sccache - Shared Compilation Cache

Sccache is a ccache-like tool. It is used as a compiler wrapper and avoids compilation when possible, storing a cache in a remote storage using the Amazon Simple Cloud Storage Service (S3) API, the Google Cloud Storage (GCS) API, or Redis.

Sccache now includes experimental Rust support.

It works as a client-server. The client spawns a server if one is not running already, and sends the wrapped command line as a request to the server, which then does the work and returns stdout/stderr for the job. The client-server model allows the server to be more efficient in its handling of the remote storage.

Sccache can also be used with local storage instead of remote.

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Table of Contents (ToC)

• Build Requirements
• Build
• Installation
• Usage
• Storage Options
• Debugging
• Interaction with GNU make jobserver
• Known Caveats

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Build Requirements

Sccache is a Rust program. Building it requires cargo (and thus rustc). sccache currently requires Rust 1.21.

We recommend you install Rust via Rustup. The generated binaries can be built so that they are very portable, see scripts/build-release.sh. By default sccache supports a local disk cache. To build sccache with support for S3 and/or Redis cache backends, add --features=all or select a specific feature by passing s3, gcs, and/or redis. Refer the Cargo Documentation for details.

Build

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

Installation

$ cargo install

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Usage

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

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

or use it with rust, like so:

$ RUSTC_WRAPPER=[path to sccache] cargo build

Sccache (tries to) support gcc, clang and MSVC. If you don't specify otherwise, sccache will use a local disk cache.

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 terminate after 10 minutes of inactivity.

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

Some notes about using sccache with Jenkins are here.

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Storage Options

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/sccache on OS X.

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 and 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>.

Set SCCACHE_REDIS to a Redis url in format redis://[:<passwd>@]<hostname>[:port][/<db>] to store the cache in a Redis instance.

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

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, set SCCACHE_GCS_KEY_PATH to the location of your JSON service account credentials. By default, SCCACHE on GCS will be read-only. To change this, set SCCACHE_GCS_RW_MODE to either READ_ONLY or READ_WRITE.

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

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Debugging

You can set the SCCACHE_LOG_LEVEL environment variable to debug or trace (not recommended, it's very verbose) to cause sccache to output more fine grained logging about what it is doing. A log file named sccache.log will be output in the current working directory whenever sccache is invoked.

Alternately, 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 RUST_LOG environment variable, the format of which is described in more detail in the env_logger documentation.

You can set the SCCACHE_ERROR_LOG environment variable to a path to cause the server process to redirect its standard error output there, in order to capture the output of unhandled panics. (The server sets RUST_BACKTRACE=1 internally.)

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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

(and possible future improvements)

• Sccache doesn't try to be smart about the command line arguments it uses when computing a key for a given compilation result (like skipping preprocessor-specific arguments)
• It doesn't support all kinds of compiler flags, and is certainly broken with a few of them. Really only the flags used during Firefox builds have been tested.
• It doesn't support ccache's direct mode.
• It doesn't support an option like CCACHE_BASEDIR.