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Performance

What You'll Learn

  • The five highest-leverage performance settings, in the order to try them
  • Why raising forks alone isn't enough at real scale
  • What actually changes once you're managing thousands of hosts, not dozens

Why Ansible's Defaults Are Conservative

forks=5, no pipelining, host key checking on — Ansible's out-of-the-box defaults are tuned for correctness on an unknown environment, not speed on a known one. A playbook that takes two minutes against 10 hosts can take hours against 2,000 with the same untouched defaults. Closing that gap deliberately is what this page is about.

The Core Levers, in Order of Leverage

  1. forks — the parallelism ceiling per task. Raise it to match control-node headroom (CPU, memory, open file descriptors) and real target capacity. See Forks, Serial, Strategy, and Throttle for the full mental model.
  2. Pipelining — eliminates the "copy module file, then execute" round trip in favor of piping module code directly. One of the single highest-leverage settings available, and free once requiretty is disabled in sudoers on the managed nodes.
  3. SSH multiplexing (ControlPersist) — reuses one SSH connection across every task in a run instead of a fresh handshake per task. Configured via ssh_args in ansible.cfg.
  4. Fact caching — skips re-gathering facts on every run when they haven't meaningfully changed. See Fact Caching.
  5. Strategy — free lets fast hosts proceed without waiting on stragglers, at the cost of losing strict per-task ordering across hosts. Only correct when a play has no real cross-host ordering dependency.

Before / After

ansible.cfg — before
[defaults]
forks = 5
ansible.cfg — after
[defaults]
forks = 20

[ssh_connection]
pipelining = True
ssh_args = -o ControlMaster=auto -o ControlPersist=60s

A playbook gathering facts and installing one package across 200 hosts, with default settings, processes hosts in batches of 5 with a fresh SSH handshake and full module-file transfer for every task. With the tuned config above, the same run uses batches of 20, reuses connections, and pipes modules directly — the difference is routinely several times faster on a real fleet, though the exact multiplier depends on network latency and target host load.

Scaling to Thousands of Hosts

  • Combine high forks, pipelining, ControlPersist, and fact caching as the baseline — before reaching for anything more exotic.
  • Use serial (see Playbooks, Plays, and Tasks) to batch extremely large runs, not just for blast-radius control but because a single control node has real ceilings on concurrent SSH connections and in-flight result memory.
  • Beyond a single control node's ceiling, distribute execution across multiple control/execution nodes rather than scaling one node vertically forever — this is what Red Hat Ansible Automation Platform's Automation Mesh is built for, if that's part of the stack.

A Note on Mitogen

You may see Mitogen referenced in older Ansible performance discussions — a third-party strategy plugin that kept a persistent Python interpreter and connection state on the control node, avoiding repeated interpreter startup cost. It was a meaningful speedup at the time. As pipelining and ControlPersist matured in Ansible core, and Mitogen's own maintenance slowed, its relevance faded — worth knowing the name if it comes up, not something to reach for by default today.

Common Mistakes

  • Raising forks without also checking SSH/OS-level connection limits on the control node — hitting a file-descriptor or connection-count wall unrelated to Ansible itself.
  • Enabling pipelining without disabling requiretty in sudoers — produces become failures that look unrelated to the actual cause.
  • Assuming strategy: free is always faster — for a play with real cross-host ordering dependencies, it produces incorrect behavior, not just different timing.
  • Tuning forks/pipelining while leaving gather_facts: true (the default) on plays that never reference a fact — wasted time that neither setting fixes.

Interview Questions

  • What are the highest-leverage Ansible performance settings, and in what order would you try them?
  • How would you scale Ansible to manage 10,000 hosts?
  • What was Mitogen, and why is it less commonly used today?

See the full reasoning walkthrough in Interview Prep: Scenario-Based Questions.

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