5 Virtual machines

In order to reduce the size of the code image which has to be transferred when an update occurs, virtual machines can be used. Because a virtual machine can be relatively high-level, the bytecode^†Bytecode is a binary representation of an executable program designed to be executed by a virtual machine. which is needed to program a virtual machine can be orders of magnitude smaller than native code. [Dun06, Lev02]

The biggest downside of virtual machines is that they incur significant overhead. This means that running a program on a virtual machine takes more processing time and power than running native code. An instruction on the Maté virtual machine can take between 1.03 and 33.5 times as long as in native code. Although the reduced code size saves energy, the overhead of the virtual machine consumes extra energy.

To reduce the overhead of the virtual machine, but keep the flexibility and small code size, [Lev05] proposed application specific virtual machines. These virtual machines have big parts of the application as virtual instructions, making the program even smaller and more high-level, while implementing most of the program in native code. This makes the program both efficient and flexible.

In [Wir06], a more dynamic and numerical approach is taken to use both virtual machine and native code. A network is equipped with a node which can do just-in-time (JIT) compiling. Ordinary nodes do not have the resources to perform this task, so the JIT compilation service is implemented by a more capable node. The JIT compilation service will compile a specified method into native code and broadcast it. The nodes use distributed method profiling to identify any hot methods, methods which take up most of the resources. These methods are compiled by the JIT compilation service and reloaded as native code. This way, a good balance between bytecode and native code is reached.