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
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