Sub-millisecond VM sandboxes using CoW memory forking
I wanted to see how fast an isolated code sandbox could start if I never had to boot a fresh VM. So instead of launching a new microVM per execution, I boot Firecracker once with Python and numpy already loaded, then snapshot the full VM state. Every execution after that creates a new KVM VM backed by a `MAP_PRIVATE` mapping of the snapshot memory, so Linux gives me copy-on-write pages automatically. That means each sandbox starts from an already-running Python process inside a real VM, runs the code, and exits. These are real KVM VMs, not containers: separate guest kernel, separate guest…
In plain words
This project demonstrates a method for creating isolated code sandboxes that start in sub-millisecond time using Firecracker microVMs. Instead of booting a fresh VM for each execution, it snapshots a pre-loaded VM with Python and numpy, then uses copy-on-write memory forking to spawn new isolated KVM instances that inherit the snapshot state. Each sandbox runs as a complete virtual machine with its own kernel and memory space, with pages copied only when modified. The implementation is written in Rust and released under Apache 2.0.
written from the facts on this page · September 2026
From the sources
In the maker’s words, at launch
I wanted to see how fast an isolated code sandbox could start if I never had to boot a fresh VM. So instead of launching a new microVM per execution, I boot Firecracker once with Python and numpy already loaded, then snapshot the full VM state. Every execution after that creates a new KVM VM backed by a `MAP_PRIVATE` mapping of the snapshot memory, so Linux gives me copy-on-write pages automatically. That means each sandbox starts from an already-running Python process inside a real VM, runs the code, and exits. These are real KVM VMs, not containers: separate guest kernel, separate guest memory, separate page tables. When a VM writes to memory, it gets a private copy of that page. The hard part was not CoW itself. The hard part was resuming the snapshotted VM correctly. Rust, Apache 2.0.
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