libstore: change max-jobs default to auto

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

@@ -1 +1,3 @@
 # Release X.Y (202?-??-??)
+
+- The `max-jobs` setting now defaults to `auto`.

src/libstore/globals.cc

@@ -145,13 +145,12 @@ 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();
 }
 
 StringSet Settings::getDefaultSystemFeatures()
@@ -266,7 +265,7 @@ 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());
+    if (str == "auto") return getMaxThreads();
     else {
         if (auto n = string2Int<decltype(value)>(str))
             return *n;

src/libstore/globals.hh

@@ -147,15 +147,14 @@ 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. `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-max-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 amount of threads available 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.