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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.[1] Originally it was used to test the Apache HTTP Server but it is generic enough to test any web server supporting HTTP/1.0 or HTTP/1.1 protocol versions.

The ab tool is written in C and it comes bundled with the standard Apache source distribution, and like the Apache web server itself, is free, open source software and distributed under the terms of the Apache License.


The original program was named zb "ZeusBench V1.0" and it was written by Adam Twiss (Zeus Technology), in 1996, in order to test performance of Zeus Web Server. Soon later Twiss licensed (donated) that program to the Apache Group so that the derived program was renamed ab "ApacheBench".

Since 1997, 1998 ab has been further developed and maintained, as a support program included in Apache HTTP server, from version 1.0 (1997,1998) to version 2.3 (2021) and later.[2]

Excerpt from comments about copyright attributions found in source code of ab program.

    ** This program is based on ZeusBench V1.0 written by Adam Twiss
    ** which is Copyright (c) 1996 by Zeus Technology Ltd. http://www.zeustech.net/

    ** HISTORY:
    **    - Originally written by Adam Twiss <adam@zeus.co.uk>, March 1996
    **      with input from Mike Belshe <mbelshe@netscape.com> and
    **      Michael Campanella <campanella@stevms.enet.dec.com>
    **    - Enhanced by Dean Gaudet <dgaudet@apache.org>, November 1997
    **    - Cleaned up by Ralf S. Engelschall <rse@apache.org>, March 1998
    **    - POST and verbosity by Kurt Sussman <kls@merlot.com>, August 1998
    **    - HTML table output added by David N. Welton <davidw@prosa.it>, January 1999
    **    - Added Cookie, Arbitrary header and auth support. <dirkx@webweaving.org>, April 1999
    ** Version 1.3d
    ** Version 2.3

Example usage[edit]

ab -n 10000 -c 10 "http://localhost/index.html"

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.[3]

For an extended example of ab output see also ab output for Squid performance tuning.

Concurrency versus threads[edit]

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. When using ApacheBench on hardware with multiple processor cores, additional instances of ApacheBench may be used in parallel to more fully saturate the target URL.

Detecting ApacheBench[edit]

The ApacheBench User Agent string is ApacheBench/MAJOR.MINOR where MAJOR and MINOR represent the major and minor version numbers of the program.[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.

See also[edit]


  1. ^ "ab - Apache HTTP server benchmarking tool". Apache. Retrieved 9 October 2014.
  2. ^ "Apache HTTP Server: ab source code, comments". Apache: HTTPd server project. 2021. Retrieved 2021-12-20.
  3. ^ "Using Apache Bench for Simple Load Testing". Pete Freitag's ColdFusion, Java and Web Development Blog. Retrieved 9 October 2014.
  4. ^ "User-agent detail: AB (Apache Bench) output". User-Agent-String-Info. Retrieved 9 October 2014.

External links[edit]