Long overflow in cache statistics

We use Guava caches inside a large enterprise application.

After a few days of runtime calling the stats() method on heavily-used caches throws an IllegalArgumentException at the equivalent line to CacheStats.java:87, presumably because the total load time has overflowed a long and looped back round to a negative value.

It looks like this is down to the load time stopwatch being started at the point that an entry is submitted for asynchronous refresh, i.e. added to the refresh queue. Our largest queues are drained in batches and end-up with usual sizes in the tens of thousands — so the nanosecond-precision load time increases very quickly under these circumstances.

I am aware that it is a design decision for the cache statistics to be monotonically increasing, so a reasonably sane solution might be to simply drop the non-negative validation as part of the CacheStats constructor, or try to somehow wrap back to zero instead of Long.MIN_VALUE. Metrics systems using similar to Graphite’s nonNegativeDerivative should handle that gracefully anyway.

NB: I think this issue also affects Caffeine caches in the same way.

[ben-manes]

LongAccumulator would work, as you can push the saturatedAdd as the accumulation function.

Class {@link LongAdder} provides analogs of the functionality of this class for the common special case of maintaining counts and sums. The call {@code new LongAdder()} is equivalent to {@code new LongAccumulator((x, y) -> x + y, 0L)}.

I think if we consider overflow non-deterministic and only promise to avoid exceptions, then it is not our responsibility to make counts correct. That means either is okay, and better handling is the user’s responsibility by providing an alternative StatsCounter (note - Guava doesn’t allow that currently).

[lowasser]

We could possibly use LongAccumulator with LongMath.saturatedAdd?