java concurrency: many writers, one reader

Question

I need to gather some statistics in my software and i am trying to make it fast and correct, which is not easy (for me!)

first my code so far with two classes, a StatsS

Answer 1

If we ignore the harvesting part and focus on the writing, the main bottleneck of the program is that the stats are locked at a very coarse level of granularity. If two threads want to update different keys, they must wait.

If you know the set of keys in advance, and can preinitialize the map so that by the time an update thread arrives the key is guaranteed to exist, you would be able to do locking on the accumulator variable instead of the whole map, or use a thread-safe accumulator object.

Instead of implementing this yourself, there are map implementations that are designed specifically for concurrency and do this more fine-grained locking for you.

One caveat though are the stats, since you would need to get locks on all the accumulators at roughly the same time. If you use an existing concurrency-friendly map, there might be a construct for getting a snapshot.