sccache

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This article has some todo items:

• Explain how to use sccache as ccache replacement

sccache helps avoid repeated recompilation for the same C, C++, and Rust object files by fetching result from a cache directory.

Compiler cache is typically useful for:

• Developers who rebuild the same/similar codebase multiple times and use /etc/portage/patches to test patches.
• Users who frequently play with USE-flag changes and end up rebuilding the same packages multiple times.
• Users who use live ebuilds extensively.
• Installing very big ebuilds, such as www-client/firefox or mail-client/thunderbird, without fear of losing multiple hours of code compilation due to a failure.

Installation

Emerge

root #emerge --ask dev-util/sccache

Configuration

Note
Unfortunately, sccache keys on the full build path, so it's not useful for ebuilds.

Initial setup

Important
Unlike ccache, current Portage has no direct sccache support.

Create the cache directory:

root #mkdir -p /var/cache/sccache

root #chown root:portage /var/cache/sccache

root #chmod 2775 /var/cache/sccache

Allow write access to the cache directory when running under sandbox:

FILE /etc/sandbox.d/20sccache

# Allow write access to sccache cache directory
SANDBOX_WRITE="/var/cache/sccache/"

Enable sccache support in make.conf:

FILE /etc/portage/make.conf

RUSTC_WRAPPER=/usr/bin/sccache
SCCACHE_DIR=/var/cache/sccache
SCCACHE_MAX_FRAME_LENGTH=104857600

From now on all builds utilizing Rust compiler will try to reuse object files from the /var/cache/sccache cache.

Note
SCCACHE_MAX_FRAME_LENGTH is needed to allow packages like www-client/firefox to compile.

Note
The setup above will only enable sccache usage for Rust.

General notes

sccache can be enabled for the current user and re-use the same cache directory:

FILE ~/.bashrc

export RUSTC_WRAPPER="/usr/bin/sccache"
export SCCACHE_DIR="/var/cache/sccache"
export SCCACHE_MAX_FRAME_LENGTH="104857600"

Useful variables and commands

Some variables:

• Variable SCCACHE_DIR points to cache root directory.

Some commands:

• Command sccache -s shows cache hit statistics:

user $SCCACHE_DIR=/var/cache/sccache sccache -s

Compile requests                      0
Compile requests executed             0
Cache hits                            0
Cache misses                          0
Cache timeouts                        0
Cache read errors                     0
Forced recaches                       0
Cache write errors                    0
Compilation failures                  0
Cache errors                          0
Non-cacheable compilations            0
Non-cacheable calls                   0
Non-compilation calls                 0
Unsupported compiler calls            0
Average cache write               0.000 s
Average cache read miss           0.000 s
Average cache read hit            0.000 s
Failed distributed compilations       0
Cache location                  Local disk: "/var/cache/sccache/"
Cache size                            5 GiB
Max cache size                       10 GiB

Note
Not like ccache, sccache can only read statistics from running instance. Those interested in statistics must manually spawn sccache server first.

See also

• Handbook:AMD64/Working/Features#Caching_compilation_objects — how to use Ccache
• Ccache — helps avoid repeated recompilation for the same C and C++ object files by fetching the result from a cache directory.