listen : backlog

listen

NAME
       listen - listen for connections on a socket

SYNOPSIS
       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       int listen(int sockfd, int backlog);

backlog

       The behavior of the backlog argument on TCP sockets changed with Linux 2.2.  Now it  speci‐
       fies the queue length for completely established sockets waiting to be accepted, instead of
       the number of incomplete connection requests.  The maximum length of the queue  for  incom‐
       plete sockets can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog.  When syncookies are
       enabled there is no logical maximum length and this setting is  ignored.   See  tcp(7)  for
       more information.

       If  the backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it
       is silently truncated to that value; the default value in this file  is  128.   In  kernels
       before 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128.

总结

SYN队列：tcp_max_syn_backlog
ACCEPT队列：min(backlog, somaxconn)

SYN队列和ACCEPT队列参考这里：

http://blog.youkuaiyun.com/russell_tao/article/details/9111769

查看backlog大小

# ss -lt

其他参考

http://blog.youkuaiyun.com/jiangguilong2000/article/details/43097497