模拟一下在BIO模式下，当线程利用率超过75%时，将自动关闭keep-alive的场景。
通过curl命令来观察，默认情况下curl会开启keep-alive选项，不过注意curl复用socket的话是在同一进程内多次访问目标同一地址时才会复用，两次执行curl的话并不会复用，比如：
$ curl http://localhost:7001/main
$ curl http://localhost:7001/main
上面连续执行curl命令并不会复用socket，socket会随着进程的消失而关闭，下次新的进程会重新创建连接。可以通过tcpdump观察，上面两次连接是不同的socket:
$ sudo tcpdump -l -i lo0 port 7001
23:43:19.236948 IP6 localhost.62625 > localhost.afs3-callback
......
23:43:26.071504 IP6 localhost.62626 > localhost.afs3-callback
......
在同一个curl进程里多次访问同一地址的话，会复用socket，通过-v参数就可以观察到：
$ curl -v http://localhost:7001/main http://localhost:7001/main
* Hostname was NOT found in DNS cache
* Trying ::1...
* Connected to localhost (::1) port 7001 (#0)
> GET /main HTTP/1.1
> User-Agent: curl/7.37.1
> Host: localhost:7001
> Accept: */*
>
< HTTP/1.1 200 OK
* Server Apache-Coyote/1.1 is not blacklisted
< Server: Apache-Coyote/1.1
< Transfer-Encoding: chunked
< Date: Mon, 18 Aug 2014 15:49:28 GMT
<
* Connection #0 to host localhost left intact
ok
* Found bundle for host localhost: 0x7fa7d8c08c50
* Re-using existing connection! (#0) with host localhost
* Connected to localhost (::1) port 7001 (#0)
> GET /main HTTP/1.1
> User-Agent: curl/7.37.1
> Host: localhost:7001
> Accept: */*
>
< HTTP/1.1 200 OK
* Server Apache-Coyote/1.1 is not blacklisted
< Server: Apache-Coyote/1.1
< Transfer-Encoding: chunked
< Date: Mon, 18 Aug 2014 15:49:28 GMT
<
* Connection #0 to host localhost left intact
ok
注意，在第二次请求开头有一句：Re-using existing connection! 表明复用了上次的socket，使用tcpdump也会看到确实是同一个socket端口连的tomcat。
默认情况下，线程池的最大线程数是200个，BIO模式下当线程利用率超过75%的时候，server会对新来的连接不再使用keep-alive。我们先模拟建立151个连接(默认开启keep-alive的)：
for i in {1..151}; do
(
{echo -ne "GET /main HTTP/1.1\nhost: localhost:7001\n\n"; sleep 20}
| telnet localhost 7001
)&;
done
上面的zsh脚本模拟了151个连接(脚本里for循环里使用后台子进程方式启动模拟任务，通过jobs命令也可查看到)，每次新建立socket并在服务器端响应后保持连接20秒(这也是服务器端默认keep-alive的超时时间)。tomcat对这151个连接保持keep-alive，BIO模式下会有151个线程耗在上面，即使socket上请求已处理完，后续没有新的请求也不会让出线程，而一直阻塞在上面。这时刚好达到了 151/200 ≈ 0.75 的临界值，那么后续建立的socket将不能再享用keep-alive
现在在这个临界值上，再执行curl命令模拟新的连接：
$ curl -v http://localhost:7001/main http://localhost:7001/main
* Hostname was NOT found in DNS cache
* Trying ::1...
* Connected to localhost (::1) port 7001 (#0)
> GET /main HTTP/1.1
> User-Agent: curl/7.37.1
> Host: localhost:7001
> Accept: */*
>
< HTTP/1.1 200 OK
* Server Apache-Coyote/1.1 is not blacklisted
< Server: Apache-Coyote/1.1
< Transfer-Encoding: chunked
< Date: Mon, 18 Aug 2014 15:22:50 GMT
< Connection: close
<
* Closing connection 0
ok
* Hostname was found in DNS cache
* Trying ::1...
* Connected to localhost (::1) port 7001 (#1)
> GET /main HTTP/1.1
> User-Agent: curl/7.37.1
> Host: localhost:7001
> Accept: */*
>
< HTTP/1.1 200 OK
* Server Apache-Coyote/1.1 is not blacklisted
< Server: Apache-Coyote/1.1
< Transfer-Encoding: chunked
< Date: Mon, 18 Aug 2014 15:22:50 GMT
< Connection: close
<
* Closing connection 1
ok
注意，这次连接有2次请求，但看不到Re-using existing connection! 关键字，每次请求结束，服务器都显式的关闭了连接，即在header里看到的：Connection: close字段。表明超过75%之后，新建立的连接都不会再使用keep-alive。
