libstore: Add load-limit setting to dynamically control parallelism

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

@@ -1,2 +1,3 @@
 # Release X.Y (202?-??-??)
 
+* Add new `load-limit` setting to control the `-lN` flag of GNU Make.

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

@@ -1069,6 +1069,9 @@ void LocalDerivationGoal::initEnv()
     /* The maximum number of cores to utilize for parallel building. */
     env["NIX_BUILD_CORES"] = (format("%d") % settings.buildCores).str();
 
+    /* If the load average is greater than N, disable parallel building. */
+    env["NIX_LOAD_LIMIT"] = (format("%d") % settings.loadLimit).str();
+
     initTmpDir();
 
     /* Compatibility hack with Nix <= 0.7: if this is a fixed-output

src/libstore/globals.hh

@@ -142,6 +142,30 @@ public:
         )",
         {"build-cores"}, false};
 
+    Setting<unsigned int> loadLimit{
+      this,
+      getDefaultCores(),
+      "load-limit",
+      R"(
+        Sets the value of the `NIX_LOAD_LIMIT` environment variable in the
+        invocation of builders. Builders can use this value at their discretion
+        to dynamically control the amount of parallelism with respect to the
+        machine's load average. For instance, in Nixpkgs, if this value is >0,
+        the builder passes the `-lN` flag to GNU Make. In this case, if the
+        load average of the machine exceeds `N`, the amount of parallelism will
+        be dynamically reduced to 1.
+
+        By default, it is set to the number of cores available to Nix.
+
+        On busy machines where Nix co-exists with other workloads, parallelism
+        may not work as intended. For example, consider a 64 core machine whose
+        load average is 24 and where Nix is limited to 8 cores. By default,
+        `-j8 -l8` will be passed to GNU Make. Since the load average exceeds 8,
+        no parallelism will take place despite the fact that 8 cores are
+        available. In this case, `load-limit` should be set to `0` to prevent
+        the `-lN` flag from being passed to GNU Make.
+      )"};
+
     /* Read-only mode.  Don't copy stuff to the store, don't change
        the database. */
     bool readOnlyMode = false;

src/nix-build/nix-build.cc

@@ -469,6 +469,7 @@ static void main_nix_build(int argc, char * * argv)
         env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = *tmp;
         env["NIX_STORE"] = store->storeDir;
         env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);
+        env["NIX_LOAD_LIMIT"] = std::to_string(settings.loadLimit);
 
         auto passAsFile = tokenizeString<StringSet>(getOr(drv.env, "passAsFile", ""));