CCache
CCache is useful for Packaging large packages with Incremental builds.
With CCache, recompile time can be reduced from many hours to a few minutes.
NixOS
On NixOS, the programs.ccache module can be used to partially enable CCache.
programs.ccache.enable = true;
However, without specifying programs.ccache.packageNames the CCache wrapper is not configured. The wrapper configuration can be added to your Nix overlays.
nixpkgs.overlays = [
(self: super: {
ccacheWrapper = super.ccacheWrapper.override {
extraConfig = ''
export CCACHE_COMPRESS=1
export CCACHE_DIR="${config.programs.ccache.cacheDir}"
export CCACHE_UMASK=007
if [ ! -d "$CCACHE_DIR" ]; then
echo "====="
echo "Directory '$CCACHE_DIR' does not exist"
echo "Please create it with:"
echo " sudo mkdir -m0770 '$CCACHE_DIR'"
echo " sudo chown root:nixbld '$CCACHE_DIR'"
echo "====="
exit 1
fi
if [ ! -w "$CCACHE_DIR" ]; then
echo "====="
echo "Directory '$CCACHE_DIR' is not accessible for user $(whoami)"
echo "Please verify its access permissions"
echo "====="
exit 1
fi
'';
};
})
];
The CCache directory also needs to be added to the builder sandboxes.
nix.settings.extra-sandbox-paths = [ config.programs.ccache.cacheDir ];
Run sudo nixos-rebuild switch to enable these options before attempting to use CCache for a derivation.
Derivation CCache
Packages can built with CCache by overriding stdenv in the derivation.
nixpkgs.overlays = [
(self: super: {
ffmpeg = super.ffmpeg.override { stdenv = super.ccacheStdenv; };
})
];
Note, that if the package is a top-level package, you can also add it to the programs.ccache.packageNames list.
programs.ccache.packageNames = [ "ffmpeg" ];
Some packages do not use stdenv directly. You may need to plumb it through other dependencies first.
System CCache
todo
Monitor CCache
The NixOS module creates a script that can be used to monitor the CCache directory without sudo.
nix-ccache --show-stats
Non-NixOS
Ceate the cache folder:
sudo mkdir -m0770 -p /nix/var/cache/ccache # Linux sudo chown --reference=/nix/store /nix/var/cache/ccache # macOS workaround for chown --reference nix-shell -p coreutils --run 'sudo chown --reference=/nix/store /nix/var/cache/ccache'
Add the path to the derivation sandbox by adding extra-sandbox-paths to nix.conf
extra-sandbox-paths = /nix/var/cache/ccache
Then configure the CCache wrapper script.
nixpkgs.overlays = [
(self: super: {
ccacheWrapper = super.ccacheWrapper.override {
extraConfig = ''
export CCACHE_COMPRESS=1
export CCACHE_DIR="/nix/var/cache/ccache"
export CCACHE_UMASK=007
if [ ! -d "$CCACHE_DIR" ]; then
echo "====="
echo "Directory '$CCACHE_DIR' does not exist"
echo "Please create it with:"
echo " sudo mkdir -m0770 '$CCACHE_DIR'"
echo " sudo chown root:nixbld '$CCACHE_DIR'"
echo "====="
exit 1
fi
if [ ! -w "$CCACHE_DIR" ]; then
echo "====="
echo "Directory '$CCACHE_DIR' is not accessible for user $(whoami)"
echo "Please verify its access permissions"
echo "====="
exit 1
fi
'';
};
})
];
Derivation CCache
Packages can built with CCache by overriding stdenv in the derivation.
nixpkgs.overlays = [
(self: super: {
ffmpeg = super.ffmpeg.override { stdenv = super.ccacheStdenv; };
})
];
Some packages do not use stdenv directly. You may need to plumb it through other dependencies first.
Monitor CCache status
# watch ccache size sudo watch du -sh /nix/var/cache/ccache # watch ccache stats sudo watch ccache --dir /nix/var/cache/ccache --show-stats
Uncacheable
TODO: why are some compilations "uncacheable" according to ccache --show-stats