Circuitry comprises memory circuitry providing a plurality of memory locations; location selection circuitry to select a set of one or more of the memory locations by which to access a data item according to a mapping relationship between an attribute of the data item and the set of one or more memory locations; the location selection circuitry being configured to initiate an allocation operation for a data item when that data item is to be newly stored by the memory circuitry and the selected set of one or more of the memory locations are already occupied by one or more other data items, the allocation operation comprising an operation to replace at least a subset of the one or more other data items from the set of one or more memory locations by the newly stored data item; and detector circuitry to detect a data access conflict in which a group of two or more data items having different respective attributes are mapped by the mapping relationship to the same set of one or more memory locations; the location selection circuitry being configured to vary the mapping relationship applicable to the attribute of at least one data item of the group of two or more data items in response to a detection by the detector circuitry of a data access conflict.