I had the needs to filter an application log file (30Mb growing) in which many applications is logging. I’ve not decided this, so don’t hang up with me :).
Well just typing M-x log4j-start-filter <my-list-of-keywords-space-separated> RET and automagically the file is showing only what I need. I can’t live without this functionality.
The counterpart of this macro is M-x log4j-stop-filter.