Compiler Flags Are Different
Unlike most Java flags, the flags to select a compiler are different: most of them do not use -XX. The standard compiler flags are simple words: -client, -server, or -d64.
The exception here is tiered compilation, which is enabled with a flag in the common format: -XX:+TieredCompilation. Tiered compilation implies that the server compiler must be used. The following command silently turns off tiered compilation, because it conflicts with the choice of the client compiler:
% java -client -XX:+TieredCompilation other_args引自 Basic Tunings: Client or Server (or Both)
这段基本正确，不过有个坑（书里后面第83页Table 4-4也有提到），那就是在64位平台上没有HotSpot Client VM，只有HotSpot Server VM。所以java -client在64位平台上会被忽略，实际启用的仍然是HotSpot Server VM而不是指定的Client VM。而且这是隐式的，没有任何警告。
例如在Linux/x86-64上运行下面例子：
$ java -client -XX:+TieredCompilation -XX:+PrintFlagsFinal -version | grep TieredCompilation
bool TieredCompilation := true {pd product}
java version "1.8.0"
Java(TM) SE Runtime Environment (build 1.8.0-b132)
Java HotSpot(TM) 64-Bit Server VM (build 25.0-b70, mixed mode)
我们还是会看到启用了多层编译的HotSpot Server VM…
