An excellent article explains usage of DTrace tool to debug Apache problem.
FTA "...Dynamic Tracing (DTrace) is a new feature in the Solaris 10 OS that is used to better understand the system and programs run on the system. It allows you to look and see what your system is doing and locate causes of poor behavior. This is done by using the "D programming language," which doesn't take much programming experience to understand. In the example, I don't even have to create any DTrace scripts as there is a package of them already made called the DTraceToolkit (by Brendan Gregg), which contains over 80 scripts...". Read complete article online.