How fast does a Space fork from a checkpoint?

A QEMU VM Space forked from a checkpoint holding 300 MB in RAM and 100 MB on disk answered its first command 328.8 ms after the fork call (p50, n = 30) on a Mac Studio (Apple M2 Max, 96 GB), measured 2026-10-25 on Cua Spaces 0.15.0.

Measured 2026-10-25Build app 0.15.0, cua 0.9.0, cua-spacesd 0.8.0n = 30Raw data (CSV)

Results

Time from the fork call until the fork answers its first command, serial. Lower is better.

Fastest

225.5 ms

p50

328.8 ms

p90

411.8 ms

p99

447.9 ms

Slowest

447.9 ms

Values shown in the chart
Measure	Value
Fastest	225.5 ms
p50	328.8 ms
p90	411.8 ms
p99	447.9 ms
Slowest	447.9 ms
Samples	30

Method

Driver on the host: rollout_bench (Rust SDK, built from the tag) against this release's daemon. A QEMU Linux VM Space (2 vCPU / 2 GiB, headless, created --forkable so its RAM is a shareable file) gets a world: 300 MB of random data in /dev/shm (guest RAM) and a 100 MB file on disk. Then a pinned memory+disk checkpoint (Sandboxes.checkpoint). Each sample = Sandboxes.fork(checkpoint) start → the fork's cua-spacesd answers a first sh -c true (fork + attach + first exec). n=30, serial, each fork deleted after its sample. A separate fork verified the world (test -s /dev/shm/state && test -s ~/state.bin: state-ok). Extra: 10 batches of fork_many x16 (16 forks started at once).

Caveats

QEMU VM kind. A gVisor fork from a checkpoint, n = 10, took p50 301 ms and p95 1,939 ms.

Environment and build

Machine	Mac Studio, Mac14,13 (Mac Studio)
Chip	Apple M2 Max, 12 cores
Memory	96 GB
OS	macOS 26.6.2 (25G83)
Where	local
Other vms	1
Network	n/a (local)
Kind	vm
Image	ghcr.io/trycua/linux:24.04-slim (QEMU disk)
Shape	2 vCPU, 2 GiB, --headless, --forkable
Build	Cua Spaces 0.15.0 DMG (https://github.com/trycua/cua/releases/download/cua-spaces-v0.15.0/cua-spaces-0.15.0-darwin-universal.dmg, sha256 8725a09ba4132aa8a1667fa48c86416c91d5dec0d3c7dc93f88f5f3e29e0aba0): bundled cua 0.9.0, Helpers/cua-spacesd 0.8.0, cua-vz
App	0.15.0
cua	0.9.0
cua_spacesd	0.8.0

Reproduce

# On an Apple silicon Mac (macOS 26), with Rust (rustup) and git:
curl -fLO https://github.com/trycua/cua/releases/download/cua-spaces-v0.15.0/cua-spaces-0.15.0-darwin-universal.dmg
hdiutil attach -nobrowse -mountpoint /tmp/cua-dmg cua-spaces-0.15.0-darwin-universal.dmg
mkdir -p ~/bench && ditto "/tmp/cua-dmg/Cua Spaces.app" ~/bench/"Cua Spaces.app" && hdiutil detach /tmp/cua-dmg
git clone --depth 1 --branch cua-spaces-v0.15.0 https://github.com/trycua/cua ~/bench/cua
(cd ~/bench/cua/libs/cua && cargo build --locked --release -p cua-sdk --example rollout_bench --example channel_bench)
export PATH="$HOME/bench/Cua Spaces.app/Contents/MacOS:$PATH" CUA_HOME=~/bench/home DO_NOT_TRACK=1 CUA_TELEMETRY=0
RB=~/bench/cua/libs/cua/target/release/examples/rollout_bench
cua daemon start --foreground &
$RB create --prefix fsrc- --n 1 --kind vm --cpus 2 --memory-mb 2048 --headless --forkable --keep
cua sb exec fsrc-0 'dd if=/dev/urandom of=/dev/shm/state bs=1M count=300; dd if=/dev/urandom of=$HOME/state.bin bs=1M count=100; sync'
$RB checkpoint --source fsrc-0 --name ckq-1 --pin
$RB fork --source ckq-1 --prefix fq- --n 30 --conc 1                    # serial forks, each deleted after its sample
for r in $(seq 1 10); do $RB fork-many --source ckq-1 --prefix fm$r- --n 16 --conc 16; $RB delete --prefix fm$r- --conc 16; done

Cua commands use the published Cua Spaces 0.15.0 release and the public trycua/cua repository at the tag cua-spaces-v0.15.0. Downloads: samples (CSV), result (JSON), this page as Markdown.

Questions

Does the fork keep the running state?

Yes. A separate fork checked that the 300 MB in /dev/shm (RAM) and the 100 MB file on disk were there.

What about many forks at once?

Ten batches of 16 forks started at once took 2,768 ms of wall time per batch at the p50.

Can I run this myself?

Yes. The commands under Reproduce use the published Cua Spaces 0.15.0 release and the public trycua/cua repository at the tag cua-spaces-v0.15.0. A `cua bench run` command that wraps them is coming; it isn't in a release yet.

Checked 2026-10-25. Sources: the samples and result files above, measured by Cua. All benchmarks.