Inside Memory Management, Part 2

Last month, I began this two-part series about memory management in Windows NT by introducing the concept of virtual memory. I reviewed the way x86 and Alpha processors use a two-level virtual address-to-physical address translation scheme. I discussed paging and introduced two powerful features of the Memory Manager: memory-mapped files and copy-on-write memory.

This month, I'll go into more detail about the internal data structures the Memory Manager uses to track the state of memory. I'll discuss working sets and the Page Frame Number (PFN) Database. I'll wrap up with a tour inside the additional data structures the Memory Manager uses to track memory shared by two or more applications, and I'll discuss Section Objects, the data structures the PFN Database uses to implement memory-mapped files.

Working Sets
The biggest effect the Memory Manager has on individual applications' performance and on the system is in its allocation of physical memory to each active process. The amount of memory the Memory Manager assigns to a process is called the working set. Every process has a working set. A special working set, called the system working set, is physical memory that belongs to parts of the NT Executive, device drivers, and the Cache Manager. If a process' working set is too small, the process will incur a high number of page faults as it accesses page table entries that describe data not present in memory but located either in the process' executable image on disk or in a paging file. For each such access, the Memory Manager must intervene and perform disk I/O to retrieve the data. If a process' working set is too large, the process will not incur page faults, but its physical memory might be holding data that the process will not access for some time, and that other processes might require. Thus, the Memory Manager must try to achieve a balance for all processes according to their memory usage patterns.

The choice NT makes for two memory-management policies--the page fetch policy and the page replacement policy--affects the way NT manages working sets. The page fetch policy is the method NT uses to bring in a process' data. NT uses the most common page fetch policy, demand paging. In demand paging, a process' data is paged-in as a process accesses it. A different page fetch policy, prefetching, requires the Memory Manager to aggressively bring in a process' data before the process asks for it. Prefetching can improve the performance of an application at the expense of other applications, because the Memory Manager's prediction of what data a process will ask for is likely to be imperfect and can waste physical memory in storing unused data. NT therefore implements a slight variation on demand paging: clustered demand paging. Instead of bringing into a process' working set only the page that a process accesses, the Memory Manager will attempt to bring in pages that surround the requested page. The idea behind clustered demand paging is that if a process accesses a page, it will likely also access the memory just preceding or following that page. The pages the Memory Manager thus brings in are known as a cluster. In NT, clusters can vary between 0 pages and 7 pages, depending on the amount of physical memory present on the system and whether the system is accessing code or data.

The page replacement policy affects the way the Memory Manager decides which page to remove from a working set. The Memory Manager must remove pages whenever physical memory is full of in-use pages and space is necessary to accommodate a page of data a process is accessing. Two characterizations for replacement policies are global and local. In a global replacement policy, the Memory Manager considers all pages of physical memory as replacement candidates, without regard to which working set the pages belong to. In a local replacement policy, the Memory Manager considers as replacement candidates only pages in the working set of the process that is accessing the page to be brought in. The disadvantage of global policy is that a rogue process can, by accessing large amounts of memory very quickly, adversely affect all other processes in the system by forcing their data out of memory. As with its page fetch policy, NT implements a variation of the replacement policy. This variation is local, because it first considers the pages in the working set of the process that is accessing the data. But the variation is also global in that it removes pages from the working sets of other processes if their memory requirements are low.

Replacement policies are further characterized by the method with which they choose a page to replace out of the set of local or global candidates. One policy, first in, first out (FIFO), removes pages in the order in which they were added to the working set (i.e., the first page added is the first page removed). An algorithm that has a more positive effect on the performance of applications is least recently used (LRU). The LRU algorithm requires the aid of the MMU to give the Memory Manager an idea of how recently a process accessed a particular page. LRU replaces first those pages that processes have not accessed for the longest period of time. On uniprocessors, NT uses the clock algorithm, a simple form of LRU replacement that is based on the limited access tracking x86 and Alpha MMUs provide. Whenever a process references a page, these MMUs set the Accessed flag bit in the page table entries (PTEs) of the page. If a page's Accessed flag is not set, the Memory Manager knows that no processes have accessed the page since the last time the Memory Manager examined the page's PTE.

The working set of every process, including the system working set, has a minimum size, a current size, and a maximum size. On a typical NT system with 64MB of physical memory, the default minimum size of a working set is 200KB, and the default maximum size is 1.4MB. Processes with the PROCESS_SET_QUOTA privilege can override these values using a Win32 API. When a process starts, it generates a large number of page faults as it references data in its virtual memory map (most often in its executable image) that it must bring into physical memory. The Memory Manager lets a process' current working set size grow as needed to its working set maximum. When a working set reaches its maximum size, the Memory Manager will allow additional pages into the working set only if plenty of free (unused) pages are available. If free pages are not available, the Memory Manager uses the replacement policies I just described to determine which page in the working set to replace. The pages in the working set are linked in a list that the Memory Manager scans. On a uniprocessor, if the Memory Manager finds a page with its Accessed flag set, the Memory Manager clears the flag and proceeds to the following pages, selecting for replacement the next page it finds with a cleared Accessed flag. Thus, the Memory Manager avoids pages that the process has accessed since the previous scan, because the Memory Manager assumes that the process will access those pages again. If the Memory Manager finds no pages to select for replacement, it restarts the scan and almost certainly finds a page whose Accessed flag it cleared on the previous pass and the process has not accessed since the first scan.

On a multiprocessor system, the Memory Manager does not clear Accessed flags during scans, because any modification to a PTE on a multiprocessor system invalidates Translation Look-aside Buffer (TLB) entries (which I described last month) on all processors. If the Memory Manager invalidates an address' cached translation by clearing its Accessed flag, the address must again undergo the slow three-step virtual memory translation process the next time a process references it. The result is an effectively random page-replacement algorithm on multiprocessors. Microsoft is developing more effective replacement algorithms for the multiprocessor version of NT 5.0.

The Balance Set Manager
The Memory Manager adjusts the sizes of working sets once every second, in response to page-in operations or when free memory drops below a certain threshold. The Memory Manager also examines all working sets to determine whether the Balance Set Manager thread needs to trim them. If free memory is plentiful, the Balance Set Manager removes pages from only the working sets of processes whose current size is above minimum that have not recently incurred many page faults. However, if the Memory Manager awakens the Balance Set Manager thread because free memory is scarce, the Balance Set Manager can trim pages from any process until it creates an adequate number of free pages--even to the point of dropping working sets below their minimum size.