Hi, please see this question http://stackoverflow.com/questions/14229793/what-does-struct-sched-domain-st... I checked following http://lwn.net/Articles/169277/ and following http://www.kernel.org/doc/Documentation/scheduler/sched-domains.txt the first line of kernel.org doc says ............. Each CPU has a "base" scheduling domain (struct sched_domain).............. and second para says " each scheduling domain spans a number of CPUs (stored in the ->span field)." third para says " Each scheduling domain must have one or more CPU groups.................. The intersection of cpumasks from any two of these groups MUST be the empty set." then some where in doc it says "Balancing within a sched domain occurs between groups. That is, each group is treated as one entity." the doc in details talks about the implementation of scheduling domains and mentions that CPUs should belong to one of the scheduling domain in a way that cpumasks intersection should be an empty set The answer of the question that I want to know is why is a scheduling domain actually needed?