src/Pure/RAW/multithreading.ML

--- a/src/Pure/RAW/multithreading.ML	Wed Mar 02 19:43:31 2016 +0100
+++ b/src/Pure/RAW/multithreading.ML	Thu Mar 03 11:12:02 2016 +0100
@@ -88,8 +88,13 @@
 
 (* options *)
 
+fun num_processors () =
+  (case Thread.numPhysicalProcessors () of
+    SOME n => n
+  | NONE => Thread.numProcessors ());
+
 fun max_threads_result m =
-  if m > 0 then m else Int.min (Int.max (Thread.numProcessors (), 1), 8);
+  if m > 0 then m else Int.min (Int.max (num_processors (), 1), 8);
 
 val max_threads = ref 1;