Atharva Pandey Gopherine

Zeroboot achieves sub-millisecond VM sandbox creation using copy-on-write forking.

The architecture decision: Zeroboot utilizes the copy-on-write (CoW) forking mechanism, where a child process is created by fork()ing an already-running VM snapshot, sharing the parent’s memory pages until either process writes, at which point the kernel copies just that page. This approach enables full isolation with near-zero startup cost. The use of CoW forking allows Zeroboot to create sandboxes at a rate of under a millisecond.

The tradeoff nobody talks about: The CoW forking approach is optimized for read-heavy workloads, but it can be defeated by write-intensive workloads. If an AI agent writes to many memory pages quickly, it can trigger a storm of page faults, each copying a 4KB page, thereby increasing the latency. For instance, an agent that allocates 100MB of heap on startup can lead to a significant number of page faults, rendering the CoW forking approach less effective.

The implementation detail: The Zeroboot implementation uses the libc library directly for fork semantics, rather than relying on a hypervisor layer. This design choice allows sandboxes to be created as Linux processes, sharing the host kernel, which results in faster sandbox creation. However, this approach also means that the isolation boundary is the process, rather than being hardware-enforced.

The engineering principle: This approach is similar to the pattern used in Redis BGSAVE, where the process is forked, and the child serializes the state while the parent continues serving. The use of CoW forking makes the fork nearly free, providing snapshot isolation at almost zero cost. If the workload is read-heavy with occasional writes, CoW forking can be an effective technique for achieving isolation.

https://github.com/zerobootdev/zeroboot