Scan does not return all the result when regionserver is busy

    try {
      Future<Pair<Result[], ScannerCallable>> f = cs.poll(timeout, TimeUnit.MILLISECONDS);
      if (f != null) {
        Pair<Result[], ScannerCallable> r = f.get(timeout, TimeUnit.MILLISECONDS);
        if (r != null && r.getSecond() != null) {
          updateCurrentlyServingReplica(r.getSecond(), r.getFirst(), done, pool);
        }
        return r == null ? null : r.getFirst(); // great we got an answer
      }
    } catch (ExecutionException e) {
      RpcRetryingCallerWithReadReplicas.throwEnrichedException(e, retries);
    } catch (CancellationException e) {
      throw new InterruptedIOException(e.getMessage());
    } catch (InterruptedException e) {
      throw new InterruptedIOException(e.getMessage());
    } catch (TimeoutException e) {
      throw new InterruptedIOException(e.getMessage());
    } finally {
      // We get there because we were interrupted or because one or more of the
      // calls succeeded or failed. In all case, we stop all our tasks.
      cs.cancelAll();
    }
    return null; // unreachable