When the system encounters a more serious shortage of primary memory, it writes all the pages of some processes to secondary memory and marks those processes as swapped. Such processes return to a state in which they can be scheduled only when the system scheduler daemon selects them to be read back into memory.
Both paging and swapping cause delay when a process is ready to run again. For processes that have strict timing requirements, this delay can be unacceptable.
To avoid swapping delays, real-time processes are never swapped, though parts of them can be paged. A program can prevent paging and swapping by locking its text and data into primary memory. For more information, see the memcntl(2) man page. How much memory can be locked is limited by how much memory is configured. Also, locking too much can cause intolerable delays to processes that do not have their text and data locked into memory.
Trade-offs between performance of real-time processes and performance of other processes depend on local needs. On some systems, process locking might be required to guarantee the necessary real-time response.
Note - See "Dispatch Latency" for information about latencies in real-time applications.