protected SleepingStopwatch() {}

    /*
     * We always hold the mutex when calling this. TODO(cpovirk): Is that important? Perhaps we need
     * to guarantee that each call to reserveEarliestAvailable, etc. sees a value >= the previous?
     * Also, is it OK that we don't hold the mutex when sleeping?
     */
    protected abstract long readMicros();

    protected abstract void sleepMicrosUninterruptibly(long micros);