Mitogen for Ansible


An extension to Ansible is included that implements connections over Mitogen, replacing embedded shell invocations with pure-Python equivalents invoked via highly efficient remote procedure calls to persistent interpreters tunnelled over SSH. No changes are required to target hosts.

The extension is stable and real-world use is encouraged. Bug reports are welcome: Ansible is huge, and only wide testing will ensure soundness.


Expect a 1.25x - 7x speedup and a CPU usage reduction of at least 2x, depending on network conditions, modules executed, and time already spent by targets on useful work. Mitogen cannot improve a module once it is executing, it can only ensure the module executes as quickly as possible.

  • One connection is used per target, in addition to one sudo invocation per user account. This is much better than SSH multiplexing combined with pipelining, as significant state can be maintained in RAM between steps, and system logs aren’t spammed with repeat authentication events.
  • A single network roundtrip is used to execute a step whose code already exists in RAM on the target. Eliminating multiplexed SSH channel creation saves 4 ms runtime per 1 ms of network latency for every playbook step.
  • Processes are aggressively reused, avoiding the cost of invoking Python and recompiling imports, saving 300-800 ms for every playbook step.
  • Code is ephemerally cached in RAM, reducing bandwidth usage by an order of magnitude compared to SSH pipelining, with around 5x fewer frames traversing the network in a typical run.
  • Fewer writes to the target filesystem occur. In typical configurations, Ansible repeatedly rewrites and extracts ZIP files to multiple temporary directories on the target. Security issues relating to temporary files in cross-account scenarios are entirely avoided.

The effect is most potent on playbooks that execute many short-lived actions, where Ansible’s overhead dominates the cost of the operation, for example when executing large with_items loops to run simple commands or write files.


  1. Thoroughly review Noteworthy Differences and Known Issues.

  2. Download and extract mitogen-0.2.5.tar.gz.

  3. Modify ansible.cfg:

    strategy_plugins = /path/to/mitogen-0.2.5/ansible_mitogen/plugins/strategy
    strategy = mitogen_linear

    The strategy key is optional. If omitted, the ANSIBLE_STRATEGY=mitogen_linear environment variable can be set on a per-run basis. Like mitogen_linear, the mitogen_free and mitogen_host_pinned strategies exists to mimic the free and host_pinned strategies.

  4. If targets have a restrictive sudoers file, add a rule like:

    deploy = (ALL) NOPASSWD:/usr/bin/python -c*
  5. Releases occur frequently and often include important fixes. Subscribe to the mitogen-announce mailing list be notified of new releases.


This demonstrates Ansible running a subset of the Mitogen integration tests concurrent to an equivalent run using the extension.


  • “With mitogen my playbook runtime went from 45 minutes to just under 3 minutes. Awesome work!”
  • “The runtime was reduced from 1.5 hours on 4 servers to just under 3 minutes. Thanks!”
  • “Oh, performance improvement using Mitogen is huge. As mentioned before, running with Mitogen enables takes 7m36 (give or take a few seconds). Without Mitogen, the same run takes 19m49! I’m not even deploying without Mitogen anymore :)”
  • Works like a charm, thank you for your quick response”
  • “I tried it out. He is not kidding about the speed increase.”
  • “I don’t know what kind of dark magic @dmw_83 has done, but his Mitogen strategy took Clojars’ Ansible runs from 14 minutes to 2 minutes. I still can’t quite believe it.”
  • “Enabling the mitogen plugin in ansible feels like switching from floppy to SSD”

Noteworthy Differences

  • Ansible 2.3-2.7 are supported along with Python 2.6, 2.7 or 3.6. Verify your installation is running one of these versions by checking ansible --version output.

  • The Ansible raw action executes as a regular Mitogen connection, precluding its use for installing Python on a target. This will be addressed soon.

  • The doas, su and sudo become methods are available. File bugs to register interest in more.

  • The docker, jail, kubectl, local, lxc, lxd, and ssh built-in connection types are supported, along with Mitogen-specific machinectl, mitogen_doas, mitogen_su, mitogen_sudo, and setns types. File bugs to register interest in others.

  • Local commands execute in a reuseable interpreter created identically to interpreters on targets. Presently one interpreter per become_user exists, and so only one local action may execute simultaneously.

