Tuning HTTP Server

Determining maximum simultaneous connections

The first tuning decision you'll need to make is determining how many simultaneous connections your IBM HTTP Server installation will need to support. Many other tuning decisions are dependent on this value.

For some IBM HTTP Server deployments, the amount of load on the web server is directly related to the typical business day, and may show a load pattern such as the following:

 

 

    Simultaneous

    connections

            |

       2000 |

            |

            |                            **********

            |                        ****          ***

       1500 |                   *****                 **

            |               ****                        ***

            |            ***                               ***

            |           *                                     **

       1000 |          *                                        **

            |         *                                           *

            |         *                                           *

            |        *                                             *

        500 |        *                                             *

            |        *                                              *

            |      **                                                *

            |   ***                                                  ***

          1 |***                                                        **

 Time of    +-------------------------------------------------------------

   day         7am  8am  9am  10am  11am  12am  1pm  2pm  3pm  4pm  5pm

For other IBM HTTP Server deployments, providing applications which are used in many time zones, load on the server varies much less during the day.

The maximum number of simultaneous connections must be based on the busiest part of the day. This maximum number of simultaneous connections is only loosely related to the number of users accessing the site. At any given moment, a single user can require anywhere from zero to four independent TCP connections.

The typical way to determine the maximum number of simultaneous connections is to monitor mod_status reports during the day until typical behavior is understood, or to use mod_mpmstats (2.0.42.2 and later).

Monitoring with mod_status

Add these directives to httpd.conf, or uncomment the ones already there: 2.           # This example is for IBM HTTP Server 2.0 and above 3.           # Similar directives are in older default configuration files. 4.             5.           Loadmodule status_module modules/mod_status.so 6.           7.           SetHandler server-status 8.           Order deny,allow 9.           Deny from all 10.       Allow from .example.com    <--- replace with "." + your domain name 11.       Request the /server-status page (http://www.example.com/server-status/) from the web server at busy times of the day and look for a line like the following: 13.       192 requests currently being processed, 287 idle workers The number of requests currently being processed is the number of simultaneous connections at this time. Taking this reading at different times of the day can be used to determine the maximum number of connections that must be handled.

Monitoring with mod_mpmstats (IBM HTTP Server 2.0.42.2 and later)

Copy the version of mod_mpmstats.so for your operating system from the ihsdiag package to the IBM HTTP Server modules directory. (Example filename: ihsdiag-1.4.1/2.0/aix/mod_mpmstats.so) Add these directives to the bottom of httpd.conf: 3.           LoadModule mpmstats_module modules/mod_mpmstats.so 4.           ReportInterval 90 Check entries like this in the error log to determine how many simultaneous connections were in use at different times of the day: 6.           [Thu Aug 19 14:01:00 2004] [notice] mpmstats: rdy 712 bsy 312 rd 121 wr 173 ka 0 log 0 dns 0 cls 18 7.           [Thu Aug 19 14:02:30 2004] [notice] mpmstats: rdy 809 bsy 215 rd 131 wr 44 ka 0 log 0 dns 0 cls 40 8.           [Thu Aug 19 14:04:01 2004] [notice] mpmstats: rdy 707 bsy 317 rd 193 wr 97 ka 0 log 0 dns 0 cls 27 9.           [Thu Aug 19 14:05:32 2004] [notice] mpmstats: rdy 731 bsy 293 rd 196 wr 39 ka 0 log 0 dns 0 cls 58

Note that if the web server has not been configured to support enough simultaneous connections, one of the following messages will be logged to the web server error log and clients will experience delays accessing the server.

Windows

[warn] Server ran out of threads to serve requests. Consider raising the ThreadsPerChild setting

Linux and Unix

[error] server reached MaxClients setting, consider raising the MaxClients setting

Check the error log for a message like this to determine if the IBM HTTP Server configuration needs to be changed.

Once the maximum number of simultaneous connections has been determined, add 25% as a safety factor. The next section discusses how to use this number in the web server configuration file.

Note: Setting of the KeepAliveTimeout can affect the apparent number of simultaneous requests being processed by the server. Increasing KeepAliveTimeout effectively reduces the number of threads available to service new inbound requests, and will result in a higher maximum number of simultaneous connections which must be supported by the web server. Decreasing KeepAliveTimeout can drive extra load on the server handling unnecessary TCP connection setup overhead. A setting of 5 to 10 seconds is reasonable for serving requests over high speed, low latency networks.