6888526: Linux getCurrentThreadCpuTime is drastically slower than Windows

Reviewed-by: dcubed, dholmes

src/os/linux/vm/globals_linux.hpp

@@ -29,9 +29,10 @@
   product(bool, UseOprofile, false,                                 \
         "enable support for Oprofile profiler")                     \
                                                                     \
-  product(bool, UseLinuxPosixThreadCPUClocks, false,                \
-          "enable fast Linux Posix clocks where available")         \
-
+  product(bool, UseLinuxPosixThreadCPUClocks, true,                 \
+          "enable fast Linux Posix clocks where available")
+// NB: The default value of UseLinuxPosixThreadCPUClocks may be
+// overridden in Arguments::parse_each_vm_init_arg.
 
 //
 // Defines Linux-specific default values. The flags are available on all

src/share/vm/runtime/arguments.cpp

@@ -2593,6 +2593,12 @@ SOLARIS_ONLY(
       FLAG_IS_DEFAULT(UseVMInterruptibleIO)) {
     FLAG_SET_DEFAULT(UseVMInterruptibleIO, true);
   }
+#ifdef LINUX
+ if (JDK_Version::current().compare_major(6) <= 0 &&
+      FLAG_IS_DEFAULT(UseLinuxPosixThreadCPUClocks)) {
+    FLAG_SET_DEFAULT(UseLinuxPosixThreadCPUClocks, false);
+  }
+#endif // LINUX
   return JNI_OK;
 }