Active Memory Manager

The Active Memory Manager, or AMM, is an experimental daemon that optimizes memory usage of workers across the Dask cluster. It is disabled by default.

Memory imbalance and duplication

Whenever a Dask task returns data, it is stored on the worker that executed the task for as long as it’s a dependency of other tasks, is referenced by a Client through a Future, or is part of a published dataset.

Dask assigns tasks to workers following criteria of CPU occupancy, Worker Resources, and locality. In the trivial use case of tasks that are not connected to each other, take the same time to compute, return data of the same size, and have no resource constraints, one will observe a perfect balance in memory occupation across workers too. In all other use cases, however, as the computation goes it could cause an imbalance in memory usage.

When a task runs on a worker and requires in input the output of a task from a different worker, Dask will transparently transfer the data between workers, ending up with multiple copies of the same data on different workers. This is generally desirable, as it avoids re-transferring the data if it’s required again later on. However, it also causes increased overall memory usage across the cluster.

Enabling the Active Memory Manager

The AMM can be enabled through the Dask configuration file:

      start: true
      interval: 2s

The above is the recommended setup and will run all enabled AMM policies (see below) every two seconds. Alternatively, you can manually start/stop the AMM from the Client or trigger a one-off iteration:

>>> client.amm.start()  # Start running every 2 seconds
>>> client.amm.stop()  # Stop running periodically
>>> client.amm.running()
>>> client.amm.run_once()


The AMM by itself doesn’t do anything. The user must enable policies which suggest actions regarding Dask data. The AMM runs the policies and enacts their suggestions, as long as they don’t harm data integrity. These suggestions can be of two types:

  • Replicate the data of an in-memory Dask task from one worker to another. This should not be confused with replication caused by task dependencies.

  • Delete one or more replicas of an in-memory task. The AMM will never delete the last replica of a task, even if a policy asks to.

There are no “move” operations. A move is performed in two passes: first a policy creates a copy; in the next AMM iteration, the same or another policy deletes the original (if the copy succeeded).

Unless a policy puts constraints on which workers should be impacted, the AMM will automatically create replicas on workers with the lowest memory usage first and delete them from workers with the highest memory usage first.

Individual policies are enabled, disabled, and configured through the Dask config:

      start: true
      interval: 2s
      - class: distributed.active_memory_manager.ReduceReplicas
      - class: my_package.MyPolicy
        arg1: foo
        arg2: bar

See below for custom policies like the one in the example above.

The default Dask config file contains a sane selection of builtin policies that should be generally desirable. You should try first with just start: true in your Dask config and see if it is fit for purpose for you before you tweak individual policies.

Built-in policies






This policy is enabled in the default Dask config. Whenever a Dask task is replicated on more than one worker and the additional replicas don’t appear to serve an ongoing computation, this policy drops all excess replicas.


This policy is incompatible with replicate() and with the broadcast=True parameter of scatter(). If you invoke replicate() to create additional replicas and then later run this policy, it will delete all replicas but one (but not necessarily the new ones).

Custom policies

Power users can write their own policies by subclassing ActiveMemoryManagerPolicy. The class should define two methods:


A custom policy may load parameters from the Dask config through __init__ parameters. If you don’t need configuration, you don’t need to implement this method.


This method accepts no parameters and is invoked by the AMM every 2 seconds (or whatever the AMM interval is). It must yield zero or more of the following suggestion tuples:

yield "replicate", <TaskState>, None

Create one replica of the target task on the worker with the lowest memory usage that doesn’t hold a replica yet. To create more than one replica, you need to yield the same command more than once.

yield "replicate", <TaskState>, {<WorkerState>, <WorkerState>, ...}

Create one replica of the target task on the worker with the lowest memory among the listed candidates.

yield "drop", <TaskState>, None

Delete one replica of the target task on the worker with the highest memory usage across the whole cluster.

yield "drop", <TaskState>, {<WorkerState>, <WorkerState>, ...}

Delete one replica of the target task on the worker with the highest memory among the listed candidates.

The AMM will silently reject unacceptable suggestions, such as:

  • Delete the last replica of a task

  • Delete a replica from a subset of workers that don’t hold any

  • Delete a replica from a worker that currently needs it for computation

  • Replicate a task that is not yet in memory

  • Create more replicas of a task than there are workers

  • Create replicas of a task on workers that already hold them

  • Create replicas on paused or retiring workers

It is generally a good idea to design policies to be as simple as possible and let the AMM take care of the edge cases above by ignoring some of the suggestions.

Optionally, the run method may retrieve which worker the AMM just selected, as follows:

ws = (yield "drop", ts, None)

The run method can access the following attributes:


The ActiveMemoryManagerExtension that the policy is attached to


Scheduler to which the suggestions will be applied. From there you can access various attributes such as tasks and workers.


