# Buggy code
def _on_running(self):
    self._execution_ref = self._get_execution_ref()

    # notify successors to propagate priority changes
    for out_key in self._succ_keys:
        self._get_operand_actor(out_key).add_running_predecessor(
            self._op_key, self.worker, _tell=True, _wait=False
        )

    @log_unhandled
    def _acceptor(data_sizes, data_shapes):
        self._allocated = False
        if not self._is_worker_alive():
            return
        self._resource_ref.deallocate_resource(
            self._session_id, self._op_key, self.worker, _tell=True, _wait=False
        )

        self._data_sizes = data_sizes
        self._data_shapes = data_shapes
        self._io_meta["data_targets"] = list(data_sizes)
        self.start_operand(OperandState.FINISHED)

    @log_unhandled
    def _rejecter(*exc):
        self._allocated = False
        # handling exception occurrence of operand execution
        exc_type = exc[0]
        self._resource_ref.deallocate_resource(
            self._session_id, self._op_key, self.worker, _tell=True, _wait=False
        )

        if self.state == OperandState.CANCELLING:
            logger.warning("Execution of operand %s cancelled.", self._op_key)
            self.free_data(OperandState.CANCELLED)
            return

        if issubclass(exc_type, ExecutionInterrupted):
            # job cancelled: switch to cancelled
            logger.warning("Execution of operand %s interrupted.", self._op_key)
            self.free_data(OperandState.CANCELLED)
        elif issubclass(exc_type, DependencyMissing):
            logger.warning(
                "Operand %s moved to UNSCHEDULED because of DependencyMissing.",
                self._op_key,
            )
            self.ref().start_operand(OperandState.UNSCHEDULED, _tell=True)
        else:
            logger.exception(
                "Attempt %d: Unexpected error %s occurred in executing operand %s in %s",
                self.retries + 1,
                exc_type.__name__,
                self._op_key,
                self.worker,
                exc_info=exc,
            )
            # increase retry times
            self.retries += 1
            if (
                not self._info["retryable"]
                or self.retries >= options.scheduler.retry_num
            ):
                # no further trial
                self.state = OperandState.FATAL
                self._exc = exc
            else:
                self.state = OperandState.READY
            self.ref().start_operand(_tell=True)

    try:
        with rewrite_worker_errors():
            if self._submit_promise is None:
                self._submit_promise = self._execution_ref.add_finish_callback(
                    self._session_id, self._op_key, _promise=True, _spawn=False
                )
            self._submit_promise.then(_acceptor, _rejecter)
    except WorkerDead:
        logger.debug(
            "Worker %s dead when adding callback for operand %s",
            self.worker,
            self._op_key,
        )
        self._resource_ref.detach_dead_workers([self.worker], _tell=True)
    finally:
        self._submit_promise = None


# Traceback (AttributeError)
AttributeError                            Traceback (most recent call last)
<ipython-input-3-a85925f048d0> in <module>
1 start=time.time()
2 df_mars=df_mars.to_gpu()
----> 3 print(df_mars.sum().to_frame(name="sum").execute())

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/core.py in execute(self, session, **kw)
580
581     def execute(self, session=None, **kw):
--> 582         self._data.execute(session, **kw)
583         return self
584

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/core.py in execute(self, session, **kw)
368
369         # no more fetch, thus just fire run
--> 370         session.run(self, **kw)
371         # return Tileable or ExecutableTuple itself
372         return self

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/session.py in run(self, *tileables, **kw)
460         tileables = tuple(mt.tensor(t) if not isinstance(t, (Entity, Base)) else t
461                           for t in tileables)
--> 462         result = self._sess.run(*tileables, **kw)
463
464         for t in tileables:

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/session.py in run(self, *tileables, **kw)
105             # set number of running cores
106             self.context.set_ncores(kw['n_parallel'])
--> 107             res = self._executor.execute_tileables(tileables, **kw)
108             return res
109

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/utils.py in _wrapped(*args, **kwargs)
406                 _kernel_mode.eager = False
407             _kernel_mode.eager_count = enter_eager_count + 1
--> 408             return func(*args, **kwargs)
409         finally:
410             _kernel_mode.eager_count -= 1

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/utils.py in inner(*args, **kwargs)
500     def inner(*args, **kwargs):
501         with build_mode():
--> 502             return func(*args, **kwargs)
503     return inner
504

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/executor.py in execute_tileables(self, tileables, fetch, n_parallel, n_thread, print_progress, mock, compose, name)
878                                    n_parallel=n_parallel or n_thread,
879                                    print_progress=print_progress, mock=mock,
--> 880                                    chunk_result=chunk_result)
881
882                 # update shape of tileable and its chunks whatever it's successful or not

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/executor.py in execute_graph(self, graph, keys, n_parallel, print_progress, mock, no_intermediate, compose, retval, chunk_result)
688             print_progress=print_progress, mock=mock, mock_max_memory=self._mock_max_memory,
689             fetch_keys=fetch_keys, no_intermediate=no_intermediate)
--> 690         res = graph_execution.execute(retval)
691         self._mock_max_memory = max(self._mock_max_memory, graph_execution._mock_max_memory)
692         if mock:

