sccache - Shared Compilation Cache

sccache is a ccache-like compiler caching tool. It is used as a compiler wrapper and avoids compilation when possible, storing cached results either on local disk or in one of several cloud storage backends.

sccache includes support for caching the compilation of C/C++ code, Rust, as well as NVIDIA's CUDA using nvcc.

sccache also provides icecream-style distributed compilation (automatic packaging of local toolchains) for all supported compilers (including Rust). The distributed compilation system includes several security features that icecream lacks such as authentication, transport layer encryption, and sandboxed compiler execution on build servers. See the distributed quickstart guide for more information.

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Table of Contents (ToC)

• Installation
• Build Requirements
• Build
• Usage
• Storage Options
  • Local
  • S3
  • Redis
  • Memcached
  • Google Cloud Storage
  • Azure
• Debugging
• Interaction with GNU make jobserver
• Known Caveats

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Installation

There are prebuilt x86-64 binaries available for Windows, Linux (a portable binary compiled against musl), and macOS on the releases page. Several package managers also include sccache packages, you can install the latest release from source using cargo, or build directly from a source checkout.

macOS

On macOS sccache can be installed via Homebrew:

brew install sccache

Windows

On Windows, sccache can be installed via scoop:

scoop install sccache

Via cargo

If you have a Rust toolchain installed you can install sccache using cargo. Note that this will compile sccache from source which is fairly resource-intensive. For CI purposes you should use prebuilt binary packages.

cargo install sccache

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Usage

Running sccache is like running ccache: prefix your compilation commands with it, like so:

sccache gcc -o foo.o -c foo.c

If you want to use sccache for caching Rust builds you can define build.rustc-wrapper in the cargo configuration file. For example, you can set it globally in $HOME/.cargo/config by adding:

[build]
rustc-wrapper = "/path/to/sccache"

Note that you need to use cargo 1.40 or newer for this to work.

Alternatively you can use the environment variable RUSTC_WRAPPER:

RUSTC_WRAPPER=/path/to/sccache cargo build

sccache supports gcc, clang, MSVC, rustc, NVCC, and Wind River's diab compiler.

If you don't specify otherwise, sccache will use a local disk cache.

sccache works using a client-server model, where the server runs locally on the same machine as the client. The client-server model allows the server to be more efficient by keeping some state in memory. The sccache command will spawn a server process if one is not already running, or you can run sccache --start-server to start the background server process without performing any compilation.

You can run sccache --stop-server to terminate the server. It will also terminate after (by default) 10 minutes of inactivity.

Running sccache --show-stats will print a summary of cache statistics.

Some notes about using sccache with Jenkins are here.

To use sccache with cmake, provide the following command line arguments to cmake 3.4 or newer:

-DCMAKE_C_COMPILER_LAUNCHER=sccache
-DCMAKE_CXX_COMPILER_LAUNCHER=sccache

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Build Requirements

sccache is a Rust program. Building it requires cargo (and thus rustc). sccache currently requires Rust 1.42.0. We recommend you install Rust via Rustup.

Build

If you are building sccache for non-development purposes make sure you use cargo build --release to get optimized binaries:

cargo build --release [--features=all|s3|redis|gcs|memcached|azure]

By default, sccache supports a local disk cache and S3. Use the --features flag to build sccache with support for other storage options. Refer the Cargo Documentation for details on how to select features with Cargo.

Building portable binaries

When building with the gcs feature, sccache will depend on OpenSSL, which can be an annoyance if you want to distribute portable binaries. It is possible to statically link against OpenSSL using the steps below before building with cargo.

Linux

You will need to download and build OpenSSL with -fPIC in order to statically link against it.

./config -fPIC --prefix=/usr/local --openssldir=/usr/local/ssl
make
make install
export OPENSSL_LIB_DIR=/usr/local/lib
export OPENSSL_INCLUDE_DIR=/usr/local/include
export OPENSSL_STATIC=yes

Build with cargo and use ldd to check that the resulting binary does not depend on OpenSSL anymore.

macOS

Just setting the below environment variable will enable static linking.

export OPENSSL_STATIC=yes

Build with cargo and use otool -L to check that the resulting binary does not depend on OpenSSL anymore.

Windows

On Windows it is fairly straightforward to just ship the required libcrypto and libssl DLLs with sccache.exe, but the binary might also depend on a few MSVC CRT DLLs that are not available on older Windows versions.

It is possible to statically link against the CRT using a .cargo/config file with the following contents.

[target.x86_64-pc-windows-msvc]
rustflags = ["-Ctarget-feature=+crt-static"]

Build with cargo and use dumpbin /dependents to check that the resulting binary does not depend on MSVC CRT DLLs anymore.

In order to statically link against both the CRT and OpenSSL, you will need to either build OpenSSL static libraries (with a statically linked CRT) yourself or get a pre-built distribution that provides these.

Then you can set environment variables which get picked up by the openssl-sys crate.

