Scheduling State¶
Overview¶
The life of a computation with Dask can be described in the following stages:
- The user authors a graph using some library, perhaps Dask.delayed or
dask.dataframe or the
submit/map
functions on the client. They submit these tasks to the scheduler. - The schedulers assimilates these tasks into its graph of all tasks to track and as their dependencies become available it asks workers to run each of these tasks.
- The worker receives information about how to run the task, communicates with its peer workers to collect dependencies, and then runs the relevant function on the appropriate data. It reports back to the scheduler that it has finished.
- The scheduler reports back to the user that the task has completed. If the user desires, it then fetches the data from the worker through the scheduler.
Most relevant logic is in tracking tasks as they evolve from newly submitted, to waiting for dependencies, to ready to run, to actively running on some worker, to finished in memory, to garbage collected. Tracking this process, and tracking all effects that this task has on other tasks that might depend on it, is the majority of the complexity of the dynamic task scheduler. This section describes the system used to perform this tracking.
For more abstract information about the policies used by the scheduler, see Scheduling Policies.
State Variables¶
We start with a description of the state that the scheduler keeps on each task. Each of the following is a dictionary keyed by task name (described below):
tasks:
{key: task}
:Dictionary mapping key to a serialized task.
A key is the name of a task, generally formed from the name of the function, followed by a hash of the function and arguments, like
'inc-ab31c010444977004d656610d2d421ec'
.The value of this dictionary is the task, which is an unevaluated function and arguments. This is stored in one of two forms:
{'function': inc, 'args': (1,), 'kwargs': {}}
; a dictionary with the
function, arguments, and keyword arguments (kwargs). However in the scheduler these are stored serialized, as they were sent from the client, so it looks more like
{'function': b'\x80\x04\x95\xcb\...', 'args': b'...', }
{'task': (inc, 1)}
: a tuple satisfying the dask graph protocol. This again is stored serialized.
These are the values that will eventually be sent to a worker when the task is ready to run.
dependencies and dependents:
{key: {keys}}
:These are dictionaries which show which tasks depend on which others. They contain redundant information. If
dependencies[a] == {b, c}
then the task with the name ofa
depends on the results of the two tasks with the names ofb
andc
. There will be complimentary entries in dependents such thata in dependents[b]
anda in dependents[c]
such asdependents[b] == {a, d}
. Keeping the information around twice allows for constant-time access for either direction of query, so we can both look up a task’s out-edges or in-edges efficiently.waiting and waiting_data:
{key: {keys}}
:These are dictionaries very similar to dependencies and dependents, but they only track keys that are still in play. For example
waiting
looks likedependencies
, tracking all of the tasks that a certain task requires before it can run. However as tasks are completed and arrive in memory they are removed from their dependents sets inwaiting
, so that when a set becomes empty we know that a key is ready to run and ready to be allocated to a worker.The
waiting_data
dictionary on the other hand holds all of the dependents of a key that have yet to run and still require that this task stay in memory in services of tasks that may depend on it (itsdependents
). When a value set in this dictionary becomes empty its task may be garbage collected (unless some client actively desires that this task stay in memory).task_state:
{key: string}
:The
task_state
dictionary holds the current state of every key. Current valid states include released, waiting, queue, stacks, no-worker, processing, memory, and erred. These states are explained further below.priority:
{key: tuple}
:The
priority
dictionary provides each key with a relative ranking. This ranking is generally a tuple of two parts. The first (and dominant) part corresponds to when it was submitted. Generally earlier tasks take precedence. The second part is determined by the client, and is a way to prioritize tasks within a large graph that may be important, such as if they are on the critical path, or good to run in order to release many dependencies. This is explained further in Scheduling PolicyA key’s priority is only used to break ties, when many keys are being considered for execution. The priority does not determine running order, but does exert some subtle influence that does significantly shape the long term performance of the cluster.
ready:
deque(key)
A deque of keys that are ready to run now but haven’t yet been sent to a worker to run. These keys show no affinity to any particular worker and so can be run equally well by anyone. This is a common pool of tasks.
stacks:
{worker: [keys]}
:Keys that are ready to run and show some affinity to a particular worker. These keys typically have dependencies that are known to be on that worker or have user-defined restrictions that require them to run on certain nodes. A worker pulls from this stack first before taking tasks from the common pool of
ready
tasks.processing:
{worker: {key: cost}}
:Keys that are currently running on a worker. This is keyed by worker address and contains the expected cost in seconds of running that task.
