Under the hood OS: Demand Paging, Page Faults and Working Set
Linux is an on demand memory system. The malloc() call immediately returns virtual memory, which is only promoted to real memory later when it is used via a page fault.
Demand Paging
In computer operating systems, demand paging (as opposed to anticipatory paging) is a method of virtual memory management. In a system that uses demand paging, the operating system copies a disk page into physical memory only if an attempt is made to access it and that page is not already in memory (i.e., if a page fault occurs). It follows that a process begins execution with none of its pages in physical memory, and many page faults will occur until most of a process’s working set of pages is located in physical memory. This is an example of a lazy loading technique.
Page Fault
A page fault (sometimes called #PF, PF or hard fault[a]) is a type of exception raised by computer hardware when a running program accesses a memory page that is not currently mapped by the memory management unit (MMU) into the virtual address spaceof a process. Logically, the page may be accessible to the process, but requires a mapping to be added to the process page tables, and may additionally require the actual page contents to be loaded from a backing store such as a disk. The processor’s MMU detects the page fault, while the exception handling software that handles page faults is generally a part of the operating system kernel. When handling a page fault, the operating system generally tries to make the required page accessible at the location in physical memory, or terminates the program in case of an illegal memory access.
Contrary to what “fault” might suggest, valid page faults are not errors, and are common and necessary to increase the amount of memory available to programs in any operating system that utilizes virtual memory.
Page Maps
pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
userspace programs to examine the page tables and related information by
reading files in /proc.
There are four components to pagemap:
* /proc/pid/pagemap. This file lets a userspace process find out which
physical frame each virtual page is mapped to. It contains one 64-bit
value for each virtual page, containing the following data (from
fs/proc/task_mmu.c, above pagemap_read):
* Bits 0-54 page frame number (PFN) if present
* Bits 0-4 swap type if swapped
* Bits 5-54 swap offset if swapped
* Bit 55 pte is soft-dirty (see Documentation/vm/soft-dirty.txt)
* Bit 56 page exclusively mapped (since 4.2)
* Bits 57-60 zero
* Bit 61 page is file-page or shared-anon (since 3.5)
* Bit 62 page swapped
* Bit 63 page presentIdle Page Flags
Working Set Size
The Working Set Size (WSS) is how much memory an application needs to keep working. Your application may have 100 Gbytes of main memory allocated and page mapped, but it is only touching 50 Mbytes each second to do its job. That’s the working set size: the “hot” memory that is frequently used. It is useful to know for capacity planning and scalability analysis.
