k7/bench/docker-perf

Docker workload benchmark — Spec 10b

Quantifies the storage tax of running Docker inside a k7 sandbox vs natively on the host. Output lands in PERFORMANCE.md under a dated section, formatted so the median + ratio numbers can be cited verbatim from any blog post.

What gets benched

Label	What it is
host	Native Docker on the Hetzner node (no k7 involved)
k7-fd	k7 sandbox, kata-firecracker-devmapper (kfd) backend, --sidecar docker (docker daemon's /var/lib/docker is an emptyDir)
k7-ql-r1	k7 sandbox, kata-qemu-longhorn (kql) backend, --sidecar docker, Longhorn replicas=1
k7-ql-r2	Same as k7-ql-r1 but Longhorn replicas=2 (requires ≥ 2-node cluster)

The four environments stack the storage path cleanly: host → no VM, no Longhorn. k7-fd → VM but no Longhorn. k7-ql-r1 → VM + one local Longhorn replica. k7-ql-r2 → VM + one local + one cross-node Longhorn replica. Subtracting k7-fd / host from k7-ql-r1 / host isolates the Longhorn tax from the kata-qemu tax.

Workloads (5 runs each, after a discarded warm-up)

Operation	Isolates
pull debian:12-slim	Network + extract + write of an external image
build (no-cache)	The headline number: apt + pip + git + 256 MB fsync
build (cached)	Sanity — should be sub-second; non-zero on k7-ql-* would mean the bind mount of /var/lib/docker is broken
run cpu (10s budget)	Kata-qemu CPU overhead (Python busy-loop)
run io (2k small + 512 MB fsync)	Write path through the storage stack
run read (venv tree cat)	Read path (mostly page-cache hits after the first call)

The Dockerfile that drives build (no-cache) is pinned at bench/docker-perf/bench.Dockerfile — apt + pip generate thousands of small files (metadata pressure), git clone is inode-heavy, and dd … conv=fsync measures sync-write throughput.

How to run it

The bench is a pytest module (not a shell script) — it reuses the exact same k7_core / test_namespace fixtures that tests/integration/test_sidecar_docker.py already uses to spin up a docker:27.5-cli sandbox with --sidecar docker. The harness times k7_core.exec_command(...) calls instead of doing anything new.

Run all three envs:

# On the cluster node (or any host that already runs the integration suite):
K7_BENCH_ENVS=host,k7-ql-r1,k7-ql-r2 \
uv run pytest -m bench tests/integration/bench_docker_perf.py -v -s

Pick a subset (useful for iterating on tooling):

K7_BENCH_ENVS=host             uv run pytest -m bench -v -s tests/integration/bench_docker_perf.py
K7_BENCH_ENVS=k7-ql-r1         uv run pytest -m bench -v -s tests/integration/bench_docker_perf.py

Switching r1 → r2 is done in-cluster by patching Longhorn's default-replica-count setting via kubectl — no k7 install reinstall, no cluster-wide downtime, and the value is restored at session teardown.

Environment variables

Var	Default	Purpose
K7_BENCH_ENVS	host,k7-ql-r1,k7-ql-r2	Comma-separated subset of envs to run
K7_BENCH_REPS	5	Reps per cell
K7_BENCH_WARMUP	1	0 to disable the warm-up
K7_BENCH_OUT	/tmp	Where logs + the aggregated CSV land

Output

Each leg writes a bench-<label>-<ts>.log of the same shape render.py already parses:

# label=k7-ql-r1
# timestamp=20260529T120000Z
# kernel=...
# docker_version=...
# longhorn_replicas=1
# columns: OPERATION  RUN  SECONDS
pull                     1 3.142
pull                     2 3.098
build_nocache            1 60.42
...

When the pytest session ends, a session-scoped fixture aggregates all logs from bench_logs into a single CSV at $K7_BENCH_OUT/bench-results-<ts>.csv. Feed that to render.py:

uv run python bench/docker-perf/render.py \
    --input /tmp/bench-results-<ts>.csv \
    --title "Docker workloads inside sandboxes — 2026-05-29 (k7 <sha>)" \
    --hardware "Hardware: 2x Hetzner AX42, Ubuntu 24.04, kernel 6.8, Docker 27.5, Longhorn 1.7.x." \
    --output - >> PERFORMANCE.md

Known confounds (acknowledge these honestly in any post)

1. Page cache — the harness drops /proc/sys/vm/drop_caches only on the host (where it has the privilege). Inside the sandbox we can't, so the run read leg is mostly a page-cache measurement, not a storage-stack one. Don't over-interpret that row.
2. Network egress for pull — Cilium FQDN egress + the sandbox's network path aren't the same as the host's. pull is here for context, not as a storage-tax signal.
3. Kata-qemu CPU overhead — the run cpu row reflects both CPU virtualization overhead and any scheduler differences. Reporting it explicitly lets readers do their own subtraction.
4. Workload immutability — the Dockerfile is committed and the base image (debian:12-slim) is captured by digest in each log header. The only real variable across re-runs is the k7 / Longhorn version.

Statistical handling

• 5 reps per cell, first warm-up run is discarded.
• Reported as median + min/max (mean is too sensitive to outliers on storage benchmarks).
• Cells with (max − min) / median > 0.30 are flagged with ⚠. Treat flagged cells as "rerun with more reps before publishing" — don't quote them.

Re-rendering without re-running

Already have a CSV (manually edited, or salvaged from a partial run)? Re-render with:

python3 bench/docker-perf/render.py \
    --input /tmp/bench-results-<ts>.csv \
    --title "Docker workloads inside sandboxes — 2026-05-29 (k7 <sha>)" \
    --hardware "Hardware: ..." \
    --output /tmp/section.md

That same output is what bench_docker_perf.py's teardown prints instructions for — re-rendering is just the path for fixing typos in the title / hardware line without re-running the bench.