max-substitution-jobs setting

src/libstore/build/substitution-goal.cc

@@ -204,7 +204,7 @@ void PathSubstitutionGoal::tryToRun()
        if maxBuildJobs == 0 (no local builds allowed), we still allow
        a substituter to run.  This is because substitutions cannot be
        distributed to another machine via the build hook. */
-    if (worker.getNrLocalBuilds() >= std::max(1U, (unsigned int) settings.maxBuildJobs)) {
+    if (worker.getNrSubstitutions() >= std::max(1U, (unsigned int) settings.maxSubstitutionJobs)) {
         worker.waitForBuildSlot(shared_from_this());
         return;
     }

src/libstore/build/worker.cc

@@ -18,6 +18,7 @@ Worker::Worker(Store & store, Store & evalStore)
 {
     /* Debugging: prevent recursive workers. */
     nrLocalBuilds = 0;
+    nrSubstitutions = 0;
     lastWokenUp = steady_time_point::min();
     permanentFailure = false;
     timedOut = false;
@@ -176,6 +177,12 @@ unsigned Worker::getNrLocalBuilds()
 }
 
 
+unsigned Worker::getNrSubstitutions()
+{
+    return nrSubstitutions;
+}
+
+
 void Worker::childStarted(GoalPtr goal, const std::set<int> & fds,
     bool inBuildSlot, bool respectTimeouts)
 {
@@ -187,7 +194,10 @@ void Worker::childStarted(GoalPtr goal, const std::set<int> & fds,
     child.inBuildSlot = inBuildSlot;
     child.respectTimeouts = respectTimeouts;
     children.emplace_back(child);
-    if (inBuildSlot) nrLocalBuilds++;
+    if (inBuildSlot) {
+        if (dynamic_cast<PathSubstitutionGoal *>(child.goal2)) nrSubstitutions++;
+        else nrLocalBuilds++;
+    }
 }
 
 
@@ -198,8 +208,13 @@ void Worker::childTerminated(Goal * goal, bool wakeSleepers)
     if (i == children.end()) return;
 
     if (i->inBuildSlot) {
-        assert(nrLocalBuilds > 0);
+        if (dynamic_cast<PathSubstitutionGoal *>(goal)) {
-        nrLocalBuilds--;
+            assert(nrSubstitutions > 0);
+            nrSubstitutions--;
+        } else {
+            assert(nrLocalBuilds > 0);
+            nrLocalBuilds--;
+        }
     }
 
     children.erase(i);
@@ -220,7 +235,9 @@ void Worker::childTerminated(Goal * goal, bool wakeSleepers)
 void Worker::waitForBuildSlot(GoalPtr goal)
 {
     debug("wait for build slot");
-    if (getNrLocalBuilds() < settings.maxBuildJobs)
+    bool isSubstitutionGoal = dynamic_cast<PathSubstitutionGoal *>(goal.get());
+    if ((!isSubstitutionGoal && getNrLocalBuilds() < settings.maxBuildJobs) ||
+        (isSubstitutionGoal && getNrSubstitutions() < settings.maxSubstitutionJobs))
         wakeUp(goal); /* we can do it right away */
     else
         addToWeakGoals(wantingToBuild, goal);

src/libstore/build/worker.hh

@@ -88,11 +88,16 @@ private:
     std::list<Child> children;
 
     /**
-     * Number of build slots occupied.  This includes local builds and
+     * Number of build slots occupied.  This includes local builds but does not
-     * substitutions but not remote builds via the build hook.
+     * include substitutions or remote builds via the build hook.
      */
     unsigned int nrLocalBuilds;
 
+    /**
+     * Number of substitution slots occupied.
+     */
+    unsigned int nrSubstitutions;
+
     /**
      * Maps used to prevent multiple instantiations of a goal for the
      * same derivation / path.
@@ -220,12 +225,16 @@ public:
     void wakeUp(GoalPtr goal);
 
     /**
-     * Return the number of local build and substitution processes
+     * Return the number of local build processes currently running (but not
-     * currently running (but not remote builds via the build
+     * remote builds via the build hook).
-     * hook).
      */
     unsigned int getNrLocalBuilds();
 
+    /**
+     * Return the number of substitution processes currently running.
+     */
+    unsigned int getNrSubstitutions();
+
     /**
      * Registers a running child process.  `inBuildSlot` means that
      * the process counts towards the jobs limit.

src/libstore/globals.cc

@@ -249,6 +249,17 @@ unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
     }
 }
 
+unsigned int MaxSubstitutionJobsSetting::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 std::max(1U, *n);
+        else
+            throw UsageError("configuration setting '%s' should be 'auto' or an integer", name);
+    }
+}
+
 
 Paths PluginFilesSetting::parse(const std::string & str) const
 {

src/libstore/globals.hh

@@ -29,6 +29,21 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
     unsigned int parse(const std::string & str) const override;
 };
 
+struct MaxSubstitutionJobsSetting : public BaseSetting<unsigned int>
+{
+    MaxSubstitutionJobsSetting(Config * options,
+        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)
+    {
+        options->addSetting(this);
+    }
+
+    unsigned int parse(const std::string & str) const override;
+};
+
 struct PluginFilesSetting : public BaseSetting<Paths>
 {
     bool pluginsLoaded = false;
@@ -159,6 +174,18 @@ public:
         )",
         {"build-max-jobs"}};
 
+    MaxSubstitutionJobsSetting maxSubstitutionJobs{
+        this, 16, "max-substitution-jobs",
+        R"(
+          This option defines the maximum number of substitution jobs that Nix
+          will try to run in parallel. The default is `16`. The minimum value
+          one can choose is `1` and lower values will be interpreted as `1`. The
+          special value `auto` causes Nix to use the number of CPUs in your
+          system. It can be overridden using the `--max-substitution-jobs`
+          command line switch.
+        )",
+        {"substitution-max-jobs"}};
+
     Setting<unsigned int> buildCores{
         this,
         getDefaultCores(),