sccache - Shared Compilation Cache Cheatsheet

sccache - Shared Compilation Cache Cheatsheet

sccache is a compiler cache (in the spirit of ccache) from Mozilla that supports Rust, C/C++, and NVCC. It wraps your compiler, hashes the inputs, and returns a cached object when it has seen the same compilation before. Its distinguishing feature is shared storage backends — S3, GCS, Redis, Azure — so a whole team and your CI fleet share one cache. That turns “CI recompiles everything from scratch every run” into “CI downloads what it already built.”

Installation

Method	Command
Cargo	cargo install sccache --locked
macOS (Homebrew)	brew install sccache
Binary	download from GitHub Releases
Verify	sccache --version

Using It with Rust

# One-off
RUSTC_WRAPPER=sccache cargo build

# Persistent (in ~/.cargo/config.toml)
[build]
rustc-wrapper = "sccache"

Variable	Purpose
RUSTC_WRAPPER=sccache	Route rustc through sccache
SCCACHE_DIR	Local cache directory
SCCACHE_CACHE_SIZE	Max local cache size (e.g. 20G)

Using It with C/C++

# CMake
cmake -DCMAKE_C_COMPILER_LAUNCHER=sccache \
      -DCMAKE_CXX_COMPILER_LAUNCHER=sccache ..

# Make
make CC="sccache gcc" CXX="sccache g++"

Checking Effectiveness

Command	Shows
sccache --show-stats	Hits, misses, cache size, errors
sccache --zero-stats	Reset counters before a test build
sccache --stop-server	Stop the background server
sccache --start-server	Start it explicitly

# Measure a cold vs warm build
sccache --zero-stats
cargo build --release
sccache --show-stats     # expect misses
cargo clean && cargo build --release
sccache --show-stats     # expect hits

The key number is cache hit rate. A low rate usually means something in the inputs is changing (flags, env vars, absolute paths).

Shared Cloud Backends

# S3
export SCCACHE_BUCKET=my-build-cache
export SCCACHE_REGION=us-east-1
export AWS_ACCESS_KEY_ID=... AWS_SECRET_ACCESS_KEY=...

# Redis
export SCCACHE_REDIS=redis://cache.internal:6379

# GCS
export SCCACHE_GCS_BUCKET=my-build-cache
export SCCACHE_GCS_KEY_PATH=/path/to/key.json

Backend	Env prefix
Local disk	SCCACHE_DIR
S3 / S3-compatible	SCCACHE_BUCKET, SCCACHE_ENDPOINT
Redis	SCCACHE_REDIS
GCS	SCCACHE_GCS_*
Azure	SCCACHE_AZURE_*
GitHub Actions	SCCACHE_GHA_ENABLED=true

CI Pattern

# GitHub Actions
env:
  SCCACHE_GHA_ENABLED: "true"
  RUSTC_WRAPPER: "sccache"
steps:
  - uses: mozilla-actions/sccache-action@v0.0.5
  - run: cargo build --release
  - run: sccache --show-stats

Things That Break Caching

Cause	Fix
Absolute paths in output	Use --remap-path-prefix
Changing RUSTFLAGS	Keep flags stable across runs
Incremental compilation	Disable it (CARGO_INCREMENTAL=0) for cache use
Proc macros / build scripts	Not always cacheable
Different compiler versions	Cache keys include the compiler

sccache vs Alternatives

Aspect	sccache	ccache	Bazel/Buck
Languages	Rust, C/C++, CUDA	C/C++	Many (full build system)
Shared cache	Cloud backends built in	Mostly local	Remote cache/execution
Setup	Wrapper env var	Wrapper	Adopt a new build system
Best for	Speeding up existing builds	C/C++ local caching	Large monorepos

Pairs well with mold — sccache cuts compile time, mold cuts link time.

Resources

• GitHub Repository
• Documentation
• GitHub Actions integration