You are correct to certain extent. JIT ( Just In Time)Compilers are not so fast also. JIT compilers can speed up the program execution 7 to 10 times over interpreting. In this case , vm compiles the bytecode to native machine code the first time the method is invoked. ( the method is compiled " just in time " for it's first use by vm. the native machine code is picked by vm next time the same method is invoked. thus , you can get faster execution. But , there are trade-offs. sometimes rather many a times , the footprint of the compiled code is more than that of interpreted code. it may also happen that the vm spends a lot of time with just in time compiling everything, vm doesn't have time to do a thorough optimization of the code. thus , jit has its own trade - off. I hope that this is helpful to you. Regarding websphere app. server on os/390 , i don't have any idea. one for thing , dbalki. As the official naming policy of this forum , the user has to register hmself/ herself with proper first name & last name. Thus , I request you to register yourself with name as per naming policy & participate in the discussion forum. SHAILESH.