ApacheBench (ab is the real program file name) is a single-threaded command line computer program used for benchmarking (measuring the performance of) HTTP web servers.
[2] This will execute 10000 HTTP GET requests, processing up to 10 requests concurrently, to the specified URL, in this example, http://localhost/index.html which requires that a web server is running on port 80 of the same computer where ab is run.
Note that ApacheBench will only use one operating system thread regardless of the concurrency level (specified by the -c parameter).
In some cases, especially when benchmarking high-capacity servers, a single instance of ApacheBench can itself be a bottleneck.
[4] It is usually not correctly categorised by web server log analysers such as Webalizer or AWStats, so running ApacheBench with a great number of requests may skew the results of the reports generated by these programs.