Compilation Modes
Inside Java HotSpot VM, there are actually two separate JIT compiler modes, which are known as C1 and C2. C1 is used for applications where quick startup and rock-solid optimization are required; GUI applications are often good candidates for this compiler. C2, on the other hand, was originally intended for long-running, predominantly server-side applications. Prior to some of the later Java SE 7 releases, these two modes were available using the -client and -server switches, respectively.

The two compiler modes use different techniques for JIT compilation, and they can output very different machine code for the same Java method. Modern Java applications, however, can usually make use of both compilation modes. To take advantage of this fact, starting with some of the later Java SE 7 releases, a new feature called tiered compilation became available. This feature uses the C1 compiler mode at the start to provide better startup performance. Once the application is properly warmed up, the C2 compiler mode takes over to provide more-aggressive optimizations and, usually, better performance. With the arrival of Java SE 8, tiered compilation is now the default behavior.
参考：
http://www.oracle.com/technetwork/articles/java/architect-evans-pt1-2266278.html