1. A shared cache memory system, comprising: a plurality of cache subdivisions; a plurality of Quality of Service identifier (QoSID) tracking tags each associated with a cache subdivision of the plurality of cache subdivisions; and a cache usage monitor configured to: associate each cache subdivision of the plurality of cache subdivisions with a QoS class of a plurality of QoS classes; receive a memory access request comprising a QoSID; access a cache subdivision corresponding to the memory access request among the plurality of cache subdivisions; determine whether the QoSID of the memory access request corresponds to the QoS class associated with the cache subdivision; update a QoSID tracking tag of the plurality of QoSID tracking tags associated with the cache subdivision corresponding to the memory access request based on the determination; and generate an approximate usage measurement for the QoS class of the plurality of QoS classes based on the plurality of QoSID tracking tags; wherein the cache usage monitor is configured to generate the approximate usage measurement for the QoS class of the plurality of QoS classes based on the plurality of OoSID tracking tags by being configured to: determine a fraction of the plurality of cache subdivisions associated with the QoS class: determine a count of the plurality of cache subdivisions associated with the QoS class for which the associated plurality of QoSID tracking tags is set: and divide the count of the plurality of cache subdivisions by the fraction to generate the approximate usage measurement.