8005489: VM hangs during GC with ParallelGC and ParallelGCThreads=0

Summary: Print an error message and exit the VM if UseParallalGC is combined with ParllelGCThreads==0. Also reviewed by vitalyd@gmail.com.
Reviewed-by: stefank, ehelin

src/share/vm/runtime/arguments.cpp

@@ -1454,30 +1454,34 @@ void Arguments::set_parallel_gc_flags() {
 
   // If no heap maximum was requested explicitly, use some reasonable fraction
   // of the physical memory, up to a maximum of 1GB.
-  if (UseParallelGC) {
-    FLAG_SET_DEFAULT(ParallelGCThreads,
-                     Abstract_VM_Version::parallel_worker_threads());
+  FLAG_SET_DEFAULT(ParallelGCThreads,
+                   Abstract_VM_Version::parallel_worker_threads());
+  if (ParallelGCThreads == 0) {
+    jio_fprintf(defaultStream::error_stream(),
+        "The Parallel GC can not be combined with -XX:ParallelGCThreads=0\n");
+    vm_exit(1);
+  }
 
-    // If InitialSurvivorRatio or MinSurvivorRatio were not specified, but the
-    // SurvivorRatio has been set, reset their default values to SurvivorRatio +
-    // 2.  By doing this we make SurvivorRatio also work for Parallel Scavenger.
-    // See CR 6362902 for details.
-    if (!FLAG_IS_DEFAULT(SurvivorRatio)) {
-      if (FLAG_IS_DEFAULT(InitialSurvivorRatio)) {
-         FLAG_SET_DEFAULT(InitialSurvivorRatio, SurvivorRatio + 2);
-      }
-      if (FLAG_IS_DEFAULT(MinSurvivorRatio)) {
-        FLAG_SET_DEFAULT(MinSurvivorRatio, SurvivorRatio + 2);
-      }
+
+  // If InitialSurvivorRatio or MinSurvivorRatio were not specified, but the
+  // SurvivorRatio has been set, reset their default values to SurvivorRatio +
+  // 2.  By doing this we make SurvivorRatio also work for Parallel Scavenger.
+  // See CR 6362902 for details.
+  if (!FLAG_IS_DEFAULT(SurvivorRatio)) {
+    if (FLAG_IS_DEFAULT(InitialSurvivorRatio)) {
+       FLAG_SET_DEFAULT(InitialSurvivorRatio, SurvivorRatio + 2);
+    }
+    if (FLAG_IS_DEFAULT(MinSurvivorRatio)) {
+      FLAG_SET_DEFAULT(MinSurvivorRatio, SurvivorRatio + 2);
     }
+  }
 
-    if (UseParallelOldGC) {
-      // Par compact uses lower default values since they are treated as
-      // minimums.  These are different defaults because of the different
-      // interpretation and are not ergonomically set.
-      if (FLAG_IS_DEFAULT(MarkSweepDeadRatio)) {
-        FLAG_SET_DEFAULT(MarkSweepDeadRatio, 1);
-      }
+  if (UseParallelOldGC) {
+    // Par compact uses lower default values since they are treated as
+    // minimums.  These are different defaults because of the different
+    // interpretation and are not ergonomically set.
+    if (FLAG_IS_DEFAULT(MarkSweepDeadRatio)) {
+      FLAG_SET_DEFAULT(MarkSweepDeadRatio, 1);
     }
   }
 }