rprocessing:
{key: {worker}}
:The reverse of the
processing
dictionary. This is all keys that are currently running with a set of all workers that are currently running them. This is redundant withprocessing
and just here for faster indexed querying. It is rare for these sets to be larger than size one but does happen in rare cases when a task might be submitted redundantly to multiple workers.who_has:
{key: {worker}}
:For keys that are in memory this shows on which workers they currently reside.
has_what:
{worker: {key}}
:This is the transpose of
who_has
, showing all keys that currently reside on each worker.released:
{keys}
The set of keys that are known, but released from memory. These have typically run to completion and are no longer necessary.
unrunnable:
{key}
The set
unrunnable
contains keys that are not currently able to run, probably because they have a user defined restriction (described below) that is not met by any available worker. These keys are waiting for an appropriate worker to join the network before computing.restrictions:
{key: {hostnames}}
:A set of hostnames per key of where that key can be run. Usually this is empty unless a key has been specifically restricted to only run on certain hosts. These restrictions don’t include a worker port. Any worker on that hostname is deemed valid.
loose_restrictions:
{key}
:Set of keys for which we are allowed to violate restrictions (see above) if not valid workers are present and the task would otherwise go into the
unrunnable
set.exceptions and tracebacks:
{key: Exception/Traceback}
:Dictionaries mapping keys to remote exceptions and tracebacks. When tasks fail we store their exceptions and tracebacks (serialized from the worker) here so that users may gather the exceptions to see the error.
exceptions_blame:
{key: key}
:If a task fails then we mark all of its dependent tasks as failed as well. This dictionary lets any failed task see which task was the origin of its failure.
suspicious_tasks:
{key: int}
Number of times a task has been involved in a worker failure. Some tasks may cause workers to fail (such as
sys.exit(0)
). When a worker fails all of the tasks on that worker are reassigned to others. This combination of behaviors can cause a bad task to catastrophically destroy all workers on the cluster, one after another. Whenever a worker fails we mark each task currently running on that worker as suspicious. If a task is involved in three failures (or some other fixed constant) then we mark the task as failed.who_wants:
{key: {client}}
:When a client submits a graph to the scheduler it also specifies which output keys it desires. Those keys are tracked here where each desired key knows which clients want it. These keys will not be released from memory and, when they complete, messages will be sent to all of these clients that the task is ready.
wants_what:
{client: {key}}
:The transpose of
who_wants
.nbytes:
{key: int}
:The number of bytes, as determined by
sizeof
, of the result of each finished task. This number is used for diagnostics and to help prioritize work.stealable:
[[key]]
A list of stacks of stealable keys, ordered by stealability. For more information see Work Stealing
Example Event and Response¶
Whenever an event happens, like when a client sends up more tasks, or when a worker finishes a task, the scheduler changes the state above. For example when a worker reports that a task has finished we perform actions like the following:
Task `key` finished by `worker`:
task_state[key] = 'memory'
who_has[key].add(worker)
has_what[worker].add(key)
nbytes[key] = nbytes
processing[worker].remove(key)
del rprocessing[key]
if key in who_wants:
send_done_message_to_clients(who_wants[key])
for dep in dependencies[key]:
waiting_data[dep].remove(key)
for dep in dependents[key]:
waiting[dep].remove(key)
if stacks[worker]:
next_task = stacks[worker].pop()
elif ready:
next_task = ready.pop()
else:
idle.add(worker)
State Transitions¶
The code presented in the section above is just for demonstration. In practice
writing this code for every possible event is highly error prone, resulting in
hard-to-track-down bugs. Instead the scheduler moves tasks between a fixed
set of states, notably 'released', 'waiting', 'queue', 'stacks', 'no-worker',
'processing', 'memory', 'error'
. Transitions between common pairs of states
are well defined and, if no path exists between a pair, the graph of
transitions can be traversed to find a valid sequence of transitions. Along
with these transitions come consistent logging and optional runtime checks that
are useful in testing.
