对于定时器事件，Libevent采用两种方法处理定时器：
1、定时器尾队列
2、时间堆
在event结构体中，定义了联合体类型ev_timeout_pos来指明定时器的位置
//仅用于定时事件处理器：当为通过定时器时，ev_next_with_common_timeout指向定时器队列中的位置
//当为其它定时器时，min_heap_idx为指向时间堆中的索引
union {
TAILQ_ENTRY(event) ev_next_with_common_timeout;
int min_heap_idx;
} ev_timeout_pos;
开发者认为有时候使用简单的链表（通用定时器）来管理定时器将具有更高的效率。如下所述：
当需要获取多个具有同一超时值的定时器时，定时器链表比时间堆效率更高，此处个人理解为：若超时时间相同，在定时器链表中，可以通过顺序遍历的方法获取超时的定时器，而采用时间堆的话，每获取一个定时器需要维护堆，时间复杂度为nlogn。
一个定时器是否是通用定时器取决于其超时值（超时值由具体超时值、幻数5以及定时器索引组成），具体由is_common_timeout函数决定：
/* Common timeouts are special timeouts that are handled as queues rather than * in the minheap. This is more efficient than the minheap if we happen to * know that we're going to get several thousands of timeout events all with * the same timeout value. * * Since all our timeout handling code assumes timevals can be copied, * assigned, etc, we can't use "magic pointer" to encode these common * timeouts. Searching through a list to see if every timeout is common could * also get inefficient. Instead, we take advantage of the fact that tv_usec * is 32 bits long, but only uses 20 of those bits (since it can never be over * 999999.) We use the top bits to encode 4 bites of magic number, and 8 bits * of index into the event_base's aray of common timeouts. */ #define MICROSECONDS_MASK COMMON_TIMEOUT_MICROSECONDS_MASK #define COMMON_TIMEOUT_IDX_MASK 0x0ff00000 #define COMMON_TIMEOUT_IDX_SHIFT 20 #define COMMON_TIMEOUT_MASK 0xf0000000 #define COMMON_TIMEOUT_MAGIC 0x50000000 #define COMMON_TIMEOUT_IDX(tv) \ (((tv)->tv_usec & COMMON_TIMEOUT_IDX_MASK)>>COMMON_TIMEOUT_IDX_SHIFT) /** Return true iff if 'tv' is a common timeout in 'base' */ static inline int is_common_timeout(const struct timeval *tv, const struct event_base *base) { int idx; if ((tv->tv_usec & COMMON_TIMEOUT_MASK) != COMMON_TIMEOUT_MAGIC) return 0; idx = COMMON_TIMEOUT_IDX(tv); return idx < base->n_common_timeouts; }
32位定时值tv->tv_usec高四位为幻数，该定时器的幻数不等于0101b时，定时器是时间堆定时器；
idx = COMMON_TIMEOUT_IDX取出定时器的20-27位（由于定时器不超过999999，只用了0-19位，因此20-31位可以用于铺助功能，这个是个编程技巧），判断idx < Reactor对象中的n_common_timeouts，该变量保存了当前Reactor对象中通用定时器队列的长度，若idx在长度范围内，则为通用定时器，否则为时间堆定时器。