CCache: Difference between revisions

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Not related to NixOS, read https://ccache.dev/manual/4.8.2.html#_cache_statistics and run ccache -vs

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 With CCache, recompile time can be reduced from many hours to a few minutes.
-== Add CCache to sandbox ==
+== NixOS ==
+On NixOS, the <code>programs.ccache</code> module can be used to partially enable CCache.
-Add to <code>/etc/nixos/configuration.nix</code>
+<syntaxhighlight lang="nix">
+programs.ccache.enable = true;
+</syntaxhighlight>
+However, without specifying <code>programs.ccache.packageNames</code> the CCache wrapper is not configured. The wrapper configuration can be added to your Nix overlays.
+<syntaxhighlight lang="nix">
+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
+      '';
+    };
+  })
+];
+</syntaxhighlight>
+The CCache directory also needs to be added to the builder sandboxes.
+<syntaxhighlight lang="nix">
+nix.settings.extra-sandbox-paths = [ config.programs.ccache.cacheDir ];
+</syntaxhighlight>
+Run <code>sudo nixos-rebuild switch</code> to enable these options before attempting to use CCache for a derivation.
+=== Derivation CCache ===
+Packages can built with CCache by overriding <code>stdenv</code> in the derivation.
 <syntaxhighlight lang="nix">
-nix.extraOptions = ''
+nixpkgs.overlays = [
-   extra-sandbox-paths = /nix/var/cache/ccache
+   (self: super: {
-'';
+    ffmpeg = super.ffmpeg.override { stdenv = super.ccacheStdenv; };
+  })
+];
 </syntaxhighlight>
-TODO: use <code>nix.sandboxPaths</code>? [https://github.com/NixOS/nixpkgs/issues/153343]
+Some packages do not use <code>stdenv</code> directly. You may need to plumb it through other dependencies first.
+Note, that if the package is a top-level package, you may instead add it to the <code>programs.ccache.packageNames</code> list.
+<syntaxhighlight lang="nix">
+programs.ccache.packageNames = [ "ffmpeg" ];
+</syntaxhighlight>
+=== System CCache ===
+todo
-Run <code>sudo nixos-rebuild switch</code>
+=== Monitor CCache ===
-Now <code>/etc/nix/nix.conf</code> should have <code>sandbox-paths = /nix/var/cache/ccache</code>
+The NixOS module creates a script that can be used to monitor the CCache directory without sudo.
-Ceate the cache folder
+<pre>
+nix-ccache --show-stats
+</pre>
+== Non-NixOS ==
+Create the cache folder:
 <pre>
@@ Line 31: / Line 95: @@
 </pre>
-== Derivation CCache ==
+Add the path to the derivation sandbox by adding <code>extra-sandbox-paths</code> to <code>nix.conf</code>
-To enable CCache only for one derivation, patch the <code>default.nix</code> file
+<pre>
+extra-sandbox-paths = /nix/var/cache/ccache
+</pre>
+Then configure the CCache wrapper script.
 <syntaxhighlight lang="nix">
-{ stdenv
+nixpkgs.overlays = [
-, ccacheStdenv
+  (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
+      '';
+    };
+  })
+];
+</syntaxhighlight>
-#stdenv.mkDerivation {
+=== Derivation CCache ===
-ccacheStdenv.mkDerivation {
+Packages can built with CCache by overriding <code>stdenv</code> in the derivation.
-  preConfigure = ''
+<syntaxhighlight lang="nix">
-    export CCACHE_DIR=/nix/var/cache/ccache
+nixpkgs.overlays = [
-    export CCACHE_UMASK=007
+   (self: super: {
-   ''
+    ffmpeg = super.ffmpeg.override { stdenv = super.ccacheStdenv; };
+  })
-# ...
+];
 </syntaxhighlight>
-Add <code>ccacheStdenv</code> to dependencies.
+Some packages do not use <code>stdenv</code> directly. You may need to plumb it through other dependencies first.
-Run <code>nix-build</code>.
+=== Monitor CCache status ===
-== System CCache ==
-todo
-== Monitor CCache status ==
 <pre>
@@ Line 67: / Line 153: @@
 # watch ccache stats
-sudo watch ccache --dir /nix/var/cache/ccache --show-stats --verbose
+sudo watch ccache --dir /nix/var/cache/ccache --show-stats
 </pre>
-== Uncacheable ==
+== Sloppiness ==
+By default, <code>stdenv</code> inserts <code>-frandom-seed</code> C compiler flag with a value that changes whenever the derivation hash has changed.
+Consequently, this behavior completely defeats any usage of <code>ccacheWrapper</code>
+To counterpart this behavior, add the following line to the ccache config (typically <code>/var/cache/ccache/ccache.conf</code>):
+<pre>
+sloppiness = random_seed
+</pre>
-TODO: why are some compilations "uncacheable" according to <code>ccache --show-stats</code>
+Be warned that this configuration option might affect reproducibility of builds, and could lead to cache poisoning.
+See [https://github.com/NixOS/nixpkgs/issues/109033 issue 109033] for more details.
 == See also ==