Tasks fall into the following states with the following allowed transitions
- Released: known but not actively computing or in memory
- Waiting: On track to be computed, waiting on dependencies to arrive in memory
- Queue (ready): Ready to be computed by any worker
- Stacks (ready): Ready to be computed by a particular preferred worker
- No-worker (ready, rare): Ready to be computed, but no appropriate worker exists
- Processing: Actively being computed by one or more workers
- Memory: In memory on one or more workers
- Erred: Task has computed and erred
- Forgotten (not actually a state): Task is no longer needed by any client and so it removed from state
Every transition between states is a separate method in the scheduler. These
task transition functions are prefixed with transition
and then have the
name of the start and finish task state like the following.
def transition_released_waiting(self, key):
def transition_processing_memory(self, key):
def transition_processing_erred(self, key):
These functions each have three effects.
- They perform the necessary transformations on the scheduler state (the 20 dicts/lists/sets) to move one key between states.
- They return a dictionary of recommended
{key: state}
transitions to enact directly afterwards on other keys. For example after we transition a key into memory we may find that many waiting keys are now ready to transition from waiting to a ready state. - Optionally they include a set of validation checks that can be turned on for testing.
Rather than call these functions directly we call the central function
transition
:
def transition(self, key, final_state):
""" Transition key to the suggested state """
This transition function finds the appropriate path from the current to the final state. It also serves as a central point for logging and diagnostics.
Often we want to enact several transitions at once or want to continually
respond to new transitions recommended by initial transitions until we reach a
steady state. For that we use the transitions
function (note the plural s
).
def transitions(self, recommendations):
recommendations = recommendations.copy()
while recommendations:
key, finish = recommendations.popitem()
new = self.transition(key, finish)
recommendations.update(new)
This function runs transition
, takes the recommendations and runs them as
well, repeating until no further task-transitions are recommended.
Stimuli¶
Transitions occur from stimuli, which are state-changing messages to the scheduler from workers or clients. The scheduler responds to the following stimuli:
- Workers
- Task finished: A task has completed on a worker and is now in memory
- Task erred: A task ran and erred on a worker
- Task missing data: A task tried to run but was unable to find necessary data on other workers
- Worker added: A new worker was added to the network
- Worker removed: An existing worker left the network
- Clients
- Update graph: The client sends more tasks to the scheduler
- Release keys: The client no longer desires the result of certain keys
Stimuli functions are prepended with the text stimulus
, and take a variety
of keyword arguments from the message as in the following examples:
def stimulus_task_finished(self, key=None, worker=None, nbytes=None,
type=None, compute_start=None, compute_stop=None,
transfer_start=None, transfer_stop=None):
def stimulus_task_erred(self, key=None, worker=None,
exception=None, traceback=None)
These functions change some non-essential administrative state and then call transition functions.
Note that there are several other non-state-changing messages that we receive from the workers and clients, such as messages requesting information about the current state of the scheduler. These are not considered stimuli.
API¶
-
class
distributed.scheduler.
Scheduler
(center=None, loop=None, max_buffer_size=8353576960.0, delete_interval=500, synchronize_worker_interval=60000, ip=None, services=None, allowed_failures=3, validate=False, steal=True, **kwargs)[source]¶ Dynamic distributed task scheduler
The scheduler tracks the current state of workers, data, and computations. The scheduler listens for events and responds by controlling workers appropriately. It continuously tries to use the workers to execute an ever growing dask graph.
All events are handled quickly, in linear time with respect to their input (which is often of constant size) and generally within a millisecond. To accomplish this the scheduler tracks a lot of state. Every operation maintains the consistency of this state.
The scheduler communicates with the outside world through Tornado IOStreams It maintains a consistent and valid view of the world even when listening to several clients at once.
A Scheduler is typically started either with the
dask-scheduler
executable:$ dask-scheduler Scheduler started at 127.0.0.1:8786
Or within a LocalCluster a Client starts up without connection information:
>>> c = Client() >>> c.cluster.scheduler Scheduler(...)
Users typically do not interact with the scheduler directly but rather with the client object
Client
.State
The scheduler contains the following state variables. Each variable is listed along with what it stores and a brief description.
