1. A method for providing computing services, comprising: 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 by said just-in-time compiler to be recompiled by said just-in-time compiler, the executable code generated by recompilation being optimized according to the set of defining parameters of the altered first virtual machine.