A computer implemented method of scheduling a plurality of virtual machines for execution by a physical computing infrastructure, each virtual machine being deployable to a subset of the infrastructure to execute a computing task, the method comprising: determining, for each virtual machine, a subset of the infrastructure and a time period for deployment of the virtual machine, so as to schedule the virtual machines to execute to completion over an aggregate of all time periods; and deploying each virtual machine by provisioning of physical resources in accordance with the determining step, wherein the determination is based on a mathematical optimisation of a risk function for each of the virtual machines corresponding to a relative risk that a virtual machine will fail to fully execute its task to completion; wherein the risk function is a function of a likelihood of failure of a task for a virtual machine and a relative impact of such failure, wherein the likelihood of failure is determined based on a probability that execution of the task will commence in accordance with the schedule, and based on a probability that execution of the task will complete in accordance with the schedule, wherein the impact of the failure is a function of a criticality of a job request, wherein the likelihood of failure is defined along a time dimension due to a processing and/or duration variability, and due to network delay variability, the likelihood of failure is additionally defined along a magnitude dimension, wherein the processing and/or duration variability is a probability of the task not starting on a server within its virtual machine time period and a probability of the task not being completed before a target time, wherein the network delay variability is a probability of having a network delay variability above or up to an acceptable criteria, wherein the magnitude dimension is a probability of the task failing the instantiation to a virtual machine time period by its target due to requirements concerning an amount of physical resources consumed, and wherein the mathematical optimisation is further arranged to minimise a quantity of physical resources in the physical computing infrastructure consumed to execute the virtual machines to completion over the aggregate time period. determining, for each virtual machine, a subset of the infrastructure and a time period for deployment of the virtual machine, so as to schedule the virtual machines to execute to completion over an aggregate of all time periods; and deploying each virtual machine by provisioning of physical resources in accordance with the determining step, wherein the determination is based on a mathematical optimisation of a risk function for each of the virtual machines corresponding to a relative risk that a virtual machine will fail to fully execute its task to completion; wherein the risk function is a function of a likelihood of failure of a task for a virtual machine and a relative impact of such failure, wherein the likelihood of failure is determined based on a probability that execution of the task will commence in accordance with the schedule, and based on a probability that execution of the task will complete in accordance with the schedule, wherein the impact of the failure is a function of a criticality of a job request, wherein the likelihood of failure is defined along a time dimension due to a processing and/or duration variability, and due to network delay variability, the likelihood of failure is additionally defined along a magnitude dimension, wherein the processing and/or duration variability is a probability of the task not starting on a server within its virtual machine time period and a probability of the task not being completed before a target time, wherein the network delay variability is a probability of having a network delay variability above or up to an acceptable criteria, wherein the magnitude dimension is a probability of the task failing the instantiation to a virtual machine time period by its target due to requirements concerning an amount of physical resources consumed, and wherein the mathematical optimisation is further arranged to minimise a quantity of physical resources in the physical computing infrastructure consumed to execute the virtual machines to completion over the aggregate time period.