关于libevent的定时器

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。

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