http://httpd.apache.org/docs/2.0/programs/ab.html
在网站开发中，有时候需要对网站进行性能测试，知道网站在一定的并发访问下的响应时间等情况，这里介绍Apache的一个工具，Apache Benchmark，可以比较方便的对网站做一些性能测试
简介:
Apache Benchmark是Apache的http服务器httpd的附带软件，到http://httpd.apache.org/下载httpd服务器，安装完成后，会在bin目录下找到ab.exe，
运行ab –h，可以看到ab的完整说明如下
Usage: ab [options] [http://]hostname[:port]/path
Options are:
-n requests Number of requests to perform
-c concurrency Number of multiple requests to make
-t timelimit Seconds to max. wait for responses
-b windowsize Size of TCP send/receive buffer, in bytes
-p postfile File containing data to POST. Remember also to set -T
-u putfile File containing data to PUT. Remember also to set -T
-T content-type Content-type header for POSTing, eg.
‘application/x-www-form-urlencoded’
Default is ‘text/plain’
-v verbosity How much troubleshooting info to print
-w Print out results in HTML tables
-i Use HEAD instead of GET
-x attributes String to insert as table attributes
-y attributes String to insert as tr attributes
-z attributes String to insert as td or th attributes
-C attribute Add cookie, eg. ‘Apache=1234. (repeatable)
-H attribute Add Arbitrary header line, eg. ‘Accept-Encoding: gzip’
Inserted after all normal header lines. (repeatable)
-A attribute Add Basic WWW Authentication, the attributes
are a colon separated username and password.
-P attribute Add Basic Proxy Authentication, the attributes
are a colon separated username and password.
-X proxy:port Proxyserver and port number to use
-V Print version number and exit
-k Use HTTP KeepAlive feature
-d Do not show percentiles served table.
-S Do not show confidence estimators and warnings.
-g filename Output collected data to gnuplot format file.
-e filename Output CSV file with percentages served
-r Don’t exit on socket receive errors.
-h Display usage information (this message)
对于最基本的使用，假设本地有一个http://localhost/phptest.php显示phpinfo，可以使用-c和-n参数，
运行如下命令测试当并发请求是10个,总共请求是100个的时候的性能数据，
ab -n 1000 -c 100 http://localhost/phptest.php
该测试在我本机结果如下：
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 localhost (be patient)
Server Software: Apache/2.2.14
Server Hostname: localhost
Server Port: 80
Document Path: /phptest.php
Document Length: 43931 bytes
Concurrency Level: 100
Time taken for tests: 7.563 seconds
Complete requests: 1000
Failed requests: 0
Write errors: 0
Total transferred: 44099000 bytes
HTML transferred: 43931000 bytes
Requests per second: 132.23 [#/sec] (mean)
Time per request: 756.250 [ms] (mean)
Time per request: 7.563 [ms] (mean, across all concurrent requests)
Transfer rate: 5694.60 [Kbytes/sec] received
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 4 16.3 0 469
Processing: 203 706 247.4 656 1547
Waiting: 172 381 168.9 328 1125
Total: 203 710 248.1 656 1563
Percentage of the requests served within a certain time (ms)
50% 656
66% 688
75% 688
80% 719
90% 1172
95% 1375
98% 1422
99% 1453
100% 1563 (longest request)
结果比较简单，重点是红色部分，意思是说在100个并发请求下，完成1000个请求，总共需要7.5秒，1000个请求全部完成，没有失败；其中，平均用时710毫秒，标准差是248毫秒，中位数【就是50%请求时间】是656毫秒，90%请求时间是1172毫秒，就是1.172秒，最慢的请求用了1.563秒；
基本上，可以通过调整这-n和-c两个参数来确定系统可以支撑的并发量。
另外，如果是动态页面，比如需要登陆以后才能看到的页面，可以先在Firefox中打开页面，然后通过firebug查看session相关的Cookie，然后使用-C的参数指定，就可以假定用户登陆了：
ab -c 100 -n 1000 -C JSESSIONID=1CBDB188AB4C901C2C3DC2C5BAEA1F47 http://localhost:9080/myapp
当然，ab还有很多缺陷，比如不能模拟多种不同类型的用户做不同的操作，不能模拟实际的用户思考时间等，缺乏图形化的工具显示结果等等。其他有些工具克服了这些缺陷。