Why Apache benchmark results are inconstant?

I run this command on ubuntu: ./ab -n 2000 -c 10 http://local开发者_开发技巧host:7000/index.html and each time I get a different number for "Time per request".(huge difference like once is 0.66 ms next time is 0.17 ms).

Why is the result unstable and how can I measure the actual performance of the Apache server?

If just the first request is slower than the following and the next results tend to be faster than it´s very likely some kind of cache which speeds up the response. In the simplest case this is just the disk cache of the os.

If you´re benchmarking on a virtual machine you probably will not get very credible results:

http://communities.vmware.com/docs/DOC-5581

Benchmarking, Profiling on Virtual Machines

Here some general best practices for benchmarking web servers:

http://www.cyberciti.biz/tips/howto-performance-benchmarks-a-web-server.html