1. An information processing system comprising: a plurality of managed servers for providing access to a storage device and for processing information; a plurality of cooling machines for cooling said servers; and a power-saving controller for sending and receiving information to and from said servers via a network to detect workload of said servers and for controlling said cooling machines and said servers in accordance with a detection result, wherein said power-saving controller controls the cooling ability of said cooling machines in accordance with the workload of said servers and also controls task allocation with respect to said servers, wherein said power-saving controller includes: a task allocation setting unit for setting up a plurality of sets of task allocations with respect to said servers from processing abilities of said servers and contents of one or more tasks to be processed on said servers; a server power calculation unit for estimating electrical power consumption of said servers in each of the set-up task allocations based on electric power characteristics with respect to the processing abilities of said servers; a heat generation amount calculation unit for estimating a heat generation amount of said servers in each of the setup task allocations based on heat generation characteristics with respect to the processing abilities of said servers; a cooling power calculation unit for setting up a cooling ability of a cooling machine of said plurality of cooling machines which is chosen to cool said servers based on the heat generation amount of said servers in each of the setup task allocations and for estimating electric power consumption of said cooling machine based on power consumption characteristics with respect to the cooling ability of said cooling machine; and a task allocation determination unit for determining in said plurality of sets of task allocations a task allocation which is reduced to a minimum in total of estimated values of said server power calculation unit and said cooling power calculation unit and for determining the task allocation to be a proper task allocation.