diff -r f97e23eaa628 -r 4b014e6c1dfe src/Pure/Concurrent/isabelle_thread.scala
--- a/src/Pure/Concurrent/isabelle_thread.scala	Wed Oct 18 18:53:26 2023 +0200
+++ b/src/Pure/Concurrent/isabelle_thread.scala	Wed Oct 18 19:05:06 2023 +0200
@@ -74,7 +74,7 @@
 
   def max_threads(): Int = {
     val m = Value.Int.unapply(System.getProperty("isabelle.threads", "0")) getOrElse 0
-    if (m > 0) m else (Runtime.getRuntime.availableProcessors max 1) min 8
+    if (m > 0) m else (Host.num_cpus() max 1) min 8
   }
 
   lazy val pool: ThreadPoolExecutor = {