Jan 29 15:47:09 topjishu kernel: __ratelimit: 1729 callbacks suppressed Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:09 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: __ratelimit: 1805 callbacks suppressed Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow Jan 29 15:47:14 topjishu kernel: TCP: time wait bucket table overflow
__ratelimit: N callbacks suppressed表示内核阻止了N条syslog消息，这是因为系统重复的日志过多（频率过高），太快输出，被内核中的net_ratelimit()限制了syslog消息。
源码参考:http://fxr.watson.org/fxr/source/lib/ratelimit.c?v=linux-2.6
这个rate limit也是Linux为了避免DoS攻击的一种机制，避免每个消息都被记录（会导致存储空间撑爆）。当内核记录消息，使用printk()通过这种机制来检查是否输出日志。
这个限制可以通过/proc/sys/kernel/printk_ratelimit和/proc/sys/kernel/printk_ratelimit_burst来调优。默认配置（RHEL6）分别是5和10。也就是说，内核允许每5秒记录10条消息。超过这个限制，内核就会抛弃日志，并记录ratelimit N: callbacks suppressed。
[root@web_server_01 ~]# cat /proc/sys/kernel/printk_ratelimit 5 [root@web_server_01 ~]# cat /proc/sys/kernel/printk_ratelimit_burst 10 [root@web_server_01 ~]#
然而，在内核的网络代码中有自己的限制配置（逻辑相同，但是是独立的配置） /proc/sys/net/core/message_cost和/proc/sys/net/core/message_burst，默认配置也是5和10。这里message_cost也是日志采样时间。
[root@web_server_01 ~]# cat /proc/sys/net/core/message_cost 5 [root@web_server_01 ~]# cat /proc/sys/net/core/message_burst 10 [root@web_server_01 ~]#
如果要关闭ratelimit机制，也就是允许每个消息都记录下来，则可以设置message_cost值为0
sysctl -w net.core.message_cost=0
不过，一旦关闭ratelimit，系统就可能存在被日志攻击的风险。