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2005-07-24

working set is not relevant


[Updated: 2005.07.24]

The key point of working set algorithm is not „virtual time“ relative to which recently used pages are defined. Virtual time is a problem rather than a solution. At the time when working set algorithm was designed, there was one to one mapping between address spaces and threads. In such setup virtual time based scanning ties memory scheduling and processor scheduling and, hence, avoids thrashing, which is a situation when one resource (processor) is scheduled to achieve its maximal utilization, ignoring constraints imposed by other resource (memory) (See Dijkstra papers EWD408 and EWD462 for as always lucid an explanation).

In modern systems there is no such one to one mapping anymore. There are multiple threads executing within the same address space. There are memory consumers that are not readily identifiable with any execution context at all: file system cache (which is merged with VM in any self-respecting kernel), general in-kernel memory allocator, etc. It means that:

  • there is no relevant notion of virtual time to be used during scanning;
  • working set control fails to bind memory and processor scheduling together, which will decrease its effectiveness.

One extreme case is to consider whole system as one entity. Then virtual time degenerates into physical one, and working set algorithm into active/inactive queue.

Moreover, working set is not page replacement algorithms at all. That is, it doesn't answer to the question „System is low on memory, what page should be reclaimed?“ Instead it vaguely points to some address space and proposes to reclaim some unknown page from it. This made sense for the time-shared server running large number of tasks with disjoint address spaces, but for the modern system working set algorithm will be most likely finger-pointing to the Mozilla (or OpenOffice, or...) on the client and Apache (or Oracle, or...) on the server. Which is completely useless, because we still don't know what page is to be reclaimed.

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