PDC10: Mysteries of Windows Memory Management Revealed: Part Two

In the last session, focusing on virtual memory, it was noted that there was almost no connection between virtual and physical memory. The only connection is that the system commit limit is the sum of physical memory and the size of the paging file(s). This session focuses on the physical memory aspects of the memory management architecture in Windows.

Physical Memory and Working Set

The working set is the physical memory that Windows gives to your process, given to you when you demand it by touching the virtual address space. A process starts with an empty working set – as it references pages that aren’t in the working set, it incurs a page fault. Many such page faults are resolved simply from memory (for example, if you load an image that is already in the file cache).

Each process has a working set default minimum and maximum. Windows doesn’t pay any attention to the maximum working set; Windows uses the minimum to commit that amount of memory, as well as to lock memory.

At some point, the memory manager decides the process is large enough; it then gives up pages to make room for new pages. If your process is ballooning, the memory manager will tend to favor it because it is obviously memory-hungry; but at some stage, it will start to pull pages out of the working set that are least recently used before it will allocate new memory to that process.

Unlike other OS, it is a local page replacement policy, meaning that it will pull the pages from the requesting process.

The working set consists of shareable and private pages; there are performance counters that measure this. Shareable working set is effectively counted as “private” if it is not shared. Note that the working set does not include trimmed memory that is still cached.

Windows Task Manager only offers the private working set figure; Process Explorer gives you a more detailed view. Remember – it’s only when you touch the memory that is committed that it becomes part of the private working set. Comparing testlimit with –m, –r and –d switches demonstrates the differences here.

Physical Memory Page Lists

The system keeps unassigned physical pages on one of several page lists in the Page Frame Number (PFN) database, each maintained as FIFO list:

• free (available for allocation but has dirty data in it);
• modified (previously belonging to a process but needing to be written to a backing store before it can be reused;
• standby (still associated with a process but not included in the working set);
• zero (zeroed out memory safe for allocation),
• ROM (read-only memory)
• bad (pages that failed internal consistency checks).

Read more: Tim Sneath

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