Limit sccache cache size to 3.5 GB to reduce transfer overhead

[agocke]

Note

This PR was generated with the assistance of GitHub Copilot.

Summary

The sccache local cache grows unboundedly (default 10 GB max) as stale entries accumulate across rolling builds. It's currently at ~6-7 GB and growing ~0.5 GB per rolling build cycle. The cache restore/save overhead is ~3 minutes per leg, eating into the ~9 minute build time savings sccache provides.

Change

Sets SCCACHE_CACHE_SIZE=3.5G to force LRU eviction of stale entries, keeping the cache close to its working set size (~3 GB for ~3,482 compiled objects).

Expected Impact

Metric	Before	After
Cache size	~6-7 GB (growing)	~3.5 GB (stable)
Restore time	~1 min	~30s
Save time	~1:45	~50s
Total overhead/leg	~3 min	~1.5 min
Net savings/leg	~6 min	~7.5 min

If hit rates drop due to premature eviction, the limit can be raised.

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[Copilot]

Pull request overview

This PR primarily updates the CoreCLR pipeline sccache setup to reduce cache transfer overhead by bounding the local cache size, but it also includes unrelated System.Security.Cryptography signature changes that don’t match the stated scope.

Changes:

• Set SCCACHE_CACHE_SIZE in the sccache setup pipeline step (currently to 3.5G).
• Adjust the Azure Pipelines Cache key/restoreKeys to exclude buildConfig.
• Mark X25519DiffieHellmanCng.DeriveRawSecretAgreementCore partial overrides as unsafe across Windows + not-supported shims.

Reviewed changes

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

File	Description
eng/pipelines/coreclr/templates/setup-sccache.yml	Sets sccache environment variables and modifies cache keying/restoration behavior.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanCng.Windows.cs	Changes the override signature to add unsafe on the Windows implementation.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanCng.cs	Changes the partial override declaration to add unsafe.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/Cng.NotSupported.cs	Changes the PNSE shim override signature to add unsafe.

Comments suppressed due to low confidence (1)

eng/pipelines/coreclr/templates/setup-sccache.yml:35

• The PR description/title talks about limiting the local sccache size, but this hunk also changes the cache key/restoreKeys to drop buildConfig (affecting cache sharing behavior between PR/rolling builds). Either update the PR description to cover this behavior change and its rationale/risks, or keep this PR focused by reverting the key change.

    #
    # buildConfig is intentionally excluded from the key: it reflects
    # the *managed* Configuration (Debug for PRs, Release for rolling
    # builds), but sccache only caches native (C/C++) compilations whose
    # flags are controlled by RuntimeConfiguration (-rc), which is
    # constant per leg regardless of buildConfig.  Omitting it lets PR
    # builds warm-start from rolling-build caches saved in the main scope.
    - task: Cache@2
      displayName: Sccache cache
      inputs:
        key: sccache | ${{ parameters.osGroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }} | "$(Build.BuildId)"
        path: $(Pipeline.Workspace)/.sccache
        restoreKeys: |
          sccache | ${{ parameters.osGroup }} | ${{ parameters.archType }} | ${{ parameters.nameSuffix }}

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[agocke]

/ba-g all affected legs have passed