libstore: change max-jobs default to auto

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

@@ -3,3 +3,5 @@
 - [URL flake references](@docroot@/command-ref/new-cli/nix3-flake.md#flake-references) now support [percent-encoded](https://datatracker.ietf.org/doc/html/rfc3986#section-2.1) characters.
 
 - [Path-like flake references](@docroot@/command-ref/new-cli/nix3-flake.md#path-like-syntax) now accept arbitrary unicode characters (except `#` and `?`).
+
+- The [`max-jobs` setting](@docroot@/command-ref/conf-file.md#conf-max-jobs) now defaults to `auto`.

src/build-remote/build-remote.cc

@@ -72,7 +72,7 @@ static int main_build_remote(int argc, char * * argv)
             settings.set(name, value);
         }
 
-        auto maxBuildJobs = settings.maxBuildJobs;
+        auto maxBuildJobs = settings.maxBuildJobs.get();
         settings.maxBuildJobs.set("1"); // hack to make tests with local?root= work
 
         initPlugins();

src/libstore/build/derivation-goal.cc

@@ -1103,7 +1103,7 @@ HookReply DerivationGoal::tryBuildHook()
         /* Send the request to the hook. */
         worker.hook->sink
             << "try"
-            << (worker.getNrLocalBuilds() < settings.maxBuildJobs ? 1 : 0)
+            << (worker.getNrLocalBuilds() < settings.maxBuildJobs.get() ? 1 : 0)
             << drv->platform
             << worker.store.printStorePath(drvPath)
             << parsedDrv->getRequiredSystemFeatures();

src/libstore/build/local-derivation-goal.cc

@@ -171,7 +171,7 @@ void LocalDerivationGoal::killSandbox(bool getStats)
 void LocalDerivationGoal::tryLocalBuild()
 {
     unsigned int curBuilds = worker.getNrLocalBuilds();
-    if (curBuilds >= settings.maxBuildJobs) {
+    if (curBuilds >= settings.maxBuildJobs.get()) {
         state = &DerivationGoal::tryToBuild;
         worker.waitForBuildSlot(shared_from_this());
         outputLocks.unlock();

src/libstore/build/worker.cc

@@ -296,7 +296,7 @@ void Worker::waitForBuildSlot(GoalPtr goal)
 {
     debug("wait for build slot");
     bool isSubstitutionGoal = goal->jobCategory() == JobCategory::Substitution;
-    if ((!isSubstitutionGoal && getNrLocalBuilds() < settings.maxBuildJobs) ||
+    if ((!isSubstitutionGoal && getNrLocalBuilds() < settings.maxBuildJobs.get()) ||
         (isSubstitutionGoal && getNrSubstitutions() < settings.maxSubstitutionJobs))
         wakeUp(goal); /* we can do it right away */
     else
@@ -371,7 +371,7 @@ void Worker::run(const Goals & _topGoals)
         if (!children.empty() || !waitingForAWhile.empty())
             waitForInput();
         else {
-            if (awake.empty() && 0U == settings.maxBuildJobs)
+            if (awake.empty() && 0U == settings.maxBuildJobs.get())
             {
                 if (getMachines().empty())
                    throw Error("unable to start any build; either increase '--max-jobs' "

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()
@@ -264,18 +263,29 @@ template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::s
     });
 }
 
-unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
+const unsigned int MaxBuildJobsSetting::get() const
 {
-    if (str == "auto") return std::max(1U, std::thread::hardware_concurrency());
+    return value.value_or(getMaxThreads());
+}
+
+std::string MaxBuildJobsSetting::to_string() const
+{
+    if (value == std::nullopt) return "auto";
+
+    return std::to_string(*value);
+}
+
+std::optional<unsigned int> MaxBuildJobsSetting::parse(const std::string & str) const
+{
+    if (str == "auto") return getMaxThreads();
     else {
-        if (auto n = string2Int<decltype(value)>(str))
+        if (auto n = string2Int<unsigned int>(str))
             return *n;
         else
             throw UsageError("configuration setting '%s' should be 'auto' or an integer", name);
     }
 }
 
-
 Paths PluginFilesSetting::parse(const std::string & str) const
 {
     if (pluginsLoaded)

src/libstore/globals.hh

@@ -14,19 +14,25 @@ namespace nix {
 
 typedef enum { smEnabled, smRelaxed, smDisabled } SandboxMode;
 
-struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
+class MaxBuildJobsSetting : public BaseSetting<std::optional<unsigned int>>
 {
+public:
+
     MaxBuildJobsSetting(Config * options,
-        unsigned int def,
+        const std::optional<unsigned int> def,
         const std::string & name,
         const std::string & description,
         const std::set<std::string> & aliases = {})
-        : BaseSetting<unsigned int>(def, true, name, description, aliases)
+        : BaseSetting<std::optional<unsigned int>>(def, true, name, description, aliases)
     {
         options->addSetting(this);
     }
 
-    unsigned int parse(const std::string & str) const override;
+    const unsigned int get() const;
+
+    std::string to_string() const override;
+
+    std::optional<unsigned int> parse(const std::string & str) const override;
 };
 
 struct PluginFilesSetting : public BaseSetting<Paths>
@@ -147,15 +153,14 @@ public:
         "the log to show if a build fails."};
 
     MaxBuildJobsSetting maxBuildJobs{
-        this, 1, "max-jobs",
+        this, std::nullopt, "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/libstore/remote-store.cc

@@ -126,7 +126,7 @@ void RemoteStore::setOptions(Connection & conn)
        << settings.keepGoing
        << settings.tryFallback
        << verbosity
-       << settings.maxBuildJobs
+       << settings.maxBuildJobs.get()
        << settings.maxSilentTime
        << true
        << (settings.verboseBuild ? lvlError : lvlVomit)

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 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.