- tasks:
{key: task}
: - Dictionary mapping key to a serialized task like the following:
{'function': b'...', 'args': b'...'}
or{'task': b'...'}
- tasks:
- dependencies:
{key: {keys}}
: - Dictionary showing which keys depend on which others
- dependencies:
- dependents:
{key: {keys}}
: - Dictionary showing which keys are dependent on which others
- dependents:
- task_state:
{key: string}
: - Dictionary listing the current state of every task among the following: released, waiting, stacks, queue, no-worker, processing, memory, erred
- task_state:
- priority:
{key: tuple}
: - A score per key that determines its priority
- priority:
- waiting:
{key: {key}}
: - Dictionary like dependencies but excludes keys already computed
- waiting:
- waiting_data:
{key: {key}}
: - Dictionary like dependents but excludes keys already computed
- waiting_data:
- ready:
deque(key)
- Keys that are ready to run, but not yet assigned to a worker
- ready:
- stacks:
{worker: [keys]}
: - List of keys waiting to be sent to each worker
- stacks:
- processing:
{worker: {key: cost}}
: - Set of keys currently in execution on each worker and their expected duration
- processing:
- stack_durations:
{worker: [ints]}
: - Expected durations of stacked tasks
- stack_durations:
- stacks_duration:
{worker: int}
: - Total duration of all tasks in each workers stack
- stacks_duration:
- rprocessing:
{key: {worker}}
: - Set of workers currently executing a particular task
- rprocessing:
- who_has:
{key: {worker}}
: - Where each key lives. The current state of distributed memory.
- who_has:
- has_what:
{worker: {key}}
: - What worker has what keys. The transpose of who_has.
- has_what:
- released:
{keys}
- Set of keys that are known, but released from memory
- released:
- unrunnable:
{key}
- Keys that we are unable to run
- unrunnable:
- restrictions:
{key: {hostnames}}
: - A set of hostnames per key of where that key can be run. Usually this is empty unless a key has been specifically restricted to only run on certain hosts. These restrictions don’t include a worker port. Any worker on that hostname is deemed valid.
- restrictions:
- loose_restrictions:
{key}
: - Set of keys for which we are allow to violate restrictions (see above) if not valid workers are present.
- loose_restrictions:
- exceptions:
{key: Exception}
: - A dict mapping keys to remote exceptions
- exceptions:
- tracebacks:
{key: list}
: - A dict mapping keys to remote tracebacks stored as a list of strings
- tracebacks:
- exceptions_blame:
{key: key}
: - A dict mapping a key to another key on which it depends that has failed
- exceptions_blame:
- suspicious_tasks:
{key: int}
- Number of times a task has been involved in a worker failure
- suspicious_tasks:
- deleted_keys:
{key: {workers}}
- Locations of workers that have keys that should be deleted
- deleted_keys:
- wants_what:
{client: {key}}
: - What keys are wanted by each client.. The transpose of who_wants.
- wants_what:
- who_wants:
{key: {client}}
: - Which clients want each key. The active targets of computation.
- who_wants:
- nbytes:
{key: int}
: - Number of bytes for a key as reported by workers holding that key.
- nbytes:
- stealable:
[[key]]
- A list of stacks of stealable keys, ordered by stealability
- stealable:
- ncores:
{worker: int}
: - Number of cores owned by each worker
- ncores:
- idle:
{worker}
: - Set of workers that are not fully utilized
- idle:
- worker_info:
{worker: {str: data}}
: - Information about each worker
- worker_info:
- host_info:
{hostname: dict}
: - Information about each worker host
- host_info:
- worker_bytes:
{worker: int}
: - Number of bytes in memory on each worker
- worker_bytes:
- occupancy:
{worker: time}
- Expected runtime for all tasks currently processing on a worker
- occupancy:
- services:
{str: port}
: - Other services running on this scheduler, like HTTP
- services:
- loop:
IOLoop
: - The running Tornado IOLoop
- loop:
- streams:
[IOStreams]
: - A list of Tornado IOStreams from which we both accept stimuli and report results
- streams:
- task_duration:
{key-prefix: time}
- Time we expect certain functions to take, e.g.
{'sum': 0.25}
- task_duration:
- coroutines:
[Futures]
: - A list of active futures that control operation
- coroutines:
- scheduler_queues:
[Queues]
: - A list of Tornado Queues from which we accept stimuli
- scheduler_queues:
- report_queues:
[Queues]
: - A list of Tornado Queues on which we report results
- report_queues:
-
add_client
(stream, client=None)[source]¶ Add client to network
We listen to all future messages from this IOStream.
-
add_keys
(stream=None, address=None, keys=())[source]¶ Learn that a worker has certain keys
This should not be used in practice and is mostly here for legacy reasons.