    Ansible usually permits up to forks simultaneous local actions. Any long-running local actions that execute for every target will experience artificial serialization, causing slowdown equivalent to task_duration * num_targets. This will be fixed soon.

  • “Module Replacer” style modules are not supported. These rarely appear in practice, and light web searches failed to reveal many examples of them.

  • Ansible permits up to forks connections to be setup in parallel, whereas in Mitogen this is handled by a fixed-size thread pool. Up to 32 connections may be established in parallel by default, this can be modified by setting the MITOGEN_POOL_SIZE environment variable.

  • The ansible_python_interpreter variable is parsed using a restrictive shell-like syntax, permitting values such as /usr/bin/env FOO=bar python, which occur in practice. Ansible documents this as an absolute path, however the implementation passes it unquoted through the shell, permitting arbitrary code to be injected.

  • Performance does not scale linearly with target count. This will improve over time.

  • SSH and become are treated distinctly when applying timeouts, and timeouts apply up to the point when the new interpreter is ready to accept messages. Ansible has two timeouts: ConnectTimeout for SSH, applying up to when authentication completes, and a separate parallel timeout up to when become authentication completes.

    For busy targets, Ansible may successfully execute a module where Mitogen would fail without increasing the timeout. For sick targets, Ansible may hang indefinitely after authentication without executing a command, for example due to a stuck filesystem IO appearing in $HOME/.profile.

New Features & Notes

Connection Delegation


Included is a preview of Connection Delegation, a Mitogen-specific implementation of stackable connection plug-ins. This enables connections via a bastion, or container connections delegated via their host machine, where reaching the host may entail further delegation.

Unlike with SSH forwarding Ansible has complete visibility of the final topology, declarative configuration via static/dynamic inventory is possible, and data can be cached and re-served, and code executed on every intermediary.

For example when targeting Docker containers on a remote machine, each module need only be uploaded once for the first task and container that requires it, then cached and served from the SSH account for every future task in any container.


Connection delegation is a work in progress, bug reports are welcome.

  • Delegated connection setup is single-threaded; only one connection can be constructed in parallel per intermediary.
  • Inferring the configuration of intermediaries may be buggy, manifesting as duplicate connections between hops, due to not perfectly replicating the configuration Ansible would normally use for the intermediary.
  • Intermediary machines cannot use login and become passwords that were supplied to Ansible interactively. If an intermediary requires a password, it must be supplied via ansible_ssh_pass, ansible_password, or ansible_become_pass inventory variables.
  • Automatic tunnelling of SSH-dependent actions, such as the synchronize module, is not yet supported. This will be added in the 0.3 series.

To enable connection delegation, set mitogen_via=<inventory name> on the command line, or as host and group variables.

# Docker container on web1.dc1 is reachable via web1.dc1.
app1.web1.dc1 ansible_host=app1 ansible_connection=docker mitogen_via=web1.dc1

# Web servers in DC1 are reachable via bastion.dc1

mitogen_via = bastion.dc1

# Web servers in DC2 are reachable via bastion.dc2

mitogen_via = bastion.dc2

# Prod bastions are reachable via a magic account on a
# corporate network gateway.
bastion.dc1 mitogen_via=prod-ssh-access@corp-gateway.internal
bastion.dc2 mitogen_via=prod-ssh-access@corp-gateway.internal


File Transfer

Normally sftp(1) or scp(1) are used to copy files by the assemble, copy, patch, script, template, and unarchive actions, or when uploading modules with pipelining disabled. With Mitogen copies are implemented natively using the same interpreters, connection tree, and routed message bus that carries RPCs.

This permits direct streaming between endpoints regardless of execution environment, without necessitating temporary copies in intermediary accounts or machines, for example when become is active, or in the presence of connection delegation. It also avoids the need to securely share temporary files between accounts and machines.

As the implementation is self-contained, it is simple to make improvements like prioritizing transfers, supporting resume, or displaying progress bars.


Transfers proceed to a hidden file in the destination directory, with content and metadata synced using fsync(2) prior to rename over any existing file. This ensures the file remains consistent at all times, in the event of a crash, or when overlapping ansible-playbook runs deploy differing file contents.

The sftp(1) and scp(1) tools may cause undetected data corruption in the form of truncated files, or files containing intermingled data segments from overlapping runs. As part of normal operation, both tools expose a window where readers may observe inconsistent file contents.


