even ignoring the fact that this isn't counting contention, 18ns is still really high overhead since it's an operation you have to do twice to read any object.

even ignoring the fact that this isn't counting contention, 18ns is still really high overhead since it's an operation you have to do twice to read any object. a read from l1 cache is about 3 cycles (1-3 ns), so 36 ns to increment and decrement a counter is far from trivial.