Read-only mapping of {WorkerState: bytes}. bytes is the expected RAM usage of the worker after all suggestions accepted so far in the current AMM iteration, from all policies, will be enacted. Note that you don’t need to access this if you are happy to always create/delete replicas on the workers with the lowest and highest memory usage respectively - the AMM will handle it for you.


Read-only mapping of {TaskState: ({<WorkerState>, ...}, {<WorkerState>, ...}). The first set contains the workers that will receive a new replica of the task according to the suggestions accepted so far; the second set contains the workers which will lose a replica.


Set of policies registered in the AMM. A policy can deregister itself as follows:

def run(self):


The following custom policy ensures that keys “foo” and “bar” are replicated on all workers at all times. New workers will receive a replica soon after connecting to the scheduler. The policy will do nothing if the target keys are not in memory somewhere or if all workers already hold a replica. Note that this example is incompatible with the ReduceReplicas built-in policy.

In (it must be accessible by the scheduler):

from distributed.active_memory_manager import ActiveMemoryManagerPolicy

class EnsureBroadcast(ActiveMemoryManagerPolicy):
    def __init__(self, key):
        self.key = key

    def run(self):
        ts = self.manager.scheduler.tasks.get(self.key)
        if not ts:
        for _ in range(len(self.manager.scheduler.workers) - len(ts.who_has)):
            yield "replicate", ts, None

Note that the policy doesn’t bother testing for edge cases such as paused workers or other policies also requesting replicas; the AMM takes care of it. In theory you could rewrite the last two lines as follows (at the cost of some wasted CPU cycles):

for _ in range(1000):
    yield "replicate", ts, None

In distributed.yaml:

      start: true
      interval: 2s
      - class: mymodule.EnsureBroadcast
        key: foo
      - class: mymodule.EnsureBroadcast
        key: bar

We could have alternatively used a single policy instance with a list of keys - the above design merely illustrates that you may have multiple instances of the same policy running side by side.

API reference

class distributed.active_memory_manager.ActiveMemoryManagerExtension(scheduler: Scheduler, policies: set[ActiveMemoryManagerPolicy] | None = None, *, register: bool = True, start: bool | None = None, interval: float | None = None)[source]

Scheduler extension that optimizes memory usage across the cluster. It can be either triggered by hand or automatically every few seconds; at every iteration it performs one or both of the following:

  • create new replicas of in-memory tasks

  • destroy replicas of in-memory tasks; this never destroys the last available copy.

There are no ‘move’ operations. A move is performed in two passes: first you create a copy and, in the next iteration, you delete the original (if the copy succeeded).

This extension is configured by the dask config section

amm_handler(comm, method: str)[source]

Scheduler handler, invoked from the Client by AMMClientProxy

run_once() None[source]

Run all policies once and asynchronously (fire and forget) enact their recommendations to replicate/drop tasks

property running: bool

Return True if the AMM is being triggered periodically; False otherwise

start() None[source]

Start executing every self.interval seconds until scheduler shutdown

stop() None[source]

Stop periodic execution

class distributed.active_memory_manager.ActiveMemoryManagerPolicy[source]

Abstract parent class

run() Generator[tuple[str, TaskState, set[WorkerState] | None], WorkerState | None, None][source]

This method is invoked by the ActiveMemoryManager every few seconds, or whenever the user invokes client.amm.run_once. It is an iterator that must emit any of the following:

  • “replicate”, <TaskState>, None

  • “replicate”, <TaskState>, {subset of potential workers to replicate to}

  • “drop”, <TaskState>, None

  • “drop”, <TaskState>, {subset of potential workers to drop from}

Each element yielded indicates the desire to create or destroy a single replica of a key. If a subset of workers is not provided, it defaults to all workers on the cluster. Either the ActiveMemoryManager or the Worker may later decide to disregard the request, e.g. because it would delete the last copy of a key or because the key is currently needed on that worker.

You may optionally retrieve which worker it was decided the key will be replicated to or dropped from, as follows:

choice = (yield "replicate", ts, None)

choice is either a WorkerState or None; the latter is returned if the ActiveMemoryManager chose to disregard the request.

The current pending (accepted) commands can be inspected on self.manager.pending; this includes the commands previously yielded by this same method.

The current memory usage on each worker, downstream of all pending commands, can be inspected on self.manager.workers_memory.

class distributed.active_memory_manager.AMMClientProxy(client: Client)[source]

Convenience accessors to operate the AMM from the dask client

Usage: client.amm.start() etc.

All methods are asynchronous if the client is asynchronous and synchronous if the client is synchronous.

class distributed.active_memory_manager.ReduceReplicas[source]

Make sure that in-memory tasks are not replicated on more workers than desired; drop the excess replicas.