Recently (2022) I designed a ring buffer for use as a 'flight recorder' type tracing system. (I.e., there is typically no reader; the writer needs to write over old records without blocking on any reader. If the reader is triggered, it flips a switch that disables the writer temporarily while the buffer contents are persisted.) In that design I subdivided the ring into several subbuffers (~8). Each subbuffer has its own equivalent of a watermark. That way, the valid portion of the ring always starts at the beginning of one of the subbuffers, and the writer could 'free' the next subbuffer worth of space trivially (without having to scan through old contents record by record). (Any write that did not fit in the current subbuffer was advanced to the start of the next one.)