I support a fair number of Apache web server instances, and periodically need to measure the time it takes Apache (and it's various modules) to process a request. On Solaris 10 hosts, I can use DTrace to retrieve this information on the fly. Since Solaris 9 and CentOS and Redhat Linux don't come with DTrace, I use a different approach on these platforms.
To get the time when each request was received by Apache, I used mod_header's "Header" directive, and "%t" option (time when a request was received, measured in milliseconds from the epoch), to add a response header with the time each request was received:
Header set X-Request-Received: %t
To get the total time Apache spent processing a request, I use mod_header's "Header" directive, and "%D" option (milliseconds spent processing the request), to add a response header with the total time Apache spent processing each request:
Header set X-Request-Processing-Time: %D
Since I don't always need the headers to be present, I like to be able to enable and disable them from the command line. The easiest way to do this is by enclosing the directives in a conditional block similar to the following:
`IfDefine RequestTime`
Header set X-Request-Received: %t
Header set X-Request-Processing-Time: %D
</IfDefine>
And using the httpd "-D" option to enable them:
$ httpd -k start -DRequestTime
After the headers are enabled, you will see entries similar to the following in each HTTP response:
$ curl -v http://192.168.1.13:8080/
About to connect() to 192.168.1.13 port 8080
< ..... >
< HTTP/1.1 200 OK
< Date: Tue, 02 Jan 2007 17:59:42TGMT:00-04:00
< Server: Apache/2.2.3 (Unix) DAV/2
< Last-Modified: Sat, 20 Nov 2004 20:16:24 GMT
< ETag: "34d37-2c-4c23b600"
< Accept-Ranges: bytes
< Content-Length: 44
< X-Request-Received: t=1167760782452525
< X-Request-Processing-Time: D=3513
< Content-Type: text/html
Similar capabilities are available for measuring request processing time on the client. Total time is helpful, but knowing how much of that time was consumed by Apache is invaluable!