Years ago, with insufficient experience, I was trying to debug a device-driver problem. I used printfs. Stupid.

Eventually I created an in-memory array, updated that with integer data which corresponded to information I wanted, and that was "fast enough" to expose the issue.

Many years later, on a Connection Machine, we discovered that asking for the current day/date would bring all processors to a screeching halt while the system propagated the correct date/time to every processor. Also while trying to profile behavior and performance.

It's not easy.