http://blog.scoutapp.com/articles/2009/07/31/understanding-load-averages
http://blog.scoutapp.com/articles/2011/02/10/understanding-disk-i-o-when-should-you-be-worried
http://juliano.info/en/Blog%3aMemory_Leak/Understanding_the_Linux_load_average
The CPU usage ratio has very little use for system load and performance evaluation. If you have either one or a hundred threads in tight loops in a single-CPU system, in both cases this metric will just return 100% CPU usage, even though in the latter case the system is a hundred times more loaded.