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/executor.py in execute(self, retval)
569         # wait until all the futures completed
570         for future in executed_futures:
--> 571             future.result()
572
573         if retval:

/opt/conda/envs/rapids/lib/python3.6/concurrent/futures/_base.py in result(self, timeout)
423                 raise CancelledError()
424             elif self._state == FINISHED:
--> 425                 return self.__get_result()
426
427             self._condition.wait(timeout)

/opt/conda/envs/rapids/lib/python3.6/concurrent/futures/_base.py in __get_result(self)
382     def __get_result(self):
383         if self._exception:
--> 384             raise self._exception
385         else:
386             return self._result

/opt/conda/envs/rapids/lib/python3.6/concurrent/futures/thread.py in run(self)
54
55         try:
---> 56             result = self.fn(*self.args, **self.kwargs)
57         except BaseException as exc:
58             self.future.set_exception(exc)

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/executor.py in _execute_operand(self, op)
441             # so we pass the first operand's first output to Executor.handle
442             first_op = ops[0]
--> 443             Executor.handle(first_op, results, self._mock)
444
445             # update maximal memory usage during execution

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/executor.py in handle(cls, op, results, mock)
639                 # The `UFuncTypeError` was introduced by numpy#12593 since v1.17.0.
640                 try:
--> 641                     return runner(results, op)
642                 except UFuncTypeError as e:
643                     raise TypeError(str(e)).with_traceback(sys.exc_info()[2]) from None

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/dataframe/reduction/core.py in execute(cls, ctx, op)
405                 cls._execute_agg(ctx, op)
406             elif op.stage == OperandStage.map:
--> 407                 cls._execute_map(ctx, op)
408             else:
409                 in_data = ctx[op.inputs[0].key]

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/dataframe/reduction/core.py in _execute_map(cls, ctx, op)
380             cls._execute_map_with_count(ctx, op)
381         else:
--> 382             cls._execute_without_count(ctx, op)
383
384     @classmethod

/opt/conda/envs/rapids/lib/python3.6/site-packages/mars/dataframe/reduction/core.py in _execute_without_count(cls, ctx, op, reduction_func)
370                 # cannot just do xdf.DataFrame(r).T
371                 # cuz the dtype will be object since pandas 1.0
--> 372                 df = xdf.DataFrame(OrderedDict((d, [v]) for d, v in r.iteritems()))
373             else:
374                 df = xdf.DataFrame(r)

AttributeError: 'Series' object has no attribute 'iteritems'

# Fixed code
def _on_running(self):
    self._execution_ref = self._get_execution_ref()

    # notify successors to propagate priority changes
    for out_key in self._succ_keys:
        self._get_operand_actor(out_key).add_running_predecessor(
            self._op_key, self.worker, _tell=True, _wait=False
        )

    @log_unhandled
    def _acceptor(data_sizes, data_shapes):
        self._allocated = False
        if not self._is_worker_alive():
            return
        self._resource_ref.deallocate_resource(
            self._session_id, self._op_key, self.worker, _tell=True, _wait=False
        )

        self._data_sizes = data_sizes
        self._data_shapes = data_shapes
        self._io_meta["data_targets"] = list(data_sizes)
        self.start_operand(OperandState.FINISHED)

    @log_unhandled
    def _rejecter(*exc):
        self._allocated = False
        # handling exception occurrence of operand execution
        exc_type = exc[0]
        self._resource_ref.deallocate_resource(
            self._session_id, self._op_key, self.worker, _tell=True, _wait=False
        )

        if self.state == OperandState.CANCELLING:
            logger.warning("Execution of operand %s cancelled.", self._op_key)
            self.free_data(OperandState.CANCELLED)
            return

        if issubclass(exc_type, ExecutionInterrupted):
            # job cancelled: switch to cancelled
            logger.warning("Execution of operand %s interrupted.", self._op_key)
            self.free_data(OperandState.CANCELLED)
        elif issubclass(exc_type, DependencyMissing):
            logger.warning(
                "Operand %s moved to UNSCHEDULED because of DependencyMissing.",
                self._op_key,
            )
            self.ref().start_operand(OperandState.UNSCHEDULED, _tell=True)
        else:
            self.handle_unexpected_failure(*exc)

    try:
        with rewrite_worker_errors():
            if self._submit_promise is None:
                self._submit_promise = self._execution_ref.add_finish_callback(
                    self._session_id, self._op_key, _promise=True, _spawn=False
                )
            self._submit_promise.then(_acceptor, _rejecter)
    except WorkerDead:
        logger.debug(
            "Worker %s dead when adding callback for operand %s",
            self.worker,
            self._op_key,
        )
        self._resource_ref.detach_dead_workers([self.worker], _tell=True)
    finally:
        self._submit_promise = None