cat /proc/sys/net/ipv4/tcp_tw_reuse 0
cat /proc/sys/net/ipv4/tcp_tw_recycle 0
cat /proc/sys/net/ipv4/tcp_timestamps 1

https://www.cnblogs.com/alchemystar/p/13801368.html

#define TCP_TIMEWAIT_LEN (60*HZ) /* how long to wait to destroy TIME-WAIT* state, about 60 seconds */

tcp_v4_rcv|->tcp_timewait_state_process/* 将time_wait状态的socket链入时间轮|->inet_twsk_schedule

/* 这边TCP_TIMEWAIT_LEN 60 * HZ */
inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,TCP_TIMEWAIT_LEN);

void inet_twsk_schedule(struct inet_timewait_sock *tw,struct inet_timewait_death_row *twdr,const int timeo, const int timewait_len)
{......// 计算时间轮的slotslot = (timeo + (1 << INET_TWDR_RECYCLE_TICK) - 1) >> INET_TWDR_RECYCLE_TICK;......// 慢时间轮的逻辑，由于没有开启TCP_TW_RECYCLE,timeo总是60*HZ(60s)// 所有都走slow_timer逻辑 if (slot >= INET_TWDR_RECYCLE_SLOTS) {/* Schedule to slow timer */if (timeo >= timewait_len) {slot = INET_TWDR_TWKILL_SLOTS - 1;} else {slot = DIV_ROUND_UP(timeo, twdr->period);if (slot >= INET_TWDR_TWKILL_SLOTS)slot = INET_TWDR_TWKILL_SLOTS - 1;}tw->tw_ttd = jiffies + timeo;// twdr->slot当前正在处理的slot// 在TIME_WAIT_LEN下，这个逻辑一般7slot = (twdr->slot + slot) & (INET_TWDR_TWKILL_SLOTS - 1);list = &twdr->cells[slot];} else{// 走短时间定时器，由于篇幅原因，不在这里赘述......}....../* twdr->period 60/8=7.5 */if (twdr->tw_count++ == 0)mod_timer(&twdr->tw_timer, jiffies + twdr->period);spin_unlock(&twdr->death_lock);
}

/*参数中的tcp_death_row即为承载时间轮处理函数的结构体*/
inet_twsk_schedule(tw,&tcp_death_row,TCP_TIMEWAIT_LEN,TCP_TIMEWAIT_LEN)
/* 具体的处理结构体 */
struct inet_timewait_death_row tcp_death_row = {....../* slow_timer时间轮处理函数 */.tw_timer = TIMER_INITIALIZER(inet_twdr_hangman, 0,(unsigned long)&tcp_death_row),/* slow_timer时间轮辅助处理函数*/.twkill_work   = __WORK_INITIALIZER(tcp_death_row.twkill_work,inet_twdr_twkill_work),/* 短时间轮处理函数 */.twcal_timer   = TIMER_INITIALIZER(inet_twdr_twcal_tick, 0,(unsigned long)&tcp_death_row),
};

void inet_twdr_hangman(unsigned long data)
{struct inet_timewait_death_row *twdr;unsigned int need_timer;twdr = (struct inet_timewait_death_row *)data;spin_lock(&twdr->death_lock);if (twdr->tw_count == 0)goto out;need_timer = 0;// 如果此slot处理的time_wait socket已经达到了100个，且还没处理完if (inet_twdr_do_twkill_work(twdr, twdr->slot)) {twdr->thread_slots |= (1 << twdr->slot);// 将余下的任务交给work queue处理schedule_work(&twdr->twkill_work);need_timer = 1;} else {/* We purged the entire slot, anything left?  */// 判断是否还需要继续处理if (twdr->tw_count)need_timer = 1;// 如果当前slot处理完了，才跳转到下一个slottwdr->slot = ((twdr->slot + 1) & (INET_TWDR_TWKILL_SLOTS - 1));}// 如果还需要继续处理，则在7.5s后再运行此函数if (need_timer)mod_timer(&twdr->tw_timer, jiffies + twdr->period);
out:spin_unlock(&twdr->death_lock);
}

注: 小时间轮(tw_cal)在tcp_tw_recycle开启的情况下会使用

注: 这里的60/8为什么需要精确到小数，而不是7。
因为实际计算的时候是拿60*HZ进行计算，
如果HZ是1024的话，那么period应该是7680,即精度精确到ms级。
所以在本文中计算的时候需要精确到小数。

public class TimeWaitSimulator {public static void main(String[] args) {double delta = (60) * 1.0 / 8;// 0表示开始清理，1表示清理完毕// 清理完毕之后slot向前推进int startPurge = 0;double sum = 0;int slot = 0;while (slot < 8) {if (startPurge == 0) {sum += delta;startPurge = 1;if (slot == 7) {// 因为假设进入work_queue之后，很快就会清理完// 所以在slot为7的时候并不需要等最后的那个purge过程7.5sSystem.out.println("slot " + slot + " has reach the last " + sum);break;}}if (startPurge == 1) {sum += delta;startPurge = 0;System.out.println("slot " + "move to next at time " + sum);// 清理完之后，slot才应该向前推进slot++;}}}
}

slot move to next at time 15.0
slot move to next at time 30.0
slot move to next at time 45.0
slot move to next at time 60.0
slot move to next at time 75.0
slot move to next at time 90.0
slot move to next at time 105.0
slot 7 has reach the last  112.5

enum tcp_tw_status
tcp_timewait_state_process(struct inet_timewait_sock *tw, struct sk_buff *skb,const struct tcphdr *th)
{......if (paws_reject)NET_INC_STATS_BH(twsk_net(tw), LINUX_MIB_PAWSESTABREJECTED);if (!th->rst) {/* In this case we must reset the TIMEWAIT timer.** If it is ACKless SYN it may be both old duplicate* and new good SYN with random sequence number <rcv_nxt.* Do not reschedule in the last case.*//* 如果有回绕校验失败的包到达的情况下,或者其实ack包 * 重置定时器到新的60s之后* /if (paws_reject || th->ack)inet_twsk_schedule(tw, &tcp_death_row, TCP_TIMEWAIT_LEN,TCP_TIMEWAIT_LEN);/* Send ACK. Note, we do not put the bucket,* it will be released by caller.*//* 向对端发送当前time wait状态应该返回的ACK */return TCP_TW_ACK;}inet_twsk_put(tw);/* 注意，这边通过paws校验的包，会返回tcp_tw_success,使得time_wait状态的* socket五元组也可以三次握手成功重新复用* /return TCP_TW_SUCCESS;
}