In multiprocessor computer systems, software lockout is the issue of performance degradation due to the idle wait times spent by the CPUs in kernel-level critical sections.
Software lockout is the major cause of scalability degradation in a multiprocessor system, posing a limit on the maximum useful number of processors.
To mitigate the phenomenon, the kernel must be designed to have its critical sections as short as possible, therefore decomposing each data structure in smaller substructures.
Conceptually, the most valid solution is to decompose each kernel data structure in smaller independent substructures, having each a shorter elaboration time.
Many uniprocessor systems with hierarchical protection domains have been estimated to spend up to 50% of the time performing "supervisor mode" operations.