Performance effect of enabling apache response time log directive

Question

What affect will enabling the response time (%D) LogFormat directive have on apache performance?

The response time is the time taken to serve a request in microseconds.

Response time can be enabled like so:

#LogFormat "%h %l %u %t \"%r\" %>s %b"    # Default LogFormat
LogFormat "%h %l %u %t \"%r\" %>s %b %D"  # LogFormat including response time

I have written a tool for monitoring and analysing backend script performance, apache-response-time, which requires response time data. I would like to warn potential users how enabling this directive will affect apache server performance.

Answer 1

Probably not worth considering in terms of HTTP responses -- only the overhead of 1 addl gettimeofday() call (and a few function calls) so something on the order of a few microseconds. Apache already notes the same thing when the request arrives.

http://www.atl.lmco.com/projects/QoS/POSIX_html/index.html lists some old microbenchmarks of gettimeofday() itself.