CCache: Difference between revisions

← Older edit

CCache (view source)

Revision as of 16:02, 18 June 2024

3,519 bytes added , 18 June

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

VisualWikitext

Xz

2

edits

@@ Line 1: / Line 1: @@
-ccache is useful for [[Packaging]] large packages with [[Incremental builds]]
+CCache is useful for [[Packaging]] large packages with [[Incremental builds]].
-with ccache, recompile time can be reduced from many hours to a few minutes
+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">
+nixpkgs.overlays = [
+  (self: super: {
+    ffmpeg = super.ffmpeg.override { stdenv = super.ccacheStdenv; };
+  })
+];
+</syntaxhighlight>
+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
+=== Monitor CCache ===
+The NixOS module creates a script that can be used to monitor the CCache directory without sudo.
 <pre>
-          nix.extraOptions = ''
+nix-ccache --show-stats
-            extra-sandbox-paths = /nix/var/cache/ccache
-          '';
 </pre>
-todo: use <code>nix.sandboxPaths</code>? [https://github.com/NixOS/nixpkgs/issues/153343]
+== Non-NixOS ==
-run <code>sudo nixos-rebuild switch</code>
-now <code>/etc/nix/nix.conf</code> should have <code>sandbox-paths = /nix/var/cache/ccache</code>
-create the cache folder
+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>
-{ stdenv
+extra-sandbox-paths = /nix/var/cache/ccache
-, ccacheStdenv
+</pre>
-# ...
-}:
-#stdenv.mkDerivation {
-ccacheStdenv.mkDerivation {
-  preConfigure = ''
-    export CCACHE_DIR=/nix/var/cache/ccache
-    export CCACHE_UMASK=007
-  ''
-# ...
+Then configure the CCache wrapper script.
-</pre>
-add <code>ccacheStdenv</code> to dependencies
+<syntaxhighlight lang="nix">
+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
+      '';
+    };
+  })
+];
+</syntaxhighlight>
-run <code>nix-build</code>
+=== Derivation CCache ===
+Packages can built with CCache by overriding <code>stdenv</code> in the derivation.
-== system ccache ==
+<syntaxhighlight lang="nix">
+nixpkgs.overlays = [
+  (self: super: {
+    ffmpeg = super.ffmpeg.override { stdenv = super.ccacheStdenv; };
+  })
+];
+</syntaxhighlight>
-todo
+Some packages do not use <code>stdenv</code> directly. You may need to plumb it through other dependencies first.
-== monitor ccache status ==
+=== Monitor CCache status ===
 <pre>
@@ Line 69: / 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 ==
-todo: why are some compilations "uncacheable" according to <code>ccache --show-stats</code>
+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>
-== see also ==
+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 ==
 * https://leanprover.github.io/lean4/doc/make/nix.html
 * [https://nixos.org/manual/nix/stable/command-ref/conf-file.html sandbox-paths in nix.conf]
+[[Category: Development]]