1. A computer program product embodied on tangible media, the computer program product comprising instructions operable to cause data processing apparatus to: store a first item in a cache with an indication of a process to generate the first item; schedule a validity period for the first item, wherein: the validity period is optimized to be less than a period of time after which the first item would be promoted from a first generation of the cache to a second generation of the cache, and invalid objects in the first generation of the cache are freed from memory more frequently than invalid objects in the second generation of the cache; and purge the first item from the cache when the validity period has expired.