VMM memory load control facility

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A process requires real-memory pages to execute. When a process references a virtual-memory page that is on disk, because it either has been paged-out or has never been read, the referenced page must be paged-in and, on average, one or more pages must be paged out (if replaced pages had been modified), creating I/O traffic and delaying the progress of the process.

The operating system attempts to steal real memory from pages that are unlikely to be referenced in the near future, through the page-replacement algorithm. A successful page-replacement algorithm allows the operating system to keep enough processes active in memory to keep the CPU busy. But at some level of competition for memory, no pages are good candidates for paging out to disk because they will all be reused in the near future by the active set of processes. This situation depends on the following:

  • Total amount of memory in the system
  • The number of processes
  • The time-varying memory requirements of each process
  • The page-replacement algorithm

When this happens, continuous paging-in and paging-out occurs. This condition is called thrashing. Thrashing results in incessant I/O to the paging disk and causes each process to encounter a page fault almost as soon as it is dispatched, with the result that none of the processes make any significant progress.

The most destructive aspect of thrashing is that, although thrashing may have been triggered by a brief, random peak in workload (such as all of the users of a system happening to press Enter in the same second), the system might continue thrashing for an indefinitely long time.

The operating system has a memory load-control algorithm that detects when the system is starting to thrash and then suspends active processes and delays the initiation of new processes for a period of time. Five parameters set rates and bounds for the algorithm. The default values of these parameters have been chosen to be "fail safe" across a wide range of workloads. In AIX Version 4, memory load control is disabled by default on systems that have available memory frames that add up to greater than or equal to 128 MB.