Sometimes you need to know the Apache PID when trying to track down segfaults or CPU usage issues.
There is a very simple way to do this by altering your LogFormat, a command format to have is the CLF (common log format):
%h %l %u %t \"%r\" %>s %b
To enable logging the pid just add the %P var like so:
[%P] %h %l %u %t \"%r\" %>s %b