A mechanism dynamically allocates physical machines (computers) for satisfying requests for the machines, such as in the context of assigning test machines for running test cases. Each request specifies one or more logical machine sets it needs, and each logical machine set has specified requirements. The physical machines are organized into a hierarchical structure that includes groups and subgroups of machines. An allocation engine performs the function of allocating available machines from a selected group in the hierarchy for a request based on the requirements of each logical machine set of the request. A group or subgroup of machines may be marked as “allocate as one unit” and all machines in it will be reserved for one request. A job corresponding to a request may have sub-jobs, and machines allocated for the sub-job are selected from the machines allocated for the parent job.