public double utilization(long t0, long t1) {
            long start = _start.getAndSet(t1);
            if (start == 0) {
                _start.compareAndSet(t1, 0);
            }
            long active = _totalDuration.getAndSet(0) + (start == 0 ? 0 : t1 - start);
            long total = t1 - Math.max(t0, _birth);

            return (double) active / (double) total;
        }