libstore: change max-jobs default to auto

doc/manual/src/release-notes/rl-next.md

@@ -28,5 +28,7 @@
 
   - The flake-specific flags `--recreate-lock-file` and `--update-input` have been removed from all commands operating on installables.
     They are superceded by `nix flake update`.
-  
+
 - Commit signature verification for the [`builtins.fetchGit`](@docroot@/language/builtins.md#builtins-fetchGit) is added as the new [`verified-fetches` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-verified-fetches).
+
+- The [`max-jobs` setting](@docroot@/command-ref/conf-file.md#conf-max-jobs) now defaults to `auto`.

src/libstore/globals.cc

@@ -148,13 +148,12 @@ std::vector<Path> getUserConfigFiles()
 
 unsigned int Settings::getDefaultCores()
 {
-    const unsigned int concurrency = std::max(1U, std::thread::hardware_concurrency());
     const unsigned int maxCPU = getMaxCPU();
 
     if (maxCPU > 0)
       return maxCPU;
     else
-      return concurrency;
+      return getMaxThreads();
 }
 
 #if __APPLE__
@@ -297,13 +296,12 @@ template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::s
 
 unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
 {
-    if (str == "auto") return std::max(1U, std::thread::hardware_concurrency());
-    else {
-        if (auto n = string2Int<decltype(value)>(str))
-            return *n;
-        else
-            throw UsageError("configuration setting '%s' should be 'auto' or an integer", name);
-    }
+    if (str == "auto") return getMaxThreads();
+
+    if (auto n = string2Int<decltype(value)>(str))
+        return *n;
+    else
+        throw UsageError("configuration setting '%s' should be 'auto' or an integer", name);
 }
 
 

src/libstore/globals.hh

@@ -150,17 +150,17 @@ public:
         "the log to show if a build fails."};
 
     MaxBuildJobsSetting maxBuildJobs{
-        this, 1, "max-jobs",
+        this, getMaxThreads(), "max-jobs",
         R"(
           This option defines the maximum number of jobs that Nix will try to
-          build in parallel. The default is `1`. The special value `auto`
-          causes Nix to use the number of CPUs in your system. `0` is useful
-          when using remote builders to prevent any local builds (except for
-          `preferLocalBuild` derivation attribute which executes locally
-          regardless). It can be overridden using the `--max-jobs` (`-j`)
-          command line switch.
+          build in parallel. The default is `auto` which causes Nix to use
+          the number of CPUs in your system.
+          It can be overridden using the `--max-jobs` / `-j` command line option.
+
+          Setting this to `0` when using remote builders will prevent any local builds.
+          The [`preferLocalBuild`](@docroot@/language/advanced-attributes.md#adv-attr-preferLocalBuild) derivation attribute will make the build always execute locally.
         )",
-        {"build-max-jobs"}};
+        {"build-max-jobs"}, "`auto`"};
 
     Setting<unsigned int> maxSubstitutionJobs{
         this, 16, "max-substitution-jobs",

src/libutil/util.cc

@@ -777,6 +777,11 @@ void drainFD(int fd, Sink & sink, bool block)
 
 //////////////////////////////////////////////////////////////////////
 
+unsigned int getMaxThreads()
+{
+    return std::max(1U, std::thread::hardware_concurrency());
+}
+
 unsigned int getMaxCPU()
 {
     #if __linux__

src/libutil/util.hh

@@ -292,6 +292,11 @@ std::string drainFD(int fd, bool block = true, const size_t reserveSize=0);
 
 void drainFD(int fd, Sink & sink, bool block = true);
 
+/**
+ * Returns the number of threads available for simultaneous execution if possible, otherwise returns 1.
+ */
+unsigned int getMaxThreads();
+
 /**
  * If cgroups are active, attempt to calculate the number of CPUs available.
  * If cgroups are unavailable or if cpu.max is set to "max", return 0.