I don’t get it. Both readIdx_ and writeIdxCached_ are “owned” by the consumer. (That being the point of a “cached” copy.) The producer might read readIdx_ occasionally, whenever we go through the whole buffer, but otherwise only the consumers access these. So why should we avoid putting them on the same cache line?

Yeap. The linked cpp ref doc has a nice example:

https://en.cppreference.com/w/cpp/thread/hardware_destructiv...

false sharing is the reason for splitting readIdx and writeIdx. But there is no reason to split readIdx and writeIdxCached.