01.介绍：
ab工具创建多个并发访问线程，模拟多个访问者同时对同一个URL进行访问的场景。测试基于URL，那么也可以对tomcat、nginx、httpd、IIS等web服务器进行测试。

02.安装ab工具
# yum install -y httpd-tools

03.性能指标
吞吐率（Requests per second）：Request per second=Complete requests/Time taken for tests；
并发连接数（The number of concurrent connections）：服务器某一时刻接受的请求数；
并发用户数（Concurrency Level）：一个用户可能同时会产生多个会话，也即连接数，多个用户同时连接则产生多个并发连接，那么并发用户数则要除以基数；
用户平均请求等待时间（Time per request）：Time per request=Time taken for tests/（Complete requests/Concurrency Level）；
服务器平均请求等待时间（Time per request:across all concurrent requests）：Time taken for/testsComplete requests。

04.测试
# ab -c80 -n500 http://www.mairoot.com/是使用最多的，-c表示并发用户数，-n表示总请求数，即是80用户一起访问500次的请求。

# ab -c80 -n500 http://www.mairoot.com/ 
This is ApacheBench, Version 2.3 <$Revision: 655654 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking www.mairoot.com (be patient)
Completed 100 requests
Completed 200 requests
Completed 300 requests
Completed 400 requests
Completed 500 requests
Finished 500 requests



Server Software: 
Server Hostname: www.mairoot.com
Server Port: 80

Document Path: /
Document Length: 0 bytes

Concurrency Level: 80
Time taken for tests: 89.798 seconds
Complete requests: 500
Failed requests: 356
 (Connect: 0, Receive: 0, Length: 356, Exceptions: 0)
Write errors: 0
Non-2xx responses: 367
Total transferred: 200749 bytes
HTML transferred: 88814 bytes
Requests per second: 5.57 [#/sec] (mean)
Time per request: 14367.709 [ms] (mean)
Time per request: 179.596 [ms] (mean, across all concurrent requests)
Transfer rate: 2.18 [Kbytes/sec] received

Connection Times (ms)
 min mean[+/-sd] median max
Connect: 38 46 5.5 44 66
Processing: 1876 13733 10671.3 10226 52514
Waiting: 0 7199 6669.8 5861 25739
Total: 1920 13778 10672.5 10270 52570

Percentage of the requests served within a certain time (ms)
 50% 10270
 66% 14802
 75% 18309
 80% 20983
 90% 27202
 95% 38675
 98% 46497
 99% 50857
 100% 52570 (longest request)
#