--- a/make/RunTestsPrebuilt.gmk	Tue Dec 05 11:12:20 2017 +0100
+++ b/make/RunTestsPrebuilt.gmk	Tue Dec 05 12:57:48 2017 +0100
@@ -211,6 +211,19 @@
   PATH_SEP:=:
 endif
 
+# Check number of cores
+ifeq ($(OPENJDK_TARGET_OS), linux)
+    NUM_CORES := $(shell $(CAT) /proc/cpuinfo  | $(GREP) -c processor)
+else ifeq ($(OPENJDK_TARGET_OS), macosx)
+    NUM_CORES := $(shell /usr/sbin/sysctl -n hw.ncpu)
+else ifeq ($(OPENJDK_TARGET_OS), solaris)
+    NUM_CORES := $(shell LC_MESSAGES=C /usr/sbin/psrinfo -v | $(GREP) -c on-line)
+else ifeq ($(OPENJDK_TARGET_OS), windows)
+    NUM_CORES := $(NUMBER_OF_PROCESSORS)
+else
+    NUM_CORES := 1
+endif
+
 ################################################################################
 # Generate the ephemeral spec file
 ################################################################################
@@ -244,6 +257,7 @@
     OPENJDK_TARGET_CPU_ARCH := $(OPENJDK_TARGET_CPU_ARCH), \
     OPENJDK_TARGET_CPU_BITS := $(OPENJDK_TARGET_CPU_BITS), \
     OPENJDK_TARGET_CPU_ENDIAN := $(OPENJDK_TARGET_CPU_ENDIAN), \
+    NUM_CORES := $(NUM_CORES), \
     include $(TOPDIR)/make/RunTestsPrebuiltSpec.gmk, \
     $(CUSTOM_NEW_SPEC_LINE), \
 )

--- a/make/RunTestsPrebuiltSpec.gmk	Tue Dec 05 11:12:20 2017 +0100
+++ b/make/RunTestsPrebuiltSpec.gmk	Tue Dec 05 12:57:48 2017 +0100
@@ -54,18 +54,14 @@
 # The "human readable" name of this configuration
 CONF_NAME := run-test-prebuilt
 
-###FIXME
-# Information about the build system
-# NUM_CORES *is* needed to calculate JOBS.
-NUM_CORES:=16
 # Number of parallel jobs to use for compilation
-JOBS?=16
-TEST_JOBS?=0
+JOBS ?= $(NUM_CORES)
+TEST_JOBS ?= 0
 
-### FIXME
-#JAVA_FLAGS:= -Duser.language=en -Duser.country=US
-#JAVA_FLAGS_BIG:= -Xms64M -Xmx1600M -XX:ThreadStackSize=1536
-#JAVA_FLAGS_SMALL:= -XX:+UseSerialGC -Xms32M -Xmx512M -XX:TieredStopAtLevel=1
+# Use hard-coded values for java flags (one size, fits all!)
+JAVA_FLAGS := -Duser.language=en -Duser.country=US
+JAVA_FLAGS_BIG:= -Xms64M -Xmx1600M -XX:ThreadStackSize=1536
+JAVA_FLAGS_SMALL:= -XX:+UseSerialGC -Xms32M -Xmx512M -XX:TieredStopAtLevel=1
 BUILD_JAVA_FLAGS := $(JAVA_FLAGS_BIG)
 
 ################################################################################
@@ -100,15 +96,15 @@
 JRE_IMAGE_DIR := $(JDK_IMAGE_DIR)
 
 ################################################################################
-# Assume target platform is same as build platform
-OPENJDK_TARGET_OS := $(OPENJDK_BUILD_OS)
-OPENJDK_TARGET_OS_TYPE := $(OPENJDK_BUILD_OS_TYPE)
-OPENJDK_TARGET_OS_ENV := $(OPENJDK_BUILD_OS_ENV)
+# Assume build platform is same as target platform
+OPENJDK_BUILD_OS := $(OPENJDK_TARGET_OS)
+OPENJDK_BUILD_OS_TYPE := $(OPENJDK_TARGET_OS_TYPE)
+OPENJDK_BUILD_OS_ENV := $(OPENJDK_TARGET_OS_ENV)
 
-OPENJDK_TARGET_CPU := $(OPENJDK_BUILD_CPU)
-OPENJDK_TARGET_CPU_ARCH := $(OPENJDK_BUILD_CPU_ARCH)
-OPENJDK_TARGET_CPU_BITS := $(OPENJDK_BUILD_CPU_BITS)
-OPENJDK_TARGET_CPU_ENDIAN := $(OPENJDK_BUILD_CPU_ENDIAN)
+OPENJDK_BUILD_CPU := $(OPENJDK_TARGET_CPU)
+OPENJDK_BUILD_CPU_ARCH := $(OPENJDK_TARGET_CPU_ARCH)
+OPENJDK_BUILD_CPU_BITS := $(OPENJDK_TARGET_CPU_BITS)
+OPENJDK_BUILD_CPU_ENDIAN := $(OPENJDK_TARGET_CPU_ENDIAN)
 
 ################################################################################
 # Java executable definitions