Fix Version/s: None
It seems to me that KeyCache can be improved in a number of ways, but first let's state the basic goal of KeyCache: to reduce the average query response time by providing an exact seek position in a file for a given key.
As it stands, KeyCache is both 100% accurate, but requires a lot of overhead per entry.
I propose to make KeyCache mostly accurate (say 99.9999%), by which I means it will always fail accurately, but may rarely return an incorrect address, and code the end users of it to be able to retry to confirm they seeked to the correct position in the file, and to retry without the cache if they did not.
The advantage of this is that we can both take the cache off-heap easily, and pack a lot more items into the cache. If we permit collisions across files and simply use the (full 128-bit) murmur hash of the key + cfId + file generation, we should get enough uniqueness to rarely have an erroneuous collision, however we will be using only 20 bytes per key, instead of the current 100 + <key length> bytes. This should allow us to answer far more queries from the key cache than before, so the positive improvement to performance should be greater than the negative drain.
For the structure I propose an associative cache, where a single contiguous address space is broken up into regions of, say, 8 entries, plus one counter. The counter tracks the recency of access of each of the entries, so that on write the least recently accessed/written can be replaced. A linear probe within the region is used to determine if the entry we're looking for is present. This should be very quick, as the entire region should fit into one or two lines of L1.
Advantage: we may see 5x bump in cache hit-rate, or even more for large keys.