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400x slower than 70ns is still only 28us. How often is the JVM calling this function?

It depends. If you’re doing continuous profiling, it’d make a call to get the current time at every method entry and exit, each of which could then add a context switch. In an absolute sense it appears to be small, but it could really add up.

This is what flame graphs are super helpful for, to see whether it’s really a problem or not.

Also, remember that every extra moment running instructions is a lost opportunity to put the CPU to sleep, so this has energy efficiency impact as well.

If you are doing continuous profiling, you are probably using a low overhead stack sampling profiler rather than recording every method entry and exit.

That's a fair point. It really depends. For example, if you're recording method run times via an observability SDK at full fidelity, this could be an issue.

If it's calling it twice per function, that's enormously expensive and this is a major win.

28us is still solid amount of time

If it's called once an hour, who cares?

Even called every frame 60 times per second, it's only 0.2% of a 60 fps time budget.

It's not a huge amount of time in absolute terms; only if it's relatively "hot."

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