One roundtrip initiates a transfer larger than 124 KiB, while smaller transfers are embedded in a 0-roundtrip pipelined call. For tools operating via SSH multiplexing, 4 roundtrips are required to configure the IO channel, followed by 6 roundtrips to transfer the file in the case of sftp, in addition to the time to start the local and remote processes.

An invocation of scp with an empty .profile over a 30 ms link takes ~140 ms, wasting 110 ms per invocation, rising to ~2,000 ms over a 400 ms UK-India link, wasting 1,600 ms per invocation.

Interpreter Reuse

Python interpreters are aggressively reused to execute modules. While this works well, it violates an unwritten assumption, and so it is possible an earlier module execution could cause a subsequent module to fail, or for unrelated modules to interact poorly due to bad hygiene, such as monkey-patching that becomes stacked over repeat invocations.

Before reporting a bug relating to a misbehaving module, please re-run with -e mitogen_task_isolation=fork to see if the problem abates. This may be set per-task, paying attention to the possibility an earlier task may be the true cause of a failure.

- name: My task.
    some_option: true
    mitogen_task_isolation: fork

If forking solves your problem, please report a bug regardless, as an internal list can be updated to prevent others bumping into the same problem.

Interpreter Recycling

There is a per-target limit on the number of interpreters. Once 20 exist, the youngest is terminated before starting any new interpreter, preventing situations like below from triggering memory exhaustion.

- hosts: corp_boxes
    user_directory: [
      # 10,000 corporate user accounts
    - name: Create user bashrc
      become: true
        ansible_become_user: "{{item}}"
        src: bashrc
        dest: "~{{item}}/.bashrc"
      with_items: "{{user_directory}}"

The youngest is chosen to preserve useful accounts like root and postgresql that often appear early in a run, however it is simple to construct a playbook that defeats this strategy. A future version will key interpreters on the identity of their creating task, avoiding useful account recycling in every scenario.

To modify the limit, set the MITOGEN_MAX_INTERPRETERS environment variable.

Standard IO

Ansible uses pseudo TTYs for most invocations to allow it to type interactive passwords, however pseudo TTYs are disabled where standard input is required or sudo is not in use. Additionally when SSH multiplexing is enabled, a string like Shared connection to localhost closed\r\n appears in stderr of every invocation.

Mitogen does not naturally require either of these, as command output is always embedded within framed messages, and it can simply call pty.openpty() in any location an interactive password must be typed.

A major downside to Ansible’s behaviour is that stdout and stderr are merged together into a single stdout variable, with carriage returns inserted in the output by the TTY layer. However ugly, the extension emulates this precisely, to avoid breaking playbooks that expect text to appear in specific variables with a particular linefeed style.

Temporary Files

Temporary file handling in Ansible is tricky, and the precise behaviour varies across major versions. A variety of temporary files and directories are created, depending on the operating mode.

In the best case when pipelining is enabled and no temporary uploads are required, for each task Ansible will create one directory below a system-supplied temporary directory returned by tempfile.mkdtemp(), owned by the target account a new-style module will execute in.

In other cases depending on the task type, whether become is active, whether the target become user is privileged, whether the associated action plugin needs to upload files, and whether the associated module needs to store files, Ansible may:

  • Create a directory owned by the SSH user either under remote_tmp, or a system-default directory,
  • Upload action dependencies such as non-new style modules or rendered templates to that directory via sftp(1) or scp(1).
  • Attempt to modify the directory’s access control list to grant access to the target user using setfacl(1), requiring that tool to be installed and a supported filesystem to be in use, or for the allow_world_readable_tmpfiles setting to be True.
  • Create a directory owned by the target user either under remote_tmp, or a system-default directory, if a new-style module needs a temporary directory and one was not previously created for a supporting file earlier in the invocation.

In summary, for each task Ansible may create one or more of:

  • ~ssh_user/<remote_tmp>/... owned by the login user,
  • $TMPDIR/ansible-tmp-... owned by the login user,
  • $TMPDIR/ansible-tmp-... owned by the login user with ACLs permitting write access by the become user,
  • ~become_user/<remote_tmp>/... owned by the become user,
  • $TMPDIR/ansible_<modname>_payload_.../ owned by the become user,
  • $TMPDIR/ansible-module-tmp-.../ owned by the become user.

Mitogen for Ansible

As Mitogen can execute new-style modules from RAM, and transfer files to target user accounts without first writing an intermediary file in any separate login account, handling is relatively simplified.

