Guava's cache does not evict elements reliably by
time. Configure a cache to have a lifetime of n
seconds, then you cannot expect that an element is
actually evicted after n seconds with Guava.
Calling Instant.now() several hundred thousand times per
second can be expensive. In my measurements >10% of the
time spend when loading new data was spend calling
Instant.now().
Fixed this by storing an Instant as static member and
updating it periodically in a separate thread.
In the last commit I added a lastAccessMap to the HotEntryCache.
This map made it much more efficient to evict entries. But it
also made and put and get operation much more expensive. Overall
that change lead to a 65% decrease in ingestion performance of
the PerformanceDB.
Fixed by removing the map again. Eviction has to look at all
elements again.
Instead of spawning a new thread for every cache, we use a single thread
that will evict entries from all caches.
The thread keeps a weak reference to the caches, so that they can be
garbage collected.
