Memory load control algorithm

The memory load control mechanism assesses, once per second, whether sufficient memory is available for the set of active processes. When a memory-overcommitment condition is detected, some processes are suspended, decreasing the number of active processes and thereby decreasing the level of memory overcommitment.

When a process is suspended, all of its threads are suspended when they reach a suspendable state. The pages of the suspended processes quickly become stale and are paged out by the page-replacement algorithm, releasing enough page frames to allow the remaining active processes to progress. During the interval in which existing processes are suspended, newly created processes are also suspended, preventing new work from entering the system. Suspended processes are not reactivated until a subsequent interval passes during which no potential thrashing condition exists. Once this safe interval has passed, the threads of the suspended processes are gradually reactivated.

Memory load-control schedo parameters specify the following:

• The system memory overcommitment threshold (v_repage_hi)
• The number of seconds required to make a safe interval (v_sec_wait)
• The individual process memory overcommitment threshold by which an individual process is qualified as a suspension candidate (v_repage_proc)
• The minimum number of active processes when processes are being suspended (v_min_process)
• The minimum number of elapsed seconds of activity for a process after reactivation (v_exempt_secs)

For information on setting and tuning these parameters, see VMM memory load control tuning with the schedo command.

Once per second, the scheduler (process 0) examines the values of all the above measures that have been collected over the preceding one-second interval, and determines if processes are to be suspended or activated. If processes are to be suspended, every process eligible for suspension by the -p and -e parameter test is marked for suspension. When that process next receives the CPU in user mode, it is suspended (unless doing so would reduce the number of active processes below the -m value). The user-mode criterion is applied so that a process is ineligible for suspension during critical system activities performed on its behalf. If, during subsequent one-second intervals, the thrashing criterion is still being met, additional process candidates meeting the criteria set by -p and -e are marked for suspension. When the scheduler subsequently determines that the safe-interval criterion has been met and processes are to be reactivated, some number of suspended processes are put on the run queue (made active) every second.

Suspended processes are reactivated by:

1. Priority
2. The order in which they were suspended

The suspended processes are not all reactivated at once. A value for the number of processes reactivated is selected by a formula that recognizes the number of then-active processes and reactivates either one-fifth of the number of then-active processes or a monotonically increasing lower bound, whichever is greater. This cautious strategy results in increasing the degree of multiprogramming roughly 20 percent per second. The intent of this strategy is to make the rate of reactivation relatively slow during the first second after the safe interval has expired, while steadily increasing the reintroduction rate in subsequent seconds. If the memory-overcommitment condition recurs during the course of reactivating processes, the following occur:

• Reactivation is halted
• The marked-to-be reactivated processes are again marked suspended
• Additional processes are suspended in accordance with the above rules