I don't get it. If the ring structure has 4 field (excluding the cold ones), and all 4 have alignment constraints, doesn't it define 4 zones in 4 cache lines, and not 2?

I expected head and cached_tail, for example, to share the same line, but since they are both 128-byte aligned, they end up in separate lines. Granted, both lines are dedicated to only one side of the producer-consumer pipe, but we're wasting cache lines for nothing.