(and to the two other responses)

If your workload is already well parallelized, then, yes 20% is quite significant. However, working to parallelize properly over 8 rather than 4 has its own costs.

The thing that bothers me most is that 800% CPU and 500% CPU on this processor are roughly equivalent at 5x100%CPU, it makes everything very hard to reason about when planning capacity.

I think you’re misunderstanding what HT is. It’s not true parallelism, it’s just hiding latency by providing some extra superscalar parallelism. You can’t expect it to give you actual linear improvements in performance because it’s just an illusion.

I understand that very well. But non of the standard tools that manage CPU understand that, and most people don't either.

If I had a nickel for every time I had to explain why "You are at 50% CPU now, but you can't actually run twice as many processes on this machine and get the same runtime", I'd be able to buy a large frapuccino or two at starbucks.

Perhaps I'm uninformed though - is there a tool like htop, which would give me an idea of how close am I to maxing out a CPU?

No there isn’t. But if you understand it I don’t get why you think 20% isn’t a good performance boost, especially considering the rate of return for power and area in silicon.

Because many people believe it is a 100% improvement, plan/budget accordingly, and then look for help.

As far as silicon/power it is nice, but IIRC (I am not involved in purchasing anymore) it used to cost over 50% in USD for those 20% in performance when you non-HT parts were common.

What a strange way to measure the benefits of a performance optimization: "how people will perceive it and then ask me for help".

You ignored the price issue, which was measurable and real, but also:

It (used to be) my job. Does "because people fall for deceptive marketing, waste money, and then waste my time trying to salvage their reputation" sound better?