2 Replies Latest reply: Feb 25, 2008 2:37 AM by 843829 RSS

    About -Xcomp -Xint or default mixed-mode

      Hi everybody,
      I've recently found out a JVM switch I was not aware of in the past, -Xcomp. The documentation says that if specified it causes 100% of the classes to be compiled with maximum optimization.

      I also know that if nothing is specified the default execution mode is mixed-mode that, if I've correctly grasped the idea, means that some classes will be compiled and other not, am I right?

      Well, that stated I would like to know if there is some kinda guideline one should follow when choosing whether to use one option rather than another. I've tried to execute the J2EE app I'm working on both with and without the -Xcomp option and I've measured a noticeable slowdown when the option is set. In a sense this is expectable, since the HotSpot has to compile some pieces of code that otherwise would have not, but I was expecting that after an initial transient subsequent operations would have been run faster. Is that right?

      In conclusion I would like to get more information about the meaning of these settings, their runtime impacts and the way they operate when set.

      Any hint/help is truly appreciated.

      Best regards,
        • 1. Re: About -Xcomp -Xint or default mixed-mode
          The JVM keeps track of how many times a method is
          executed by the interpreter and compiles the method
          once the number of executions have exceeded a
          threshold. As you say -Xcomp causes every method
          to be compiled. That includes methods that are
          executed only once. For those you pay the price
          for compiling and don't get any benefit because
          the method is never executed again. The assumption
          is that methods that are performance critical are
          executed often and will get compiled. I would say
          in your case since you don't see improved performance
          that assumption is correct. I don't have any more details
          about these setting because I just use the default
          • 2. Re: About -Xcomp -Xint or default mixed-mode
            Thank you very much jon999,
            your answer has clarified me the topic. Furthermore I've found an interesting HotSpot-related FAQ on java.sun.com that do completes the picture.

            Best regards,