why not just synchronize on the arena, itself?

because we want to minimize the overhead as much as possible. With this it is possible to allocate out of different PoolSubPage pools concurrently ;)

These are different for different requested sizes

OK as long as you're confident that this will fix the race, LGTM

yes I am :) You can also run the test-case that is included. It will fail before and now pass :)

we are still vulnerable to overflow here. Also why switch to currentTimeMillis?

Why are we vulnerable to overflow ? If we not have enough left after adding stuff to to System.currentTimeMillis() I think we have bigger problems.

@Scottmitch actually I think we not even need this.... Let me update

Maybe we do have bigger problems, but lets not make those problems worse 😄

We could use something like this:

long start = nanoTime();
long expireTime = // 30 seconds...or w/e you need

while (!isExpired(start, expireTime)) {
...
}

private static boolean isExpired(long start, long expireTime) {
  return nanoTime() - start > expireTime;
}