Multi-Stage Build Size Reduction

Problem Statement

A multi-stage build compiles wheels, CUDA extensions and datasets in a fat builder stage, then copies only the finished artefacts into a slim runtime stage. Everything else in the builder is discarded — it never reaches the registry. Compute how much that actually saves.

Background

The shipped image is the runtime stage: its base image, its own layers, plus whatever COPY --from=builder pulled across. The builder's compilers, caches and intermediate object files are left behind.

The single-stage baseline used here is the honest comparison: you would have done everything in one image on top of the builder's base, so you pay for every builder layer plus the runtime stage's non-base layers.

multi_stage_mb  = sum(runtime_layers) + copied_mb
single_stage_mb = sum(builder_layers) + sum(runtime_layers[1:])
saved_mb        = single_stage_mb - multi_stage_mb
reduction_pct   = 100 * saved_mb / single_stage_mb

Note runtime_layers[1:] — the single-stage build does not carry a second base image.

Your Task

Implement:

def multi_stage_report(builder_layers, runtime_layers, copied_mb):

Return a dict with keys "multi_stage_mb", "single_stage_mb", "saved_mb" and "reduction_pct", in that order, each rounded to 2 decimal places.

Input Format

• builder_layers: list of numbers, MB added by each builder-stage layer (index 0 is the builder base image).
• runtime_layers: list of numbers, MB added by each runtime-stage layer (index 0 is the runtime base image).
• copied_mb: number, total MB copied from the builder into the runtime stage.

Output Format

• A dict with the four keys above, values rounded to 2 decimals.

Sample

print(multi_stage_report([900.0, 15.0, 640.0], [110.0, 4.0], 52.0))

Output:

{'multi_stage_mb': 166.0, 'single_stage_mb': 1559.0, 'saved_mb': 1393.0, 'reduction_pct': 89.35}

The runtime stage is 114 MB plus a 52 MB virtualenv copied from the builder, so 166 MB ships. The one-stage equivalent drags the 900 MB toolchain base and the 640 MB build cache along: 1559 MB.