For channels and other things in the local OS process, you should have a good idea. Give or take being in major swap problems, etc., but I think of that as an "

For channels and other things in the local OS process, you should have a good idea. Give or take being in major swap problems, etc., but I think of that as an "already lost" situation; at that point the problem isn't the expense of moving things between threads or local processes anyhow. When you get out into discussing network matters, it gets a lot fuzzier. You may "know" that you're only .025ms away from something in some specific system, but your TCP-based API doesn't necessarily "know" that and will happily use the exact same code to do something that takes 4 orders of magnitude larger. That's enough difference to be problematic.