See the following example for using pre-built libraries from Shining Light Productions, assuming an installation in C:\OpenSSL-Win64:

set OPENSSL_LIB_DIR=C:\OpenSSL-Win64\lib\VC\static
set OPENSSL_INCLUDE_DIR=C:\OpenSSL-Win64\include
set OPENSSL_LIBS=libcrypto64MT:libssl64MT

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Storage Options

Local

sccache defaults to using local disk storage. You can set the SCCACHE_DIR environment variable to change the disk cache location. By default it will use a sensible location for the current platform: ~/.cache/sccache on Linux, %LOCALAPPDATA%\Mozilla\sccache on Windows, and ~/Library/Caches/Mozilla.sccache on MacOS.

The default cache size is 10 gigabytes. To change this, set SCCACHE_CACHE_SIZE, for example SCCACHE_CACHE_SIZE="1G".

S3

If you want to use S3 storage for the sccache cache, you need to set the SCCACHE_BUCKET environment variable to the name of the S3 bucket to use.

You can use AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY to set the S3 credentials. Alternately, you can set AWS_IAM_CREDENTIALS_URL to a URL that returns credentials in the format supported by the EC2 metadata service, and credentials will be fetched from that location as needed. In the absence of either of these options, credentials for the instance's IAM role will be fetched from the EC2 metadata service directly.

If you need to override the default endpoint you can set SCCACHE_ENDPOINT. To connect to a minio storage for example you can set SCCACHE_ENDPOINT=<ip>:<port>. If your endpoint requires TLS, set SCCACHE_S3_USE_SSL=true.

You can also define a prefix that will be prepended to the keys of all cache objects created and read within the S3 bucket, effectively creating a scope. To do that use the SCCACHE_S3_KEY_PREFIX environment variable. This can be useful when sharing a bucket with another application.

Redis

Set SCCACHE_REDIS to a Redis url in format redis://[:<passwd>@]<hostname>[:port][/<db>] to store the cache in a Redis instance. Redis can be configured as a LRU (least recently used) cache with a fixed maximum cache size. Set maxmemory and maxmemory-policy according to the Redis documentation. The allkeys-lru policy which discards the least recently accessed or modified key fits well for the sccache use case.

Memcached

Set SCCACHE_MEMCACHED to a Memcached url in format tcp://<hostname>:<port> ... to store the cache in a Memcached instance.

Google Cloud Storage

To use Google Cloud Storage, you need to set the SCCACHE_GCS_BUCKET environment variable to the name of the GCS bucket. If you're using authentication, either set SCCACHE_GCS_KEY_PATH to the location of your JSON service account credentials or SCCACHE_GCS_CREDENTIALS_URL with a URL that returns the oauth token. By default, SCCACHE on GCS will be read-only. To change this, set SCCACHE_GCS_RW_MODE to either READ_ONLY or READ_WRITE.

Azure

To use Azure Blob Storage, you'll need your Azure connection string and an existing Blob Storage container name. Set the SCCACHE_AZURE_CONNECTION_STRING environment variable to your connection string, and SCCACHE_AZURE_BLOB_CONTAINER to the name of the container to use. Note that sccache will not create the container for you - you'll need to do that yourself.

Important: The environment variables are only taken into account when the server starts, i.e. only on the first run.

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Debugging

You can run the server manually in foreground mode by running SCCACHE_START_SERVER=1 SCCACHE_NO_DAEMON=1 sccache, and send logging to stderr by setting the SCCACHE_LOG environment variable for example

SCCACHE_LOG=debug SCCACHE_START_SERVER=1 SCCACHE_NO_DAEMON=1 sccache

Alternately, you can set the SCCACHE_ERROR_LOG environment variable to a path and set SCCACHE_LOG to get the server process to redirect its logging there (including the output of unhandled panics, since the server sets RUST_BACKTRACE=1 internally).

SCCACHE_ERROR_LOG=/tmp/sccache_log.txt SCCACHE_LOG=debug sccache

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Interaction with GNU make jobserver

sccache provides support for a GNU make jobserver. When the server is started from a process that provides a jobserver, sccache will use that jobserver and provide it to any processes it spawns. (If you are running sccache from a GNU make recipe, you will need to prefix the command with + to get this behavior.) If the sccache server is started without a jobserver present it will create its own with the number of slots equal to the number of available CPU cores.

This is most useful when using sccache for Rust compilation, as rustc supports using a jobserver for parallel codegen, so this ensures that rustc will not overwhelm the system with codegen tasks. Cargo implements its own jobserver (see the information on NUM_JOBS in the cargo documentation) for rustc to use, so using sccache for Rust compilation in cargo via RUSTC_WRAPPER should do the right thing automatically.

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Known Caveats

General

• Absolute paths to files must match to get a cache hit. This means that even if you are using a shared cache, everyone will have to build at the same absolute path (i.e. not in $HOME) in order to benefit each other. In Rust this includes the source for third party crates which are stored in $HOME/.cargo/registry/cache by default.

Rust

• Crates that invoke the system linker cannot be cached. This includes bin, dylib, cdylib, and proc-macro crates. You may be able to improve compilation time of large bin crates by converting them to a lib crate with a thin bin wrapper.
• Incrementally compiled crates cannot be cached. By default, in the debug profile Cargo will use incremental compilation for workspace members and path dependencies. You can disable incremental compilation.

More details on Rust caveats