Limit the number of forks getting Java versions (#41386)

To reduce configuration time, we fork some threads to compute the Java
version for the various configured Javas. However, as the number of
JAVA${N}_HOME variable increases, the current implementation creates as
many threads as there are such variables, which could be more than the
number of physical cores on the machine. It is not likely that we would
see benefits to trying to execute all of these once beyond the number of
physical cores (maybe simultaneous multi-threading helps though, who
knows. Therefore, this commit limits the parallelization here to the
number number of physical cores.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -197,9 +197,10 @@ class BuildPlugin implements Plugin<Project> {
                 }
             }
 
+            final int numberOfPhysicalCores = numberOfPhysicalCores(project.rootProject)
             if (javaVersions.isEmpty() == false) {
 
-                ExecutorService exec = Executors.newFixedThreadPool(javaVersions.size())
+                ExecutorService exec = Executors.newFixedThreadPool(numberOfPhysicalCores)
                 Set<Future<Void>> results = new HashSet<>()
 
                 javaVersions.entrySet().stream()
@@ -247,7 +248,7 @@ class BuildPlugin implements Plugin<Project> {
             project.rootProject.ext.inFipsJvm = inFipsJvm
             project.rootProject.ext.gradleJavaVersion = JavaVersion.toVersion(gradleJavaVersion)
             project.rootProject.ext.java9Home = "${-> findJavaHome("9")}"
-            project.rootProject.ext.defaultParallel = findDefaultParallel(project.rootProject)
+            project.rootProject.ext.defaultParallel = numberOfPhysicalCores
         }
 
         project.targetCompatibility = project.rootProject.ext.minimumRuntimeVersion
@@ -1013,7 +1014,7 @@ class BuildPlugin implements Plugin<Project> {
         }
     }
 
-    private static int findDefaultParallel(Project project) {
+    private static int numberOfPhysicalCores(Project project) {
         if (project.file("/proc/cpuinfo").exists()) {
             // Count physical cores on any Linux distro ( don't count hyper-threading )
             Map<String, Integer> socketToCore = [:]
@@ -1026,7 +1027,7 @@ class BuildPlugin implements Plugin<Project> {
                     if (name == "physical id") {
                         currentID = value
                     }
-                    // Number  of cores not including hyper-threading
+                    // number of cores not including hyper-threading
                     if (name == "cpu cores") {
                         assert currentID.isEmpty() == false
                         socketToCore[currentID] = Integer.valueOf(value)
@@ -1044,8 +1045,11 @@ class BuildPlugin implements Plugin<Project> {
                 standardOutput = stdout
             }
             return Integer.parseInt(stdout.toString('UTF-8').trim())
+        } else {
+            // guess that it is half the number of processors (which is wrong on systems that do not have simultaneous multi-threading)
+            // TODO: implement this on Windows
+            return Runtime.getRuntime().availableProcessors() / 2
         }
-        return Runtime.getRuntime().availableProcessors() / 2
     }
 
     private static configurePrecommit(Project project) {