Limit the number of threads used for CTW

compiler/src/jdk.graal.compiler.test/src/jdk/graal/compiler/hotspot/test/LibGraalCompilationDriver.java

@@ -117,8 +117,8 @@ public class LibGraalCompilationDriver {
  private final boolean multiThreaded;
 
  /**
- * Number of threads to use for multithreaded compilation. If 0, the value of
- * {@code Runtime.getRuntime().availableProcessors()} is used instead.
+ * Number of threads to use for multithreaded compilation. If 0, a good default value is picked
+ * by {@link #getThreadCount()}.
  */
  private final int numThreads;
 
@@ -694,7 +694,12 @@ private int getThreadCount() {
  if (multiThreaded) {
  threadCount = numThreads;
  if (threadCount == 0) {
- threadCount = Runtime.getRuntime().availableProcessors();
+ /*
+ * On very large machine there might be hundreds of processors and the compiler
+ * doesn't really scale well enough for that so limit the max number of threads. 32
+ * was picked as that seemed to scale well enough in testing.
+ */
+ threadCount = Math.min(32, Runtime.getRuntime().availableProcessors());
  }
  }
  return threadCount;