-
add_plugin
(plugin)[source]¶ Add external plugin to scheduler
See https://distributed.readthedocs.io/en/latest/plugins.html
-
add_worker
(stream=None, address=None, keys=(), ncores=None, name=None, coerce_address=True, nbytes=None, now=None, host_info=None, **info)[source]¶ Add a new worker to the cluster
-
broadcast
(stream=None, msg=None, workers=None, hosts=None, nanny=False)[source]¶ Broadcast message to workers, return all results
-
change_worker_cores
(stream=None, worker=None, diff=0)[source]¶ Add or remove cores from a worker
This is used when a worker wants to spin off a long-running task
-
clear_data_from_workers
()[source]¶ Send delete signals to clear unused data from workers
This watches the
.deleted_keys
attribute, which stores a set of keys to be deleted from each worker. This function is run periodically by the._delete_periodic_callback
to actually remove the data.This runs every
self.delete_interval
milliseconds.
-
close
(stream=None, fast=False)[source]¶ Send cleanup signal to all coroutines then wait until finished
See also
-
coerce_address
(addr)[source]¶ Coerce possible input addresses to canonical form
Handles lists, strings, bytes, tuples, or aliases
-
correct_time_delay
(worker, msg)[source]¶ Apply offset time delay in message times.
Clocks on different workers differ. We keep track of a relative “now” through periodic heartbeats. We use this known delay to align message times to Scheduler local time. In particular this helps with diagnostics.
Operates in place
-
ensure_occupied
()[source]¶ Run ready tasks on idle workers
Work stealing policy
If some workers are idle but not others, if there are no globally ready tasks, and if there are tasks in worker stacks, then we start to pull preferred tasks from overburdened workers and deploy them back into the global pool in the following manner.
We determine the number of tasks to reclaim as the number of all tasks in all stacks times the fraction of idle workers to all workers. We sort the stacks by size and walk through them, reclaiming half of each stack until we have enough task to fill the global pool. We are careful not to reclaim tasks that are restricted to run on certain workers.
-
ensure_occupied_queue
(worker, count)[source]¶ Send at most count tasks from the ready queue to the specified worker
-
ensure_occupied_stacks
(worker)[source]¶ Send tasks to worker while it has tasks and free cores
These tasks may come from the worker’s own stacks or from the global ready deque.
We update the idle workers set appropriately.
-
feed
(stream, function=None, setup=None, teardown=None, interval=1, **kwargs)[source]¶ Provides a data stream to external requester
Caution: this runs arbitrary Python code on the scheduler. This should eventually be phased out. It is mostly used by diagnostics.
-
handle_messages
(in_queue, report, client=None)[source]¶ The master client coroutine. Handles all inbound messages from clients.
This runs once per Client IOStream or Queue.
See also
Scheduler.worker_stream
- The equivalent function for workers
-
handle_queues
(scheduler_queue, report_queue)[source]¶ Register new control and report queues to the Scheduler
Queues are not in common use. This may be deprecated in the future.
-
issaturated
(worker, latency=0.005)[source]¶ Determine if a worker has enough work to avoid being idle
A worker is saturated if the following criteria are met
- It is working on at least as many tasks as it has cores
- The expected time it will take to complete all of its currently assigned tasks is at least a full round-trip time. This is relevant when it has many small tasks
-
rebalance
(stream=None, keys=None, workers=None)[source]¶ Rebalance keys so that each worker stores roughly equal bytes
Policy
This orders the workers by what fraction of bytes of the existing keys they have. It walks down this list from most-to-least. At each worker it sends the largest results it can find and sends them to the least occupied worker until either the sender or the recipient are at the average expected load.
-
remove_worker
(stream=None, address=None, safe=False)[source]¶ Remove worker from cluster
We do this when a worker reports that it plans to leave or when it appears to be unresponsive. This may send its tasks back to a released state.
-
replicate
(stream=None, keys=None, n=None, workers=None, branching_factor=2, delete=True)[source]¶ Replicate data throughout cluster
This performs a tree copy of the data throughout the network individually on each piece of data.
Parameters: keys: Iterable
list of keys to replicate
n: int
Number of replications we expect to see within the cluster
branching_factor: int, optional
The number of workers that can copy data in each generation
See also
-
report
(msg)[source]¶ Publish updates to all listening Queues and Streams
If the message contains a key then we only send the message to those streams that care about the key.
