Speed up bazel CI builds via smarter caching

[smolkaj]

The bazel builds we run for continuous integration could likely be sped up significantly by using a smarter caching scheme, like P4Runtime and p4-constraints.

[Sarthak-Developer-Coder]

Speeding up Bazel CI builds through smarter caching (#5142) can significantly reduce build times and resource consumption. Here’s how to optimize Bazel’s caching strategy for CI:

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1. Enable Remote Caching

Bazel supports remote caching, which allows build outputs to be shared across different CI runs, preventing redundant compilations.

Solution: Use a Remote Cache (e.g., Google Cloud, S3, or BuildBuddy)

Modify your CI configuration to use a remote cache:

bazel build --remote_cache=<your-remote-cache-url> --remote_upload_local_results

If using Google Cloud Storage, configure it as follows:

bazel build --remote_cache=grpc://bazel-cache.example.com

For AWS S3, use:

bazel build --experimental_remote_cache=your-s3-bucket-url

👉 Benefit: Cache reuse across different CI jobs.

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2. Use Persistent Disk Caching in CI

To store build artifacts locally, enable disk caching in your CI pipeline:

bazel build --disk_cache=/cache/bazel

In a Docker-based CI, mount a persistent volume for /cache/bazel to ensure caches persist between builds.

👉 Benefit: Faster builds for frequently changed code.

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3. Enable Remote Execution (Optional)

If you have a distributed build environment, configure Bazel Remote Execution to offload computations.

bazel build --remote_executor=grpc://build-cluster.example.com

👉 Benefit: Speed up builds by distributing workloads.

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4. Reduce Unnecessary Rebuilds

Modify bazel.rc to enable smart dependency tracking:

build --repository_cache=~/.bazel-cache
build --config=ci

Add --config=ci in your CI pipeline:

bazel test --config=ci //...

👉 Benefit: Avoid rebuilding unchanged dependencies.

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5. Optimize CI Cache Keys

For GitHub Actions, cache Bazel’s outputs:

- name: Cache Bazel
  uses: actions/cache@v3
  with:
    path: ~/.cache/bazel
    key: ${{ runner.os }}-bazel-${{ github.sha }}
    restore-keys: |
      ${{ runner.os }}-bazel-

👉 Benefit: Restores previous builds for faster execution.

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6. Use --keep_state_after_build

Preserve intermediate build state between CI steps:

bazel build --keep_state_after_build

👉 Benefit: Reduces recomputation in multi-step builds.

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7. Enable Build Profiling

To identify slow targets, generate a build profile:

bazel build --profile=bazel-profile.json

Analyze with:

bazel analyze-profile bazel-profile.json

👉 Benefit: Identifies bottlenecks in CI builds.

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🚀 Next Steps

Would you like help setting this up for a specific CI provider (GitHub Actions, GitLab, Jenkins)? Let me know!