Technique for controlling use of a cache to store prefetcher metadata

Abstract

An apparatus comprises prefetch circuitry, and a cache having a plurality of entries to store data for access by processing circuitry and blocks of metadata for reference by the prefetch circuitry. The prefetch circuitry can detect one or more access sequences in dependence on training inputs derived from demand accesses processed by the cache in response to memory access operations performed by the processing circuitry. On detecting a given access sequence, this causes an associated given block of metadata providing information indicative of the given access sequence to be stored in a selected entry of the cache. Eviction control circuitry, responsive to a victimisation event, performs an operation to select a victim entry in the cache, the victim entry being selected from one or more candidate victim entries. Each entry has an associated age indication value used to determine whether that entry is allowed to be a candidate victim entry, and the eviction control circuitry is arranged to perform a dynamic ageing operation to determine an ageing control value used to control updating of the associated age indication value for any entry storing a block of metadata. The dynamic ageing operation is arranged to determine the ageing control value in dependence on at least a training rate indication for the prefetch circuitry, where the training rate indication is indicative of a number of training inputs per memory access operation performed by the processing circuitry.