Support auto-configuration of build-max-jobs

"build-max-jobs" and the "-j" option can now be set to "auto" to use
the number of CPUs in the system. (Unlike build-cores, it doesn't use
0 to imply auto-configuration, because a) magic values are a bad idea
in general; b) 0 is a legitimate value used to disable local
building.)

Fixes #1198.

doc/manual/command-ref/conf-file.xml

@@ -101,9 +101,9 @@ flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>
 
     <listitem><para>This option defines the maximum number of jobs
     that Nix will try to build in parallel.  The default is
-    <literal>1</literal>.  You should generally set it to the number
-    of CPUs in your system (e.g., <literal>2</literal> on an Athlon 64
-    X2).  It can be overridden using the <option
+    <literal>1</literal>. The special value <literal>auto</literal>
+    causes Nix to use the number of CPUs in your system.  It can be
+    overridden using the <option
     linkend='opt-max-jobs'>--max-jobs</option> (<option>-j</option>)
     command line switch.</para></listitem>
 

doc/manual/command-ref/opt-common.xml

@@ -93,8 +93,9 @@
   <term><option>-j</option></term>
 
   <listitem><para>Sets the maximum number of build jobs that Nix will
-  perform in parallel to the specified number.  The default is
-  specified by the <link
+  perform in parallel to the specified number.  Specify
+  <literal>auto</literal> to use the number of CPUs in the system.
+  The default is specified by the <link
   linkend='conf-build-max-jobs'><literal>build-max-jobs</literal></link>
   configuration setting, which itself defaults to
   <literal>1</literal>.  A higher value is useful on SMP systems or to

src/libmain/shared.cc

@@ -167,14 +167,17 @@ struct LegacyArgs : public MixCommonArgs
             settings.set("build-fallback", "true");
         });
 
+        mkFlag1('j', "max-jobs", "jobs", "maximum number of parallel builds", [=](std::string s) {
+            settings.set("build-max-jobs", s);
+        });
+
         auto intSettingAlias = [&](char shortName, const std::string & longName,
             const std::string & description, const std::string & dest) {
             mkFlag<unsigned int>(shortName, longName, description, [=](unsigned int n) {
                 settings.set(dest, std::to_string(n));
             });
         };
 
-        intSettingAlias('j', "max-jobs", "maximum number of parallel builds", "build-max-jobs");
         intSettingAlias(0, "cores", "maximum number of CPU cores to use inside a build", "build-cores");
         intSettingAlias(0, "max-silent-time", "number of seconds of silence before a build is killed", "build-max-silent-time");
         intSettingAlias(0, "timeout", "number of seconds before a build is killed", "build-timeout");

src/libstore/globals.cc

@@ -147,7 +147,14 @@ int Settings::get(const string & name, int def)
 void Settings::update()
 {
     _get(tryFallback, "build-fallback");
-    _get(maxBuildJobs, "build-max-jobs");
+
+    auto s = get("build-max-jobs", std::string("1"));
+    if (s == "auto")
+        maxBuildJobs = std::max(1U, std::thread::hardware_concurrency());
+    else
+        if (!string2Int(s, maxBuildJobs))
+            throw Error("configuration setting ‘build-max-jobs’ should be ‘auto’ or an integer");
+
     _get(buildCores, "build-cores");
     _get(thisSystem, "system");
     _get(maxSilentTime, "build-max-silent-time");