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

Initial setup

Note
Current portage has no direct sccache support like ccache 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 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

Done! From now on all builds utilizing Rust compiler will try to reuse object files from /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