1. A method of optimizing just-in-time (‘JIT’) compiling for a software application executing on a compute node, the compute node having installed upon it a software Virtual Machine (‘VM’) capable of supporting the software application, the method comprising: identifying a plurality of portions within the software application; for each portion of the plurality of portions, assigning, by an application manager, one of a plurality of a JIT levels to the portion of the software application, wherein the plurality of JIT levels excludes a level in which no JIT compiling is carried out, including: establishing the JIT level for the portion of the software application in dependence upon both a historic JIT profile for the portion of the software application specifying, for each previous execution, one of a plurality of JIT levels previously used when jitting the portion of the software application and previous execution performance of the portion of the software application including a previous performance value and a number of times the particular portion was invoked; wherein each portion of the plurality of portions of the application is assigned a different JIT level; and jitting, by the VM installed on the compute node, the portion of the software application in dependence upon the JIT level assigned to that portion of the software application.