Just a quick note about strace, the ancient Linux system-call tracing tool.

It can trace multiple processes at once: simply start it with multiple -p arguments (the numbers give the processes' PIDs):

sudo strace -p 2916 -p 2929 -p 2930 -p 2931 -p 2932 -o /tmp/strace.log

This is great for tracing daemons which use separate worker processes, for example Apache with mpm-prefork.