mlock() allows to lock the (current or further used) address space to physical memory. It therefor disables paging for the selected memory (or all for mlockall()). Real-time programs often uses the ability to fix their memory to avoid unpredictable situations. Think about a welding- or laser robot with out-swapped memory …

mlockall() is the big brother of mlock(): you specify that all currently (MCL_CURRENT) or future touched (MCL_FUTURE) pages are locked. This includes code (text segment), data and stack, shared libraries, user space kernel data, shared memory and memory-mapped areas.

Occasional I utilize mlockall() for performance measurements to ensure test reliability. Bundled with SCHED_FIFO (a realtime scheduling policy) you obtain a much cleaner measurement ( exclude IO operations completely (writes to disks, network, whatever)). If you follow this tips you will get clean measurements – but that’s another topic).

But one surprising fact appear by deeper analyses of an algorithm (it was a fast, SIMD enriched Hamming distance calculation – but this doesn’t matter here): the mlock() free version of the program achieve better results then the mlock() version.

w/o mlock():

w mlock():

See the irregular area in the lower figure? The peaks are all higher (and therefore disadvantages for the algorithm).