Right now each thread creates its own (thread private) SimpleLRUCache,
holding up to 1024 terms.
This is rather wasteful, since if there are a high number of threads
that come through Lucene, you're multiplying the RAM usage. You're
also cutting way back on likelihood of a cache hit (except the known
multiple times we lookup a term within-query, which uses one thread).
In NRT search we open new SegmentReaders (on tiny segments) often
which each thread must then spend CPU/RAM creating & populating.
Now that we are on 1.5 we can use java.util.concurrent.*, eg
ConcurrentHashMap. One simple approach could be a double-barrel LRU
cache, using 2 maps (primary, secondary). You check the cache by
first checking primary; if that's a miss, you check secondary and if
you get a hit you promote it to primary. Once primary is full you
clear secondary and swap them.
Or... any other suggested approach?