code: protected final void updateLastNotReadyTime()
    {
      if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) 
        SibTr.entry(this, tc, "updateLastNotReadyTime");
      if (readAhead)
      { // only relevant when prefetching
        synchronized (this)
        {
          // it is possible that both the CP and the RCD try to transition to !ready concurrently.
          // we only remember the time from the first transition.
          if (!usable)
          {
            lastNotReadyTime = System.currentTimeMillis();
            usable = true;
          }
        }
      }
      if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) 
        SibTr.exit(this, tc, "updateLastNotReadyTime");
    }
docs: Update the time when the consumer became not ready