1. A method for providing computing services, comprising computer-executed actions of: allocating a respective virtual machine to each of a plurality of clients of a cloud computer service, each virtual machine having a respective set of defining parameters; wherein a first virtual machine of a first client of said plurality of clients includes a just-in-time compiler supporting execution of at least one application on said first virtual machine, said just-in-time compiler generating executable code for execution on said first virtual machine, said executable code generated by said just-in-time compiler being optimized according to the set of defining parameters of said first virtual machine; receiving a request from said first client to alter the set of defining parameters of said first virtual machine; altering the set of defining parameters of said first virtual machine responsive to said request from said first client to create an altered first virtual machine; responsive to altering the set of defining parameters of said first virtual machine, causing at least one code module of an application executing in said first virtual machine which was previously compiled to executable code by said just-in-time compiler to be recompiled to executable code by said just-in-time compiler, wherein the recompilation comprises: with respect to each of a plurality of code modules of said application executing in said first virtual machine which were previously compiled by said just-in-time compiler, making a selective determination whether to recompile the respective code module, wherein said selective determination is based at least in part on an amount of difference of at least one parameter of said set of defining parameters between an original value of the at least one parameter and an altered value of the at least one parameter; and recompiling the respective code module according to a result of said selective determination, the executable code generated by recompilation being optimized according to the set of defining parameters of the altered first virtual machine.