-
scatter
(stream=None, data=None, workers=None, client=None, broadcast=False, timeout=2)[source]¶ Send data out to workers
See also
-
start_ipython
(stream=None)[source]¶ Start an IPython kernel
Returns Jupyter connection info dictionary.
-
steal_time_ratio
(key, bandwidth=100000000.0)[source]¶ The compute to communication time ratio of a key
Returns: ratio: The compute/communication time ratio of the task
loc: The self.stealable bin into which this key should go
-
stimulus_missing_data
(keys=None, key=None, worker=None, ensure=True, **kwargs)[source]¶ Mark that certain keys have gone missing. Recover.
-
stimulus_task_erred
(key=None, worker=None, exception=None, traceback=None, **kwargs)[source]¶ Mark that a task has erred on a particular worker
-
stimulus_task_finished
(key=None, worker=None, **kwargs)[source]¶ Mark that a task has finished execution on a particular worker
-
transition
(key, finish, *args, **kwargs)[source]¶ Transition a key from its current state to the finish state
Returns: Dictionary of recommendations for future transitions See also
Scheduler.transitions
- transitive version of this function
Examples
>>> self.transition('x', 'waiting') {'x': 'ready'}
-
transitions
(recommendations)[source]¶ Process transitions until none are left
This includes feedback from previous transitions and continues until we reach a steady state
-
update_data
(stream=None, who_has=None, nbytes=None, client=None)[source]¶ Learn that new data has entered the network from an external source
See also
Scheduler.mark_key_in_memory
-
update_graph
(client=None, tasks=None, keys=None, dependencies=None, restrictions=None, priority=None, loose_restrictions=None)[source]¶ Add new computations to the internal dask graph
This happens whenever the Client calls submit, map, get, or compute.
-
work_steal
()[source]¶ Steal tasks from saturated workers to idle workers
This moves tasks from the bottom of the stacks of over-occupied workers to the stacks of idling workers.
See also
-
worker_stream
(worker)[source]¶ Listen to responses from a single worker
This is the main loop for scheduler-worker interaction
See also
Scheduler.handle_messages
- Equivalent coroutine for clients
-
distributed.scheduler.
decide_worker
(dependencies, stacks, stack_duration, processing, who_has, has_what, restrictions, loose_restrictions, nbytes, ncores, key)[source]¶ Decide which worker should take task
>>> dependencies = {'c': {'b'}, 'b': {'a'}} >>> stacks = {'alice:8000': ['z'], 'bob:8000': []} >>> processing = {'alice:8000': set(), 'bob:8000': set()} >>> who_has = {'a': {'alice:8000'}} >>> has_what = {'alice:8000': {'a'}} >>> nbytes = {'a': 100} >>> ncores = {'alice:8000': 1, 'bob:8000': 1} >>> restrictions = {} >>> loose_restrictions = set()
We choose the worker that has the data on which ‘b’ depends (alice has ‘a’)
>>> decide_worker(dependencies, stacks, processing, who_has, has_what, ... restrictions, loose_restrictions, nbytes, ncores, 'b') 'alice:8000'
If both Alice and Bob have dependencies then we choose the less-busy worker
>>> who_has = {'a': {'alice:8000', 'bob:8000'}} >>> has_what = {'alice:8000': {'a'}, 'bob:8000': {'a'}} >>> decide_worker(dependencies, stacks, processing, who_has, has_what, ... restrictions, loose_restrictions, nbytes, ncores, 'b') 'bob:8000'
Optionally provide restrictions of where jobs are allowed to occur
>>> restrictions = {'b': {'alice', 'charlie'}} >>> decide_worker(dependencies, stacks, processing, who_has, has_what, ... restrictions, loose_restrictions, nbytes, ncores, 'b') 'alice:8000'
If the task requires data communication, then we choose to minimize the number of bytes sent between workers. This takes precedence over worker occupancy.
>>> dependencies = {'c': {'a', 'b'}} >>> who_has = {'a': {'alice:8000'}, 'b': {'bob:8000'}} >>> has_what = {'alice:8000': {'a'}, 'bob:8000': {'b'}} >>> nbytes = {'a': 1, 'b': 1000} >>> stacks = {'alice:8000': [], 'bob:8000': []}
>>> decide_worker(dependencies, stacks, processing, who_has, has_what, ... {}, set(), nbytes, ncores, 'c') 'bob:8000'