me->new_conn_queue == NULL) {
perror("Failed to allocate memory for connection queue");
exit(EXIT_FAILURE);
}
cq_init(me->new_conn_queue);
//初始化线程锁
if (pthread_mutex_init(&me->stats.mutex, NULL) != 0) {
perror("Failed to initialize mutex");
exit(EXIT_FAILURE);
}
me->suffix_cache = cache_create("suffix", SUFFIX_SIZE, sizeof(char*),
NULL, NULL);
if (me->suffix_cache == NULL) {
fprintf(stderr, "Failed to create suffix cache\n");
exit(EXIT_FAILURE);
}
}
create_worker方法:
/*
* Creates a worker thread.
*/
//这个方法是真正的创建工作线程
static void create_worker(void *(*func)(void *), void *arg) {
pthread_t thread;
pthread_attr_t attr;
int ret;
pthread_attr_init(&attr);
//这边真正的创建线程
if ((ret = pthread_create(&thread, &attr, func, arg)) != 0) {
fprintf(stderr, "Can't create thread: %s\n",
strerror(ret));
exit(1);
}
}
worker_libevent方法:
/*
* Worker thread: main event loop
*/
static void *worker_libevent(void *arg) {
LIBEVENT_THREAD *me = arg;
/* Any per-thread setup can happen here; thread_init() will block until
* all threads have finished initializing.
*/
/* set an indexable thread-specific memory item for the lock type.
* this could be unnecessary if we pass the conn *c struct through
* all item_lock calls...
*/
me->item_lock_type = ITEM_LOCK_GRANULAR;
pthread_setspecific(item_lock_type_key, &me->item_lock_type);
register_thread_initialized();
//这个方法主要是开启事件的循环
//每个线程中都会有自己独立的event_base和事件的循环机制
//memcache的每个工作线程都会独立处理自己接管的连接
event_base_loop(me->base, 0);
return NULL;
}
thread_libevent_process方法:
static void thread_libevent_process(int fd, short which, void *arg) {
LIBEVENT_THREAD *me = arg;
CQ_ITEM *item;
char buf[1];
//回调函数中回去读取pipe中的信息
//主线程中如果有新的连接,会向其中一个线程的pipe中写入1
//这边读取pipe中的数据,如果为1,则说明从pipe中获取的数据是正确的
if (read(fd, buf, 1) != 1)
if (settings.verbose > 0)
fprintf(stderr, "Can't read from libevent pipe\n");
switch (buf[0]) {
case 'c':
//从工作线程的队列中获取一个CQ_ITEM连接信息
item = cq_pop(me->new_conn_queue);
//如果item不为空,则需要进行连接的接管
if (NULL != item) {
//conn_new这个方法非常重要,主要是创建socket的读写等监听事件。
//init_state 为初始化的类型,主要在drive_machine中通过这个状态类判断处理类型
conn *c = conn_new(item->sfd, item->init_state, item->event_flags,
item->read_buffer_size, item->transport, me->base);
if (c == NULL) {
if (IS_UDP(item->transport)) {
fprintf(stderr, "Can't listen for events on UDP socket\n");
exit(1);
} else {
if (settings.verbose > 0) {
fprintf(stderr, "Can't listen for events on fd %d\n",
item->sfd);
}
close(item->sfd);
}
} else {
c->thread = me;
}
cqi_free(item);
}
break;
/* we were told to flip the lock type and report in */
case 'l':
me->item_lock_type = ITEM_LOCK_GRANULAR;
register_thread_initialized();
break;
case 'g':
me->item_lock_type = ITEM_LOCK_GLOBAL;
register_thread_initialized();
break;
}
}
conn_new方法(主要看两行):
//我们发现这个方法中又在创建event了,这边实际上是监听socket的读写等事件
//主线程主要是监听用户的socket连接事件;工作线程主要监听socket的读写事件
//当用户socket的连接有数据传递过来的时候,就会调用event_handler这个回调函数
event_set(&c->event, sfd, event_flags, event_handler, (void *)c);
event_base_set(base, &c->event);
c->ev_flags = event_flags;
//将事件添加到libevent的loop循环中
if (event_add(&c->event, 0) == -1) {
perror("event_add");
return NULL;
}
event_handler方法:
void event_handler(const int fd, const short which, void *arg) {
conn *c;
//组装conn结构
c = (conn *)arg;
assert(c != NULL);
c->which = which;
/* sanity */
if (fd != c->sfd) {
if (settings.verbose > 0)
fprintf(stderr, "Catastrophic: event fd doesn't match conn fd!\n");
conn_close(c);
return;
}
//最终转交给了drive_machine这个方法
//memcache的大部分的网络事件都是由drive_machine这个方法来处理的
//drive_machine这个方法主要通过c->state这个事件的类型来处理不同类型的事件
drive_machine(c);
/* wait for next event */
return;
}
然后继续看最重要的,也是核心的处理事件的方法drive_machine(状态机),监听socket连接、监听socket的读写、断开连接等操作都是在drive_machine这个方法中实现的。而这些操作都是通过c->state这个状态来判断不同的操作类型。
static void drive_machine(conn *c) {
//....................
