# Buggy code
def _on_ready(self):
    self.worker = None
    self._execution_ref = None

    # if under retry, give application a delay
    delay = options.scheduler.retry_delay if self.retries else 0
    # Send resource application. Submit job when worker assigned
    try:
        new_assignment = self._assigner_ref.get_worker_assignments(
            self._session_id, self._info
        )
    except DependencyMissing:
        logger.warning(
            "DependencyMissing met, operand %s will be back to UNSCHEDULED.",
            self._op_key,
        )
        self._assigned_workers = set()
        self.ref().start_operand(OperandState.UNSCHEDULED, _tell=True)
        return

    chunk_sizes = self._chunk_meta_ref.batch_get_chunk_size(
        self._session_id, self._input_chunks
    )
    if any(v is None for v in chunk_sizes):
        logger.warning(
            "DependencyMissing met, operand %s will be back to UNSCHEDULED.",
            self._op_key,
        )
        self._assigned_workers = set()
        self.ref().start_operand(OperandState.UNSCHEDULED, _tell=True)
        return

    new_assignment = [a for a in new_assignment if a not in self._assigned_workers]
    self._assigned_workers.update(new_assignment)
    logger.debug(
        "Operand %s assigned to run on workers %r, now it has %r",
        self._op_key,
        new_assignment,
        self._assigned_workers,
    )

    data_sizes = dict(zip(self._input_chunks, chunk_sizes))

    dead_workers = set()
    serialized_exec_graph = self._graph_ref.get_executable_operand_dag(self._op_key)
    for worker_ep in new_assignment:
        try:
            with rewrite_worker_errors():
                self._get_execution_ref(address=worker_ep).enqueue_graph(
                    self._session_id,
                    self._op_key,
                    serialized_exec_graph,
                    self._io_meta,
                    data_sizes,
                    self._info["optimize"],
                    succ_keys=self._succ_keys,
                    _delay=delay,
                    _promise=True,
                ).then(functools.partial(self._handle_worker_accept, worker_ep))
        except WorkerDead:
            logger.debug(
                "Worker %s dead when submitting operand %s into queue",
                worker_ep,
                self._op_key,
            )
            dead_workers.add(worker_ep)
            self._assigned_workers.difference_update([worker_ep])
    if dead_workers:
        self._resource_ref.detach_dead_workers(list(dead_workers), _tell=True)
        if not self._assigned_workers:
            self.ref().start_operand(_tell=True)


# 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 _on_ready(self):
    self.worker = None
    self._execution_ref = None

    # if under retry, give application a delay
    delay = options.scheduler.retry_delay if self.retries else 0
    # Send resource application. Submit job when worker assigned
    try:
        new_assignment = self._assigner_ref.get_worker_assignments(
            self._session_id, self._info
        )
    except DependencyMissing:
        logger.warning(
            "DependencyMissing met, operand %s will be back to UNSCHEDULED.",
            self._op_key,
        )
        self._assigned_workers = set()
        self.ref().start_operand(OperandState.UNSCHEDULED, _tell=True)
        return

    chunk_sizes = self._chunk_meta_ref.batch_get_chunk_size(
        self._session_id, self._input_chunks
    )
    if any(v is None for v in chunk_sizes):
        logger.warning(
            "DependencyMissing met, operand %s will be back to UNSCHEDULED.",
            self._op_key,
        )
        self._assigned_workers = set()
        self.ref().start_operand(OperandState.UNSCHEDULED, _tell=True)
        return

    new_assignment = [a for a in new_assignment if a not in self._assigned_workers]
    self._assigned_workers.update(new_assignment)
    logger.debug(
        "Operand %s assigned to run on workers %r, now it has %r",
        self._op_key,
        new_assignment,
        self._assigned_workers,
    )

    data_sizes = dict(zip(self._input_chunks, chunk_sizes))

    dead_workers = set()
    serialized_exec_graph = self._graph_refs[0].get_executable_operand_dag(self._op_key)
    for worker_ep in new_assignment:
        try:
            with rewrite_worker_errors():
                self._get_execution_ref(address=worker_ep).enqueue_graph(
                    self._session_id,
                    self._op_key,
                    serialized_exec_graph,
                    self._io_meta,
                    data_sizes,
                    self._info["optimize"],
                    succ_keys=self._succ_keys,
                    _delay=delay,
                    _promise=True,
                ).then(functools.partial(self._handle_worker_accept, worker_ep))
        except WorkerDead:
            logger.debug(
                "Worker %s dead when submitting operand %s into queue",
                worker_ep,
                self._op_key,
            )
            dead_workers.add(worker_ep)
            self._assigned_workers.difference_update([worker_ep])
    if dead_workers:
        self._resource_ref.detach_dead_workers(list(dead_workers), _tell=True)
        if not self._assigned_workers:
            self.ref().start_operand(_tell=True)