In a complex system it isn't always easy to reason about the behavior. Even if you fully understand the cpu, the compiler, the kernel, the disks etc. the way th

In a complex system it isn't always easy to reason about the behavior. Even if you fully understand the cpu, the compiler, the kernel, the disks etc. the way they interact in a real application isn't easy to hold in your head or to deal with using theoretical tools. I've spent a lot of time optimizing video codecs and while it is maybe possible to reason about these without a profiler it's much faster to profile. The profiler isn't going to tell you what you need to do though.