Temporary directories must exist to maintain compatibility with Ansible, as many modules introspect sys.argv to find a directory where they may write files, however only one directory exists for the lifetime of each interpreter, its location is consistent for each account, and it is always privately owned by that account.

During startup, the persistent remote interpreter tries the paths below until one is found that is writeable and lives on a filesystem with noexec disabled:

  1. $variable and tilde-expanded remote_tmp setting from ansible.cfg
  2. $variable and tilde-expanded system_tmpdirs setting from ansible.cfg
  3. TMPDIR environment variable
  4. TEMP environment variable
  5. TMP environment variable
  6. /tmp
  7. /var/tmp
  8. /usr/tmp
  9. Current working directory

The directory is created at startup and recursively destroyed during interpeter shutdown. Subdirectories are automatically created and destroyed by the controller for each task that requires them.

Round-trip Avoidance

Mitogen avoids many round-trips due to temporary file handling that are present in regular Ansible:

  • During task startup, it is not necessary to wait until the target has succeeded in creating a temporary directory. Instead, any failed attempt to create the directory will cause any subsequent RPC belonging to the same task to fail with the error that occurred.
  • As temporary directories are privately owned by the target user account, operations relating to modifying the directory to support cross-account access are avoided.
  • An explicit work-around is included to avoid the copy and template actions needlessly triggering a round-trip to set their temporary file as executable.
  • During task shutdown, it is not necessary to wait to learn if the target has succeeded in deleting a temporary directory, since any error that may occur can is logged asynchronously via the logging framework, and the persistent remote interpreter arranges for all subdirectories to be destroyed during interpreter shutdown.

Process Environment Emulation

Since Ansible discards processes after each module invocation, follow-up tasks often (but not always) receive a new environment that will usually include changes made by previous tasks. As such modifications are common, for compatibility the extension emulates the existing behaviour as closely as possible.

Some scenarios exist where emulation is impossible, for example, applying nsswitch.conf changes when nscd is not in use. If future scenarios appear that cannot be solved through emulation, the extension will be updated to automatically restart affected interpreters instead.

DNS Resolution

Modifications to /etc/resolv.conf cause the glibc resolver configuration to be reloaded via res_init(3). This isn’t necessary on some Linux distributions carrying glibc patches to automatically check /etc/resolv.conf periodically, however it is necessary on at least Debian and BSD derivatives.


When become: true is active or SSH multiplexing is disabled, modifications by previous tasks to /etc/environment and $HOME/.pam_environment are normally reflected, since the content of those files is reapplied by PAM via pam_env on each authentication of sudo or sshd.

Both files are monitored for changes, and changes are applied where it appears safe to do so:

  • New keys are added if they did not otherwise exist in the inherited environment, or previously had the same value as found in the file before it changed.
  • Given a key (such as http_proxy) added to the file where no such key exists in the environment, the key will be added.
  • Given a key (such as PATH) where an existing environment key exists with a different value, the update or deletion will be ignored, as it is likely the key was overridden elsewhere after pam_env ran, such as by /etc/profile.
  • Given a key removed from the file that had the same value as the existing environment key, the key will be removed.

How Modules Execute

Ansible usually modifies, recompresses and reuploads modules every time they run on a target, work that must be repeated by the controller for every playbook step.

With the extension any modifications are done on the target, allowing pristine copies of modules to be cached, reducing the necessity to re-transfer modules for each invocation. Unmodified modules are uploaded once on first use and cached in RAM for the remainder of the run.

Native executables detected using a complex heuristic. Arguments are supplied as a JSON file whose path is the sole script parameter.
Module Replacer
Python scripts detected by the presence of #<<INCLUDE_ANSIBLE_MODULE_COMMON>> appearing in their source. This type is not yet supported.
Python scripts detected by the presence of from ansible.module_utils. appearing in their source. Arguments are supplied as JSON written to sys.stdin of the target interpreter.
Detected by the presence of INCLUDE_ANSIBLE_MODULE_JSON_ARGS appearing in the script source. The interpreter directive (#!interpreter) is adjusted to match the corresponding value of {{ansible_*_interpreter}} if one is set. Arguments are supplied as JSON mixed into the script as a replacement for INCLUDE_ANSIBLE_MODULE_JSON_ARGS.
Detected by the presence of WANT_JSON appearing in the script source. The interpreter directive is adjusted as above. Arguments are supplied as a JSON file whose path is the sole script parameter.
Old Style
Files not matching any of the above tests. The interpreter directive is adjusted as above. Arguments are supplied as a file whose path is the sole script parameter. The format of the file is "key=repr(value)[ key2=repr(value2)[ ..]] ".

