Use the Firecracker balloon with deflate_on_oom and free-page-reporting to safely overcommit and reclaim idle guest memory
domain: firecracker-microvm.github.io · 5 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
The balloon lets the host reclaim guest memory (inflate) and give it back later (deflate); install it pre-boot via PUT /balloon and adjust target size anytime via PATCH /balloon {amount_mib}
Set deflate_on_oom=true so that when a guest process allocation would trigger OOM, the kernel takes pages from the balloon instead of invoking the OOM killer - a guest-side memory safety valve
Enable free_page_reporting so the guest continuously reports ranges it is not using, which the host can reclaim proactively
free_page_hinting is a Developer-Preview feature that lets the HOST trigger reclaim of guest-reported pages (host-initiated)
Operate the balloon through the API: GET /balloon returns config, PATCH /balloon and PATCH /balloon/statistics resize target and stats interval at runtime
Known gotchas
The virtio balloon is paravirtualized and REQUIRES guest-driver cooperation - it is not a security boundary; a compromised guest driver can ignore the balloon and use all memory it was booted with
Prepare the host for the worst case (provision host memory for full boot size, or swap, or monitor + kill/migrate the VM) even while using the balloon
deflate_on_oom does not cover kernel-internal allocations (caches) or overcommit-refused user allocations; running with vm.overcommit_memory=1 globally is not recommended
Guest needs kernel CONFIG_MEMORY_BALLOON=y and CONFIG_VIRTIO_BALLOON=y
Official doc: https://raw.githubusercontent.com/firecracker-microvm/firecracker/main/docs/ballooning.md
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