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

add ccache to sandbox

add to /etc/nixos/configuration.nix

          nix.extraOptions = ''
            extra-sandbox-paths = /nix/var/cache/ccache
          '';

todo: use nix.sandboxPaths? [1]

run sudo nixos-rebuild switch

now /etc/nix/nix.conf should have sandbox-paths = /nix/var/cache/ccache

create 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'

derivation ccache

to enable ccache only for one derivation

patch the default.nix file

{ stdenv
, ccacheStdenv
# ...
}:

#stdenv.mkDerivation {
ccacheStdenv.mkDerivation {

  preConfigure = ''
    export CCACHE_DIR=/nix/var/cache/ccache
    export CCACHE_UMASK=007
  ''

# ...

add ccacheStdenv to dependencies

run nix-build

system ccache

todo

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 --verbose

uncacheable

todo: why are some compilations "uncacheable" according to ccache --show-stats