源码memcached.c中，main入口函数。
第一步，根据memcached的启动参数做校验参数、配置。
main函数中，几乎600行代码都是这些参数校验。吐槽一个。
第二步，初始化。
2.1：初始化主线程的libevent。
main_base = event_init();
2.2：初始化memcached的stats信息。
在文本协议的memcached中，我们nc/telent后输入stats命令，会很快地输出一些当前memcached的信息的。这些就是stats信息。并不是输入stats的时候才遍历统计出来的。而是已经保存好了这份信息。代码调用在main函数中的：
stats_init();
具体的统计信息，可以在memcached.h这个文件中找到：
/**
* Global stats.
*/
struct stats {
pthread_mutex_t mutex;
unsigned int curr_items;
unsigned int total_items;
uint64_t curr_bytes;
unsigned int curr_conns;
unsigned int total_conns;
uint64_t rejected_conns;
uint64_t malloc_fails;
unsigned int reserved_fds;
unsigned int conn_structs;
uint64_t get_cmds;
uint64_t set_cmds;
uint64_t touch_cmds;
uint64_t get_hits;
uint64_t get_misses;
uint64_t touch_hits;
uint64_t touch_misses;
uint64_t evictions;
uint64_t reclaimed;
time_t started; /* when the process was started */
bool accepting_conns; /* whether we are currently accepting */
uint64_t listen_disabled_num;
unsigned int hash_power_level; /* Better hope it‘s not over 9000 */
uint64_t hash_bytes; /* size used for hash tables */
bool hash_is_expanding; /* If the hash table is being expanded */
uint64_t expired_unfetched; /* items reclaimed but never touched */
uint64_t evicted_unfetched; /* items evicted but never touched */
bool slab_reassign_running; /* slab reassign in progress */
uint64_t slabs_moved; /* times slabs were moved around */
uint64_t lru_crawler_starts; /* Number of item crawlers kicked off */
bool lru_crawler_running; /* crawl in progress */
uint64_t lru_maintainer_juggles; /* number of LRU bg pokes */
};
2.3：hash桶初始化
代码main函数中的：
assoc_init(settings.hashpower_init);
在memcached中，保存着一份hash表用来存放memcached key。默认这个hash表是2^16(65536)个key。后续会根据规则动态扩容这个hash表的。如果希望启动的时候，这个hash表更大，可以-o 参数调节。
hash表中， memcached key作为key，value是item指针，并不是item value。
2.4：初始化connection。
conn_init()
也就是 memcached启动参数中的-c参数，默认1024。
为了更快地找到connection的fd（文件描述符），实际上申请的connection会比配置的更大一点。
/*
* Initializes the connections array. We don‘t actually allocate connection
* structures until they‘re needed, so as to avoid wasting memory when the
* maximum connection count is much higher than the actual number of
* connections.
*
* This does end up wasting a few pointers‘ worth of memory for FDs that are
* used for things other than connections, but that‘s worth it in exchange for
* being able to directly index the conns array by FD.
*/
static void conn_init(void) {
/* We‘re unlikely to see an FD much higher than maxconns. */
int next_fd = dup(1);
int headroom = 10; /* account for extra unexpected open FDs */
struct rlimit rl;
max_fds = settings.maxconns + headroom + next_fd;
/* But if possible, get the actual highest FD we can possibly ever see. */
if (getrlimit(RLIMIT_NOFILE, &rl) == 0) {
max_fds = rl.rlim_max;
} else {
fprintf(stderr, "Failed to query maximum file descriptor; "
"falling back to maxconns\n");
}
close(next_fd);
if ((conns = calloc(max_fds, sizeof(conn *))) == NULL) {
fprintf(stderr, "Failed to allocate connection structures\n");
/* This is unrecoverable so bail out early. */
exit(1);
}
}
2.5：初始化slabs。
在2.3的hash桶中初始化的是key。slabs初始化的是这些key对应的value。下面摘录关键代码：
while (++i < MAX_NUMBER_OF_SLAB_CLASSES-1 && size <= settings.item_size_max / factor) {
/* Make sure items are always n-byte aligned */
if (size % CHUNK_ALIGN_BYTES)
size += CHUNK_ALIGN_BYTES - (size % CHUNK_ALIGN_BYTES);
slabclass[i].size = size;
slabclass[i].perslab = settings.item_size_max / slabclass[i].size;
size *= factor;
if (settings.verbose > 1) {
fprintf(stderr, "slab class %3d: chunk size %9u perslab %7u\n",
i, slabclass[i].size, slabclass[i].perslab);
}
}
在初始化slab的时候，下一个slab的size（chunk size）总是大于等于当前slab的size的。
2.6：初始化worker线程。
memcached_thread_init(settings.num_threads, main_base);
worker线程和main线程，组成了libevent的reactor模式。
2.7：定时器
clock_handler(0, 0, 0);
用于对比对象是否过期。
第三步、libevent主线程监听事件。
/* enter the event loop */
if (event_base_loop(main_base, 0) != 0) {
retval = EXIT_FAILURE;
}
第四步、关闭hash桶线程。
在2.3的初始化步骤中，有线程操作。这里明确关闭这个线程。
void stop_assoc_maintenance_thread() {
mutex_lock(&maintenance_lock);
do_run_maintenance_thread = 0;
pthread_cond_signal(&maintenance_cond);
mutex_unlock(&maintenance_lock);
/* Wait for the maintenance thread to stop */
pthread_join(maintenance_tid, NULL);
}
memcached启动的主要流程就是这些了。
源码github上有，见：https://github.com/memcached/memcached/blob/master/memcached.c 文件有点大，可能浏览器卡顿一下。这里肯定就不贴出来了^_^