assert(c != NULL);
while (!stop) {
//这边通过state来处理不同类型的事件
switch(c->state) {
//这边主要处理tcp连接,只有在主线程的下,才会执行listening监听操作。
case conn_listening:
addrlen = sizeof(addr);
#ifdef HAVE_ACCEPT4
if (use_accept4) {
sfd = accept4(c->sfd, (struct sockaddr *)&addr, &addrlen, SOCK_NONBLOCK);
} else {
sfd = accept(c->sfd, (struct sockaddr *)&addr, &addrlen);
}
#else
sfd = accept(c->sfd, (struct sockaddr *)&addr, &addrlen);
#endif
//......................
if (settings.maxconns_fast &&
stats.curr_conns + stats.reserved_fds >= settings.maxconns - 1) {
str = "ERROR Too many open connections\r\n";
res = write(sfd, str, strlen(str));
close(sfd);
STATS_LOCK();
stats.rejected_conns++;
STATS_UNLOCK();
} else {
dispatch_conn_new(sfd, conn_new_cmd, EV_READ | EV_PERSIST,
DATA_BUFFER_SIZE, tcp_transport);
}
stop = true;
break;
//连接等待
case conn_waiting:
//.........
break;
//读取事件
//例如有用户提交数据过来的时候,工作线程监听到事件后,最终会调用这块代码
case conn_read:
res = IS_UDP(c->transport) ? try_read_udp(c) : try_read_network(c);
switch (res) {
case READ_NO_DATA_RECEIVED:
conn_set_state(c, conn_waiting);
break;
case READ_DATA_RECEIVED:
conn_set_state(c, conn_parse_cmd);
break;
case READ_ERROR:
conn_set_state(c, conn_closing);
break;
case READ_MEMORY_ERROR: /* Failed to allocate more memory */
/* State already set by try_read_network */
break;
}
break;
case conn_parse_cmd :
if (try_read_command(c) == 0) {
/* wee need more data! */
conn_set_state(c, conn_waiting);
}
break;
case conn_new_cmd:
/* Only process nreqs at a time to avoid starving other
connections */
--nreqs;
if (nreqs >= 0) {
reset_cmd_handler(c);
} else {
pthread_mutex_lock(&c->thread->stats.mutex);
c->thread->stats.conn_yields++;
pthread_mutex_unlock(&c->thread->stats.mutex);
if (c->rbytes > 0) {
/* We have already read in data into the input buffer,
so libevent will most likely not signal read events
on the socket (unless more data is available. As a
hack we should just put in a request to write data,
because that should be possible ;-)
*/
if (!update_event(c, EV_WRITE | EV_PERSIST)) {
if (settings.verbose > 0)
fprintf(stderr, "Couldn't update event\n");
conn_set_state(c, conn_closing);
break;
}
}
stop = true;
}
break;
case conn_nread:
//....................
break;
case conn_swallow:
//....................
break;
case conn_write:
//.................
break;
//连接关闭
case conn_closing:
if (IS_UDP(c->transport))
conn_cleanup(c);
else
conn_close(c);
stop = true;
break;
case conn_closed:
/* This only happens if dormando is an idiot. */
abort();
break;
case conn_max_state:
assert(false);
break;
}
}
return;
}
主线程的socket server主要通过server_sockets这个方法创建。而server_sockets中主要调用了server_socket这个方法,我们可以看下server_socket这个方法:
/**
* Create a socket and bind it to a specific port number
* @param interface the interface to bind to
* @param port the port number to bind to
* @param transport the transport protocol (TCP / UDP)
* @param portnumber_file A filepointer to write the port numbers to
* when they are successfully added to the list of ports we
* listen on.
