Process management

LWP state transition

Applications that have strict fixed priority constraints may need to prevent processes and LWPs from being swapped or paged out to secondary memory. Here's a simplified overview of UNIX system LWP states and the transitions between states:

LWP state transition diagram

An active LWP is normally in one of the five states in the diagram. The arrows show how it changes states.

An LWP is running if it is assigned to a CPU. An LWP is preempted--that is, removed from the running state--by the scheduler if an LWP with a higher priority becomes runnable. An LWP is also preempted if it consumes its entire time slice and An LWP of equal priority is runnable.
An LWP is runnable in memory if it is in primary memory and ready to run, but is not assigned to a CPU.
An LWP is sleeping in memory if it is in primary memory but is waiting for a specific event before it can continue execution. For example, an LWP is sleeping if it is waiting for an I/O operation to complete, for a locked resource to be unlocked, or for a timer to expire. When the event occurs, the process is sent a wakeup; if the reason for its sleep is gone, the LWP becomes runnable.
An LWP is runnable and swapped if it is not waiting for a specific event but has had its whole address space written to secondary memory to make room in primary memory for other LWPs.
An LWP is sleeping and swapped if it is both waiting for a specific event and has had its whole address space written to secondary memory to make room in primary memory for other processes or LWPs.

If a machine does not have enough primary memory to hold all its active processes and LWPs, it must page or swap some address space to secondary memory:

When the system is short of primary memory, it writes individual pages of some processes and LWPs to secondary memory but leaves those processes and LWPs runnable. When an LWP runs, if it accesses those pages, it must sleep while the pages are read back into primary memory.
When the system gets into a more serious shortage of primary memory, it writes all the pages of some processes and LWPs to secondary memory and marks those processes and LWPs as swapped. Such processes and LWPs get back into a schedulable state only by being chosen by the system scheduler demon process, then read back into memory.

Both paging and swapping, and especially swapping, introduce delay when a process or LWP is ready to run again. For processes and LWPs that have strict timing requirements, this delay can be unacceptable. To avoid swapping delays, fixed priority processes and LWPs are never swapped, though parts of them may be paged. An application can prevent paging and swapping by locking its text and data into primary memory. For more information see memcntl(2). Of course, how much can be locked is limited by how much memory is configured. Also, locking too much can cause intolerable delays to processes and LWPs that do not have their text and data locked into memory. Tradeoffs between performance of fixed priority processes and LWPs and performance of other processes and LWPs depend on local needs. On some systems, process locking may be required to guarantee the necessary fixed priority response.