From 2894831ecd67296cc351362e3287671d67cd5fc1 Mon Sep 17 00:00:00 2001
From: Gregory Szorc <gregory.szorc@gmail.com>
Date: Tue, 24 Mar 2026 11:00:15 +0800
Subject: [PATCH] ci: optimize Docker image loading

Loading images step currently takes ~30s +- a ton of variance.

Linux CI jobs are currently taking 8-20m typically. So image builds
can be up to ~6% of wall time.

I've been playing around with further increasing Depot runner sizes.
That delivers a considerable wall time reduction (<5m builds possible).
This increases the percentage overhead of any "CI taxes," like restoring
caches and pulling/loading container images.

This commit implements a pair of optimizations to enable the image
loading step to take as little as 5s:

* We parallelize the image decompression + loading using simple
  `&` + `wait` shell features.
* We filter out images that aren't needed for the current job.

The filtering isn't fully lean - we don't currently filter out
images related to cross-compiling.

And we still incur overhead to download images we don't load.

Those optimizations will wait for another day.
---
 .github/workflows/linux.yml | 44 +++++++++++++++++++++++++++++--------
 1 file changed, 35 insertions(+), 9 deletions(-)

diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index 189790fd7..bea616b76 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -2,7 +2,7 @@ name: linux
 
 on:
   push:
-    branches: [main]
+    branches: [ main ]
   pull_request:
 
 concurrency:
@@ -12,7 +12,7 @@ concurrency:
 env:
   FORCE_COLOR: 1
 
-permissions: {}
+permissions: { }
 
 jobs:
   crate-build:
@@ -248,15 +248,41 @@ jobs:
 
       - name: Load Docker Images
         run: |
+          set -euo pipefail
+          
+          # We need to keep the image-*.tar file since it is used as a
+          # Makefile dependency.
+          load() {
+            image="${1%.tar.zst}"
+            echo "decompressing ${image}.tar.zst"
+            zstd -d --rm "${image}.tar.zst"
+            docker load --input "${image}.tar"
+          }
+          
+          # Avoid loading images that aren't used.
+          case "$(uname -m)" in
+          aarch64)
+            want_suffix=linux_aarch64.tar.zst
+            ;;
+          x86_64)
+            want_suffix=linux_x86_64.tar.zst
+            ;;
+          *)
+            echo "unsupported host arch: $(uname -m)"
+            exit 1
+            ;;
+          esac
+          
           for f in build/image-*.tar.zst; do
-            echo "decompressing $f"
-            zstd -d --rm ${f}
-          done
-
-          for f in build/image-*.tar; do
-            echo "loading $f"
-            docker load --input $f
+            if [[ "$f" == *"${want_suffix}" ]]; then
+              load "${f}" &
+            else
+              echo "skipping ${f}"
+              rm "${f}"
+            fi
           done
+          
+          wait
 
       - name: Build
         if: ${{ ! matrix.dry-run }}