-# Redirect config output to cout so that gets diffed too (in case
-# that's the source of the problem).
-$simargs .= " --root:config_output_file=cout";
+# Run individual invocations in separate dirs so output and intermediate
+# files (particularly config.py and config.ini) don't conflict.
+$dir1 = "tracediff-$$-1";
+$dir2 = "tracediff-$$-2";
+mkdir($dir1) or die "Can't create dir $dir1\n";
+mkdir($dir2) or die "Can't create dir $dir2\n";