Runtime Patches

Three small runtime patches are employed in to hook into desirable locations, in order to override uses of shell, the module executor, and the mechanism for selecting a connection plug-in. While it is hoped the patches can be avoided in future, for interesting versions of Ansible deployed today this simply is not possible, and so they continue to be required.

The patches are concise and behave conservatively, including by disabling themselves when non-Mitogen connections are in use. Additional third party plug-ins are unlikely to attempt similar patches, so the risk to an established configuration should be minimal.

Flag Emulation

Mitogen re-parses sudo_flags, become_flags, and ssh_flags using option parsers extracted from sudo(1) and ssh(1) in order to emulate their equivalent semantics. This allows:

  • robust support for common ansible.cfg tricks without reconfiguration, such as forwarding SSH agents across sudo invocations,
  • reporting on conflicting flag combinations,
  • reporting on unsupported flag combinations,
  • internally special-casing certain behaviour (like recursive agent forwarding) without boring the user with the details,
  • avoiding opening the extension up to untestable scenarios where users can insert arbitrary garbage between Mitogen and the components it integrates with,
  • precise emulation by an alternative implementation, for example if Mitogen grew support for Paramiko.

Connection Types

Matching Ansible, connection variables are treated on a per-task basis, causing establishment of additional reuseable interpreters as necessary to match the configuration of each task.


doas can be used as a connection method that supports connection delegation, or as a become method.

When used as a become method:

  • ansible_python_interpreter
  • ansible_become_exe: path to doas binary.
  • ansible_become_user (default: root)
  • ansible_become_pass (default: assume passwordless)
  • ansible.cfg: timeout

When used as the mitogen_doas connection method:

  • The inventory hostname has no special meaning.
  • ansible_user: username to use.
  • ansible_password: password to use.
  • ansible_python_interpreter


Like docker except connection delegation is supported.

  • ansible_host: Name of Docker container (default: inventory hostname).
  • ansible_user: Name of user within the container to execute as.

FreeBSD Jail

Like jail except connection delegation is supported.

  • ansible_host: Name of jail (default: inventory hostname).
  • ansible_user: Name of user within the jail to execute as.

Kubernetes Pod

Like kubectl except connection delegation is supported.

  • ansible_host: Name of pod (default: inventory hostname).
  • ansible_user: Name of user to authenticate to API as.


Like local except connection delegation is supported.

  • ansible_python_interpreter

Process Model

Ansible usually executes local connection commands as a transient subprocess of the forked worker executing a task. With the extension, the local connection exists as a persistent subprocess of the connection multiplexer.

This means that global state mutations made to the top-level Ansible process that are normally visible to newly forked subprocesses, such as vars plug-ins that modify the environment, will not be reflected when executing local commands without additional effort.

During execution the extension presently mimics the working directory and process environment inheritence of regular Ansible, however it is possible some additional differences exist that may break existing playbooks.


Connect to classic LXC containers, like lxc except connection delegation is supported, and lxc-attach is always used rather than the LXC Python bindings, as is usual with lxc.

  • ansible_python_interpreter
  • ansible_host: Name of LXC container (default: inventory hostname).
  • mitogen_lxc_attach_path: path to lxc-attach command if not available
    on the system path.


Connect to modern LXD containers, like lxd except connection delegation is supported. The lxc command must be available on the host machine.

  • ansible_python_interpreter
  • ansible_host: Name of LXC container (default: inventory hostname).
  • mitogen_lxc_path: path to lxc command if not available on the system path.


Like the machinectl third party plugin except connection delegation is supported. This is a light wrapper around the setns method.

  • ansible_host: Name of Docker container (default: inventory hostname).
  • ansible_user: Name of user within the container to execute as.
  • mitogen_machinectl_path: path to machinectl command if not available as /bin/machinectl.


The setns method connects to Linux containers via setns(2). Unlike Docker, LXC, and LXD the namespace transition is handled internally, ensuring optimal throughput to the child. This is necessary for Machinectl where only PTY channels are supported.

