src/Pure/System/platform.scala

--- a/src/Pure/System/platform.scala	Thu Sep 10 16:04:12 2020 +0200
+++ b/src/Pure/System/platform.scala	Thu Sep 10 21:07:58 2020 +0200
@@ -70,20 +70,35 @@
   val jvm_name: String = System.getProperty("java.vm.name", "")
 
 
+  /* memory status */
+
+  sealed case class Memory_Status(heap_size: Long, heap_free: Long)
+  {
+    def heap_used: Long = (heap_size - heap_free) max 0
+    def heap_used_fraction: Double =
+      if (heap_size == 0) 0.0 else heap_used.toDouble / heap_size
+  }
+
+  def memory_status(): Memory_Status =
+  {
+    val heap_size = Runtime.getRuntime.totalMemory()
+    val heap_used = heap_size - Runtime.getRuntime.freeMemory()
+    Memory_Status(heap_size, heap_used)
+  }
+
+
   /* JVM statistics */
 
   def jvm_statistics(): Properties.T =
   {
-    val heap_size = Runtime.getRuntime.totalMemory()
-    val heap_used = heap_size - Runtime.getRuntime.freeMemory()
-
+    val status = memory_status()
     val threads = Thread.activeCount()
     val workers = Isabelle_Thread.pool.getPoolSize
     val workers_active = Isabelle_Thread.pool.getActiveCount
 
     List(
-      "java_heap_size" -> heap_size.toString,
-      "java_heap_used" -> heap_used.toString,
+      "java_heap_size" -> status.heap_size.toString,
+      "java_heap_used" -> status.heap_used.toString,
       "java_threads_total" -> threads.toString,
       "java_workers_total" -> workers.toString,
       "java_workers_active" -> workers_active.toString)