# Buggy code
def create_operand_actors(self, _clean_io_meta=True, _start=True):
    """
    Create operand actors for all operands
    """
    logger.debug("Creating operand actors for graph %s", self._graph_key)

    chunk_graph = self.get_chunk_graph()
    operand_infos = self._operand_infos

    op_refs = dict()
    initial_keys = []
    for op_key in self._op_key_to_chunk:
        chunks = self._op_key_to_chunk[op_key]
        op = chunks[0].op

        op_name = type(op).__name__
        op_info = operand_infos[op_key]

        io_meta = self._collect_operand_io_meta(chunk_graph, chunks)
        op_info["op_name"] = op_name
        op_info["io_meta"] = io_meta

        if io_meta["predecessors"]:
            state = "UNSCHEDULED"
        else:
            initial_keys.append(op_key)
            state = "READY"
        op_info["retries"] = 0
        op_info["state"] = state

        position = None
        if op_key in self._terminal_chunk_op_tensor:
            position = OperandPosition.TERMINAL
        elif not io_meta["predecessors"]:
            position = OperandPosition.INITIAL

        op_cls = get_operand_actor_class(type(op))
        op_uid = op_cls.gen_uid(self._session_id, op_key)
        scheduler_addr = self.get_scheduler(op_uid)
        op_refs[op_key] = self.ctx.create_actor(
            op_cls,
            self._session_id,
            self._graph_key,
            op_key,
            op_info,
            position=position,
            uid=op_uid,
            address=scheduler_addr,
            wait=False,
        )
        op_info["state"] = getattr(OperandState, state.upper())
        if _clean_io_meta:
            del op_info["io_meta"]

    self.state = GraphState.RUNNING

    if _start:
        op_refs = dict((k, v.result()) for k, v in op_refs.items())
        start_futures = [
            op_refs[op_key].start_operand(_tell=True, _wait=False)
            for op_key in initial_keys
        ]
        [future.result() for future in start_futures]


# Traceback (SystemError)
In [1]: from mars.deploy.local import new_cluster

In [2]: import mars.tensor as mt

In [3]: cluster = new_cluster(scheduler_n_process=2, worker_n_process=2)
/Users/travis/miniconda3/conda-bld/arrow-cpp_1540410566532/work/cpp/src/plasma/store.cc:971: Allowing the Plasma store to use up to 3.43597GB of memory.
/Users/travis/miniconda3/conda-bld/arrow-cpp_1540410566532/work/cpp/src/plasma/store.cc:1001: Starting object store with directory /tmp and huge page support disabled
/Users/travis/miniconda3/conda-bld/arrow-cpp_1540410566532/work/cpp/src/plasma/eviction_policy.cc:85: There is not enough space to create this object, so evicting 0 objects to free up 0 bytes. The number of bytes in use (before this eviction) is 2130706432.

In [4]: a = mt.ones((3, 3), chunk_size=2)

In [5]: cluster.session.run(a)
Out[5]:
array([[1., 1., 1.],
[1., 1., 1.],
[1., 1., 1.]])

In [6]: cluster.session.run(a.dot(a))
Failed to start graph execution.
Traceback (most recent call last):
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 249, in execute_graph
self.create_operand_actors()
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 679, in create_operand_actors
op_refs = dict((k, v.result()) for k, v in op_refs.items())
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 679, in <genexpr>
op_refs = dict((k, v.result()) for k, v in op_refs.items())
File "src/gevent/event.py", line 457, in gevent._event.AsyncResult.result
File "src/gevent/event.py", line 381, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 409, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 399, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 379, in gevent._event.AsyncResult._raise_exception
File "/Users/hekaisheng/miniconda3/lib/python3.6/site-packages/gevent/_compat.py", line 47, in reraise
raise value.with_traceback(tb)
File "mars/actors/pool/gevent_pool.pyx", line 984, in mars.actors.pool.gevent_pool.Communicator._on_receive_create_actor
self.pool.create_actor(message.actor_cls, message.actor_ref.uid,
File "mars/actors/pool/gevent_pool.pyx", line 241, in mars.actors.pool.gevent_pool.LocalActorPool.create_actor
raise ActorAlreadyExist('Actor {0} already exist, cannot create'.format(actor.uid))
mars.actors.errors.ActorAlreadyExist: Actor s:operator$8a307641-78a8-43a9-a612-8edca02ca309$058707656a679b3a0ab03c10d40abb0e already exist, cannot create
Traceback (most recent call last):
File "src/gevent/greenlet.py", line 716, in gevent._greenlet.Greenlet.run
File "mars/actors/pool/gevent_pool.pyx", line 88, in mars.actors.pool.gevent_pool.ActorExecutionContext.fire_run
cpdef object fire_run(self):
File "mars/actors/pool/gevent_pool.pyx", line 91, in mars.actors.pool.gevent_pool.ActorExecutionContext.fire_run
with self.lock:
File "mars/actors/pool/gevent_pool.pyx", line 102, in mars.actors.pool.gevent_pool.ActorExecutionContext.fire_run
raise
File "mars/actors/pool/gevent_pool.pyx", line 96, in mars.actors.pool.gevent_pool.ActorExecutionContext.fire_run
res = actor.on_receive(message_ctx.message)
File "mars/actors/core.pyx", line 108, in mars.actors.core.FunctionActor.on_receive
File "mars/actors/core.pyx", line 110, in mars.actors.core.FunctionActor.on_receive
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 249, in execute_graph
self.create_operand_actors()
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 679, in create_operand_actors
op_refs = dict((k, v.result()) for k, v in op_refs.items())
File "/Users/hekaisheng/Documents/mars_dev/mars/mars/scheduler/graph.py", line 679, in <genexpr>
op_refs = dict((k, v.result()) for k, v in op_refs.items())
File "src/gevent/event.py", line 457, in gevent._event.AsyncResult.result
File "src/gevent/event.py", line 381, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 409, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 399, in gevent._event.AsyncResult.get
File "src/gevent/event.py", line 379, in gevent._event.AsyncResult._raise_exception
File "/Users/hekaisheng/miniconda3/lib/python3.6/site-packages/gevent/_compat.py", line 47, in reraise
raise value.with_traceback(tb)
File "mars/actors/pool/gevent_pool.pyx", line 984, in mars.actors.pool.gevent_pool.Communicator._on_receive_create_actor
self.pool.create_actor(message.actor_cls, message.actor_ref.uid,
File "mars/actors/pool/gevent_pool.pyx", line 241, in mars.actors.pool.gevent_pool.LocalActorPool.create_actor
raise ActorAlreadyExist('Actor {0} already exist, cannot create'.format(actor.uid))
mars.actors.errors.ActorAlreadyExist: Actor s:operator$8a307641-78a8-43a9-a612-8edca02ca309$058707656a679b3a0ab03c10d40abb0e already exist, cannot create
2019-01-24T12:05:09Z <Greenlet "Greenlet-0" at 0x119218448: <built-in method fire_run of mars.actors.pool.gevent_pool.ActorExecutionContext object at 0x319839688>> failed with ActorAlreadyExist

