Runtime runs every sandbox as a Firecracker microVM on dedicated servers it operates itself. Each one gets its own kernel, without a second layer of virtualization underneath.
Bare metal against a virtual machine
| Question | Bare-metal server | Virtual machine |
|---|---|---|
| Who owns the hardware | You, for the rental period | Shared; the provider slices it |
| Hypervisor | Yours to install, or none | The provider's |
| Hardware features | All of them: /dev/kvm, performance counters |
Only what the provider passes through |
| Start time | Minutes to hours, when a machine is provisioned | Seconds to minutes |
| Billing | The whole machine, busy or not | The slice, usually by the second |
Why sandbox providers care
A microVM needs hardware virtualization, which Linux exposes as KVM. Inside
another provider's VM, KVM is often missing or slow. Firecracker's getting-started
guide states this directly: "EC2 only supports KVM on .metal instance types".
The project runs its tests on those instance types. So a sandbox service built
on microVMs either rents bare metal or owns its servers.
Owning the servers also sets the price floor. A provider that resells another cloud's VMs pays that cloud's margin first. A provider that runs its own hardware can bill the CPU a sandbox uses, rather than the vCPUs it holds. That is how Runtime charges $0.025 per active vCPU-hour (pricing).
What you get on Runtime
You do not rent the whole machine. You get a slice of it with its own kernel. The host enforces CPU, memory, disk and network rules, outside the guest (security). A paid sandbox takes up to 16 vCPUs and 64 GiB. It pauses with its memory kept. While it waits, a 2 vCPU, 4 GiB sandbox costs $0.03125 an hour.
Related
Sources
Checked 25 October 2026.