A utility program must be installed to discover the PID of the container’s root process.

  • mitogen_kind: one of docker, lxc, lxd or machinectl.
  • ansible_host: Name of container as it is known to the corresponding tool (default: inventory hostname).
  • ansible_user: Name of user within the container to execute as.
  • mitogen_docker_path: path to Docker if not available on the system path.
  • mitogen_lxc_path: path to LXD’s lxc command if not available as lxc-info.
  • mitogen_lxc_info_path: path to LXC classic’s lxc-info command if not available as lxc-info.
  • mitogen_machinectl_path: path to machinectl command if not available as /bin/machinectl.


Su can be used as a connection method that supports connection delegation, or as a become method.

When used as a become method:

  • ansible_python_interpreter
  • ansible_su_exe, ansible_become_exe
  • ansible_su_user, ansible_become_user (default: root)
  • ansible_su_pass, ansible_become_pass (default: assume passwordless)
  • su_flags, become_flags
  • ansible.cfg: timeout

When used as the mitogen_su connection method:

  • The inventory hostname has no special meaning.
  • ansible_user: username to su as.
  • ansible_password: password to su as.
  • ansible_python_interpreter


Sudo can be used as a connection method that supports connection delegation, or as a become method.

When used as a become method:

  • ansible_python_interpreter
  • ansible_sudo_exe, ansible_become_exe
  • ansible_sudo_user, ansible_become_user (default: root)
  • ansible_sudo_pass, ansible_become_pass (default: assume passwordless)
  • sudo_flags, become_flags
  • ansible.cfg: timeout

When used as the mitogen_sudo connection method:

  • The inventory hostname has no special meaning.
  • ansible_user: username to sudo as.
  • ansible_password: password to sudo as.
  • sudo_flags, become_flags
  • ansible_python_interpreter


Like ssh except connection delegation is supported.

  • ansible_ssh_timeout
  • ansible_host, ansible_ssh_host
  • ansible_user, ansible_ssh_user
  • ansible_port, ssh_port
  • ansible_ssh_executable, ssh_executable
  • ansible_ssh_private_key_file
  • ansible_ssh_pass, ansible_password (default: assume passwordless)
  • ssh_args, ssh_common_args, ssh_extra_args
  • mitogen_ssh_debug_level: integer between 0..3 indicating the SSH client debug level. Ansible must also be run with ‘-vvv’ to view the output.
  • mitogen_ssh_compression: True to enable SSH compression, otherwise False. This will change to off by default in a future release. If you are targetting many hosts on a fast network, please consider disabling SSH compression.


Diagnostics and logging package output on targets are usually discarded. With Mitogen, these are captured and forwarded to the controller where they can be viewed with -vvv. Basic high level logs are produced with -vvv, with logging of all IO on the controller with -vvvv or higher.

While uncaptured standard IO and the logging package on targets is forwarded, it is not possible to receive IO activity logs, as the forwarding process would would itself generate additional IO.

To receive a complete trace of every process on every machine, file-based logging is necessary. File-based logging can be enabled by setting MITOGEN_ROUTER_DEBUG=1 in your environment. When file-based logging is enabled, one file per context will be created on the local machine and every target machine, as /tmp/mitogen.<pid>.log.

Common Problems

The most common bug reports fall into the following categories, so it is worth checking whether you can categorize a problem using the tools provided before reporting it:

Missed/Incorrect Configuration Variables

In some cases Ansible may support a configuration variable that Mitogen does not yet support, or Mitogen supports, but the support is broken. For example, Mitogen may pick the wrong username or SSH parameters.

To detect this, use the special mitogen_get_stack action described below to verify the settings Mitogen has chosen for the connection make sense.

Process Environment Differences

Mitogen’s process model differs significantly to Ansible’s in many places. In the past, bugs have been reported because Ansible plug-ins modify an environment variable after Mitogen processes are started.

If your task’s failure may relate to the process environment in some way, for example, SSH_AUTH_SOCK, LC_ALL or PATH, then an environment difference may explain it. Environment differences are always considered bugs in the extension, and are very easy to repair, so even if you find a workaround, please report them to avoid someone else encountering the same problem.

Variable Expansion Differences

To avoid many classes of bugs, Mitogen avoids shell wherever possible. Ansible however is traditionally built on shell, and it is often difficult to tell just how many times a configuration parameter will pass through shell expansion and quoting, and in what context before it is used.