---------------------------------------------------------------------------
SystemError                               Traceback (most recent call last)
<ipython-input-6-d3363e4685e3> in <module>()
----> 1 cluster.session.run(a.dot(a))

~/Documents/mars_dev/mars/mars/session.py in run(self, *tensors, **kw)
115         # execute the non-executed tensors
116         if idx_to_run_tensors:
--> 117             execute_result = self._sess.run(*idx_to_run_tensors.values(), **kw)
118             if execute_result:
119                 # fetch is True

~/Documents/mars_dev/mars/mars/deploy/local/session.py in run(self, *tensors, **kw)
80             if graph_state == GraphState.FAILED:
81                 # TODO(qin): add traceback
---> 82                 raise SystemError('Graph execution failed with unknown reason')
83
84         if 0 < timeout < time.time() - exec_start_time:

SystemError: Graph execution failed with unknown reason

# Fixed code
def create_operand_actors(self, _clean_io_meta=True, _start=True):
    """
    Create operand actors for all operands
    """
    logger.debug("Creating operand actors for graph %s", self._graph_key)

    chunk_graph = self.get_chunk_graph()
    operand_infos = self._operand_infos

    op_refs = dict()
    initial_keys = []
    for op_key in self._op_key_to_chunk:
        chunks = self._op_key_to_chunk[op_key]
        op = chunks[0].op

        op_name = type(op).__name__
        op_info = operand_infos[op_key]

        io_meta = self._collect_operand_io_meta(chunk_graph, chunks)
        op_info["op_name"] = op_name
        op_info["io_meta"] = io_meta

        if io_meta["predecessors"]:
            state = "UNSCHEDULED"
        else:
            initial_keys.append(op_key)
            state = "READY"
        op_info["retries"] = 0
        op_info["state"] = state

        position = None
        if op_key in self._terminal_chunk_op_tensor:
            position = OperandPosition.TERMINAL
        elif not io_meta["predecessors"]:
            position = OperandPosition.INITIAL

        op_cls = get_operand_actor_class(type(op))
        op_uid = op_cls.gen_uid(self._session_id, op_key)
        scheduler_addr = self.get_scheduler(op_uid)

        # if operand actor exists, the behavior depends on the existing operand state.
        op_ref = self.ctx.actor_ref(op_uid, address=scheduler_addr)
        if self.ctx.has_actor(op_ref):
            op_ref.append_graph(self._graph_key, op_info, position=position)
            op_refs[op_key] = op_ref
        else:
            op_refs[op_key] = self.ctx.create_actor(
                op_cls,
                self._session_id,
                self._graph_key,
                op_key,
                op_info,
                position=position,
                uid=op_uid,
                address=scheduler_addr,
                wait=False,
            )

        op_info["state"] = getattr(OperandState, state.upper())
        if _clean_io_meta:
            del op_info["io_meta"]

    self.state = GraphState.RUNNING

    if _start:
        op_refs = dict(
            (k, v) if isinstance(v, ActorRef) else (k, v.result())
            for k, v in op_refs.items()
        )
        start_futures = [
            op_refs[op_key].start_operand(_tell=True, _wait=False)
            for op_key in initial_keys
        ]
        [future.result() for future in start_futures]