SOLR-2906 introduced an LFU cache and in-progress SOLR-3393 makes it O(1). The discussions seem to indicate that the higher hit rate (vs LRU) is offset by the slower performance of the implementation. An original goal appeared to be to introduce ARC, a patented algorithm that uses ghost entries to retain history information.
My analysis of Window TinyLfu indicates that it may be a better option. It uses a frequency sketch to compactly estimate an entry's popularity. It uses LRU to capture recency and operate in O(1) time. When using available academic traces the policy provides a near optimal hit rate regardless of the workload.
I'm getting ready to release the policy in Caffeine, which Solr already has a dependency on. But, the code is fairly straightforward and a port into Solr's caches instead is a pragmatic alternative. More interesting is what the impact would be in Solr's workloads and feedback on the policy's design.