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General: Understanding Memory - System Working Set

Windows OS functions also consumes RAM, so the system has a working set that needs to be controlled and managed like any other process. In this section we discuss the components of the system working set and look at how it is managed.

Both system code and device driver code occupies memory. In addition, the OS allocates data structures in two areas of memory: a pool for non-pageable storage and a pool for pageable storage. Data structures accessed by OS and driver functions when interrupts are disabled must be resident in RAM at the time they are referenced. These data structure are usually allocated from the non-pageable pool so that they reside permanently in RAM. The Pool Nonpages Bytes counter in the Memory object shows the amount of RAM currently allocated that is permanently resident in RAM.

Mainly, though, most system data structure are pageable: they are created in a pageable pool of storage and subject to page replacement like the virtual memory pages of any other process. The Windows OS maintains a working set of active pages in RAM that are subject to the same page replacement policy as ordinary process address spaces. The Cache Bytes counter provides the total number of resident pages in the current system working set. Cache Bytes is the sum of the System Cache Resident Bytes, System Driver Resident Bytes, System Code Resident Bytes, and Pool Paged Resident Bytes counters. The OS’s working set is known as the cache because it also includes resident pages of the Windows file cache, the OS function that typically consumes more RAM than any other.

The system working set is subject to the same local page replacement policy as ordinary process working sets. By default, the system cache has a minimum working set of about 4.8MB and a maximum working set of a little more than 2000 pages, or about 8MB. Just like ordinary processes, when the OS is running at its maximum working set value and it references a page that is not currently resident (causing a page fault), the new page displaces an older page from the system cache and the older page is moved to the Standby List. Just like ordinary processes, when the Balance Set manager thread responsible for page trimming detects that the OS is running at its current working set maximum and at least 4MB of RAM available, it adjusts the system working set upwards 20 pages at a time.

A registry parameter called LargeSystemCache can be set to change the system working set maximum value. When the LargeSystemCache is turned on, the system working set maximum is set to approximately 80% of the total amount of RAM installed.

Turning on the LargeSystemCache setting, which boosts the value of the system’s maximum working set size, favors the system working set over the working sets of other processes. It allows the working set of the system, which includes the Windows file cache, to grow relatively unconstrained in relation to other process working sets and to absorb the bulk of the system’s uninstalled RAM.

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