Due to this, in some circumstances Mitogen may parse some expanded variables differently, for example, in the wrong user account. Careful review of -vvv and mitogen_ssh_debug_level logs can reveal this. For example in the past, Mitogen used a different method of expanding ~/.ssh/id_rsa, causing authentication to fail when ansible-playbook was run via sudo -E.

External Tool Integration Differences

Mitogen reimplements any aspect of Ansible that involves integrating with SSH, sudo, Docker, or related tools. For this reason, sometimes its support for those tools differs or is less mature than in Ansible.

In the past Mitogen has had bug reports due to failing to recognize a particular variation of a login or password prompt on an exotic or non-English operating system, or confusing a login banner for a password prompt. Careful review of -vvv logs help identify these cases, as Mitogen logs all strings it receives during connection, and how it interprets them.

The mitogen_get_stack Action

When a Mitogen strategy is loaded, a special mitogen_get_stack action is available that returns a concise description of the connection configuration as extracted from Ansible and passed to the core library. Using it, you can learn whether a problem lies in the Ansible extension or deeper in library code.

The action may be used in a playbook as mitogen_get_stack: just like a regular module, or directly from the command-line:

$ ANSIBLE_STRATEGY=mitogen_linear ansible -m mitogen_get_stack -b -k k3
SSH password:
k3 | SUCCESS => {
    "changed": true,
    "result": [
            "kwargs": {
                "check_host_keys": "enforce",
                "connect_timeout": 10,
                "hostname": "k3",
                "identities_only": false,
                "identity_file": null,
                "password": "mysecretpassword",
                "port": null,
                "python_path": null,
                "ssh_args": [
                "ssh_debug_level": null,
                "ssh_path": "ssh",
                "username": null
            "method": "ssh"
            "enable_lru": true,
            "kwargs": {
                "connect_timeout": 10,
                "password": null,
                "python_path": null,
                "sudo_args": [
                "sudo_path": null,
                "username": "root"
            "method": "sudo"

Each object in the list represents a single ‘hop’ in the connection, from nearest to furthest. Unlike in Ansible, the core library treats become steps and SSH steps identically, so they are represented distinctly in the output.

The presence of null means no explicit value was extracted from Ansible, and either the Mitogen library or SSH will choose a value for the parameter. In the example above, Mitogen will choose /usr/bin/python for python_path, and SSH will choose 22 for port, or whatever Port it parses from ~/.ssh/config. Note the presence of null may indicate the extension failed to extract the correct value.

When using mitogen_get_stack to diagnose a problem, pay special attention to ensuring the invocation exactly matches the problematic task. For example, if the failing task has delegate_to: or become: enabled, the mitogen_get_stack invocation must include those statements in order for the output to be accurate.

If a playbook cannot start at all, you may need to temporarily use gather_facts: no to allow the first task to proceed. This action does not create connections, so if it is the first task, it is still possible to review its output.

The mitogen_ssh_debug_level Variable

Mitogen has support for capturing SSH diagnostic logs, and integrating them into the regular debug log output produced when -vvv is active. This provides a single audit trail of every component active during SSH authentication.

Particularly for authentication failures, setting this variable to 3, in combination with -vvv, allows review of every parameter passed to SSH, and review of every action SSH attempted during authentication.

For example, this method can be used to ascertain whether SSH attempted agent authentication, or what private key files it was able to access and which it tried.

Post-authentication Bootstrap Failure

If logging indicates Mitogen was able to authenticate, but some error occurred after authentication preventing the Python bootstrap from completing, it can be immensely useful to temporarily replace ansible_python_interpreter with a wrapper that runs Python under strace:

$ ssh badbox

badbox$ cat >
strace -o /tmp/strace-python.$$ -ff -s 100 python "$@"

badbox$ chmod +x
badbox$ logout

$ ansible-playbook site.yml \
    -e ansible_python_interpreter=./ \
    -l badbox

This will produce a potentially large number of log files under /tmp/. The lowest-numbered traced PID is generally the main Python interpreter. The most intricate bootstrap steps happen there, any error should be visible near the end of the trace.

It is also possible the first stage bootstrap failed. That is usually the next lowest-numbered PID and tends to be the smallest file. Even if you can’t ascertain the problem with your configuration from these logs, including them in a bug report can save days of detective effort.

