1. A method of clearing obsolete entries from a first one of a plurality of mapping caches, each of the plurality of mapping caches being associated with a corresponding one of a plurality of processing units of a computing device, each of the caches being used to translate virtual addresses to physical addresses and storing mappings based on an address translation map, the method comprising: maintaining a counter; updating said counter each time the first one of the plurality of mapping caches is flushed; recording said counter's value in response to a change in the address translation map; determining, based on a comparison of said counter's value with the recorded counter value, that the first one of the plurality of mapping caches has not definitely been flushed since said change in the address translation map occurred; and flushing the first one of the plurality of mapping caches.