To Keep or Not to Keep - The Volatility of Replacement Policy Metadata in Hybrid Caches

The field of Non-Volatile Memory (NVM) is one of the most actively researched topics regarding disruptive memory technologies. A promising application of NVM lies in the hybridization of caches, combining both NVM and conventional SRAM to realize trade-offs, as NVM is expected to cause significant write overheads while offering a high density. Hybrid caches have gained importance in the field of intermittent computing on batteryless embedded devices, with non-volatile cache lines generally withstanding power outages. Replacement policies are employed to determine replacement candidates and whether to place data in volatile or non-volatile cache sections. Almost all known replacement policies require some type of metadata. Storing this metadata in NVM may generate additional write overheads as it is frequently accessed, while volatile metadata is lost after power outages. Losing this metadata may lead to unsuitable replacement decisions after regaining power, however it also cancels out any previously accumulated mispredictions. This paper examines the effect of different technological implementations of cache replacement policies on intermittently powered embedded systems employing hybrid caches. Our experiments show that the choice of technology for the replacement policy is an important design decision. Depending on the selected approach to store the metadata of the policies, we observe up to 28% in possible energy savings.