Diagnosing Hangs

If you encounter a hang, the MITOGEN_DUMP_THREAD_STACKS=<secs> environment variable arranges for each process on each machine to dump each thread stack into the logging framework every secs seconds, which is visible when running with -vvv.

However, certain controller hangs may render MITOGEN_DUMP_THREAD_STACKS ineffective, or occur too infrequently for interactive reproduction. In these cases faulthandler may be used:

  1. For Python 2, pip install faulthandler. This is unnecessary on Python 3.
  2. Once the hang occurs, observe the process tree using pstree or ps --forest.
  3. The most likely process to be hung is the connection multiplexer, which can easily be identified as the parent of all SSH client processes.
  4. Send kill -SEGV <pid> to the multiplexer PID, causing it to print all thread stacks.
  5. File a bug including a copy of the stacks, along with a description of the last task executing prior to the hang.

It is possible the hang occurred in a process on a target. If strace is available, look for the host name not listed in Ansible output as reporting a result for the most recent task, log into it, and use strace -ff -p <pid> on each process whose name begins with mitogen::

$ strace -ff -p 29858
strace: Process 29858 attached with 3 threads
[pid 29864] futex(0x55ea9be52f60, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 0, NULL, 0xffffffff <unfinished ...>
[pid 29860] restart_syscall(<... resuming interrupted poll ...> <unfinished ...>
[pid 29858] futex(0x55ea9be52f60, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 0, NULL, 0xffffffff


This shows one thread waiting on IO (poll) and two more waiting on the same lock. It is taken from a real example of a deadlock due to a forking bug. Please include any such information for all processes that you are able to collect in any bug report.

Getting Help

Some users and developers hang out on the #mitogen channel on the FreeNode IRC network.

Sample Profiles

The summaries below may be reproduced using data and scripts maintained in the pcaps branch. Traces were recorded using Ansible 2.5.14.

Trivial Loop: Local Host

This demonstrates Mitogen vs. SSH pipelining to the local machine running bench/loop-100-items.yml, executing a simple command 100 times. Most Ansible controller overhead is isolated, characterizing just module executor and connection layer performance. Mitogen requires 63x less bandwidth and 5.9x less time.


Unlike in SSH pipelining where payloads are sent as a single compressed block, by default Mitogen enables SSH compression for its uncompressed RPC data. In many-host scenarios it may be desirable to disable compression. This has negligible impact on footprint, since program code is separately compressed and sent only once. Compression also benefits SSH pipelining, but the presence of large precompressed per-task payloads may present a more significant CPU burden during many-host runs.


In a detailed trace, improved interaction with the host machine is visible. In this playbook because no forks were required to start SSH clients from the worker process executing the loop, the worker’s memory was never marked read-only, thus avoiding a major hidden performance problem - the page fault rate is more than halved.

File Transfer: UK to France

This playbook was used to compare file transfer performance over a ~26 ms link. It uses the with_filetree loop syntax to copy a directory of 1,000 0-byte files to the target.

  Secs CPU Secs Sent Received Roundtrips
Mitogen 98.54 43.04 815 KiB 447 KiB 3.79
SSH Pipelining 1,483.54 329.37 99,539 KiB 6,870 KiB 57.01

Roundtrips is the approximate number of network roundtrips required to describe the runtime that was consumed. Due to Mitogen’s built-in file transfer support, continuous reinitialization of an external scp/sftp client is avoided, permitting large with_filetree copies to become practical without any special casing within the playbook or the Ansible implementation.

DebOps: UK to India

This is an all-green run of 246 tasks from the DebOps 0.7.2 common.yml playbook over a ~370 ms link between the UK and India. The playbook touches a wide variety of modules, many featuring unavoidable waits for slow computation on the target.

More tasks of a wider variety are featured than previously, placing strain on Mitogen’s module loading and in-memory caching. By running over a long-distance connection, it highlights behaviour of the connection layer in the presence of high latency.

Mitogen requires 14.5x less bandwidth and 4x less time.


Django App: UK to India

This short playbook features only 23 steps executed over the same ~370 ms link as previously, with many steps running unavoidably expensive tasks like building C++ code, and compiling static web site assets.

Despite the small margin for optimization, Mitogen still manages 6.2x less bandwidth and 1.8x less time.