49.9. MAP_NORESERVE and Swap Space Overcommitting

Some applications create large (usually private anonymous) mappings, but use only a small part of the mapped region. For example, certain types of scientific applications allocate a very large array, but operate on only a few widely separated elements of the array (a so-called sparse array).

If the kernel always allocated (or reserved) enough swap space for the whole of such mappings, then a lot of swap space would potentially be wasted. Instead, the kernel can reserve swap space for the pages of a mapping only as they are actually required (i.e., when the application accesses a page). This approach is called lazy swap reservation, and has the advantage that the total virtual memory used by applications can exceed the total size of RAM plus swap space.