*/
static int server_socket(const char *interface,
int port,
enum network_transport transport,
FILE *portnumber_file) {
//创建一个新的事件
//我们发现上面的工作线程也是调用这个方法,但是区别是这个方法指定了state的类型为:conn_listening
//注意这边有一个conn_listening,这个参数主要是指定调用drive_machine这个方法中的conn_listen代码块。
if (!(listen_conn_add = conn_new(sfd, conn_listening,
EV_READ | EV_PERSIST, 1,
transport, main_base))) {
fprintf(stderr, "failed to create listening connection\n");
exit(EXIT_FAILURE);
}
listen_conn_add->next = listen_conn;
listen_conn = listen_conn_add;
}
conn_new方法:
//我们发现这个方法中又在创建event了,这边实际上是监听socket的读写等事件
//主线程主要是监听用户的socket连接事件;工作线程主要监听socket的读写事件
//当用户socket的连接有数据传递过来的时候,就会调用event_handler这个回调函数
event_set(&c->event, sfd, event_flags, event_handler, (void *)c);
event_base_set(base, &c->event);
c->ev_flags = event_flags;
//将事件添加到libevent的loop循环中
if (event_add(&c->event, 0) == -1) {
perror("event_add");
return NULL;
}
然后我们跟踪进入event_handler这个方法,并且进入drive_machine这个方法,我们上面说过server_socket这个方法中传递的state参数为默认写死的conn_listening这个状态,所以我们详细看drive_machine中关于conn_listening这块逻辑的代码。
case conn_listening:
addrlen = sizeof(addr);
#ifdef HAVE_ACCEPT4
//我们可以看到下面的代码是accept,接受客户端的socket连接的代码
if (use_accept4) {
sfd = accept4(c->sfd, (struct sockaddr *)&addr, &addrlen, SOCK_NONBLOCK);
} else {
sfd = accept(c->sfd, (struct sockaddr *)&addr, &addrlen);
}
#else
sfd = accept(c->sfd, (struct sockaddr *)&addr, &addrlen);
#endif
if (sfd == -1) {
if (use_accept4 && errno == ENOSYS) {
use_accept4 = 0;
continue;
}
perror(use_accept4 ? "accept4()" : "accept()");
if (errno == EAGAIN || errno == EWOULDBLOCK) {
/* these are transient, so don't log anything */
stop = true;
} else if (errno == EMFILE) {
if (settings.verbose > 0)
fprintf(stderr, "Too many open connections\n");
accept_new_conns(false);
stop = true;
} else {
perror("accept()");
stop = true;
}
break;
}
if (!use_accept4) {
if (fcntl(sfd, F_SETFL, fcntl(sfd, F_GETFL) | O_NONBLOCK) < 0) {
perror("setting O_NONBLOCK");
close(sfd);
break;
}
}
if (settings.maxconns_fast &&
stats.curr_conns + stats.reserved_fds >= settings.maxconns - 1) {
str = "ERROR Too many open connections\r\n";
res = write(sfd, str, strlen(str));
close(sfd);
STATS_LOCK();
stats.rejected_conns++;
STATS_UNLOCK();
} else {
//如果客户端用socket连接上来,则会调用这个分发逻辑的函数
//这个函数会将连接信息分发到某一个工作线程中,然后工作线程接管具体的读写操作
dispatch_conn_new(sfd, conn_new_cmd, EV_READ | EV_PERSIST,
DATA_BUFFER_SIZE, tcp_transport);
}
stop = true;
break;
dispatch_conn_new方法:
/*
* Dispatches a new connection to another thread. This is only ever called
* from the main thread, either during initialization (for UDP) or because
* of an incoming connection.
*/
void dispatch_conn_new(int sfd, enum conn_states init_state, int event_flags,
int read_buffer_size, enum network_transport transport) {
//每个连接连上来的时候,都会申请一块CQ_ITEM的内存块,用于存储连接的基本信息
CQ_ITEM *item = cqi_new();
char buf[1];
//如果item创建失败,则关闭连接
if (item == NULL) {
close(sfd);
/* given that malloc failed this may also fail, but let's try */
fprintf(stderr, "Failed to allocate memory for connection object\n");
return ;
}
//这个方法非常重要。主要是通过求余数的方法来得到当前的连接需要哪个线程来接管
//而且last_thread会记录每次最后一次使用的工作线程,每次记录之后就可以让工作线程进入一个轮询,保证了每个工作线程处理的连接数的平衡
int tid = (last_thread + 1) % settings.num_threads;
//获取线程的基本结构
LIBEVENT_THREAD *thread = threads + tid;
last_thread = tid;
item->sfd = sfd;
item->init_state = init_state;
item->event_flags = event_flags;
item->read_buffer_size = read_buffer_size;
item->transport = transport;
//向工作线程的队列中放入CQ_ITEM
cq_push(thread->new_conn_queue, item);
MEMCACHED_CONN_DISPATCH(sfd, thread->thread_id);
buf[0] = 'c';
//向工作线程的pipe中写入1
//工作线程监听到pipe中有写入数据,工作线程接收到通知后,就会向thread->new_conn_queue队列中pop出一个item,然后进行连接的接管操作
if (write(thread->notify_send_fd, buf, 1) != 1) {
perror("Writing to thread notify pipe");
}
}