Date:  26 August 2008

 The key to using
memory efficiently is virtual memory management. Consider both Windows and a
UNIX/Linux operating system. Compare and contrast how each implements virtual
memory. Describe how each one handles page faults, page sizes and how it
reconciles thrashing issues. Cite your sources.

 

Answer:

 Virtual
memory makes the system appear to have more memory than it actually has by
sharing it between competing processes as they need it. Systems
that use this technique make programming of applications easier and use physical
memory (e.g.

RAM) more
efficiently than those without virtual memory.

 Compare
and contrast how each implements virtual memory 

Windows

UNIX/Linux

Windows NT and its variants employ a dynamically allocated page file for memory management. A page file is allocated on disk, for less frequently accessed objects in memory, leaving more RAM available to actively used objects. This scheme suffers from slow-downs due to disk fragmentation, which hampers the speed at which the objects can be brought back into memory when they are needed. Windows can be configured to place the page file on a separate partition; doing this negates the disk-fragmentation issues, but introduces an I/O slowdown due to the seek time involved in switching back and forth between the two partitions. However, the main reason this is not done by default is that, if the page file is on a separate partition, then Windows cannot create a memory dump in the event of a Stop Error. The ideal solution performance-wise is to have the page file on a separate hard drive to the primary one, which eliminates both defragmentation and I/O issues. In memory management, for example, Windows NT 5.0 and its successors use clustered paging, a working set memory analogue and a free memory manager that fires up exactly once per second, •         Paging –        Available –        Reserved –        Committed

Most hard drive installations of Linux utilize a "swap partition", where the disk space allocated for paging is separate from general data, and is used strictly for paging operations. This reduces slowdown due to disk fragmentation from general use. As with Windows, for best performance the swap partition should be placed on a separate hard drive to the primary one. Unix uses an adaptive page specific algorithm to control paging. In Unix, there is no working set equivalent, and the free memory manager runs when needed

 

Paging are
performed when a program tries to access pages that do not currently reside in
RAM

 

Thrashing is swapping out a
piece of a process just before that piece is needed.
The processor spends most of its time swapping pieces rather than
executing user instructions

 

Windows	UNIX/Linux
Page faults Windows uses structured exception handling to report page fault-based invalid accesses as access violation exceptions. Recent Windows versions also write a minidump (similar in principle to a core dump) describing the state of the crashed process for later analysis alongside such less-technical error messages	Typically use signals, to report these error conditions to programs. . UNIX operating systems typically report these conditions to the user with error messages such as "segmentation violation", or "bus error".
Page sizes Win32-based operating system, such as Windows 9x, NT use the system function GetSystemInfo()from kernel32.dll. Windows NT provides a page-based virtual memory management scheme that allows applications to realize a 32-bit linear address space for 4 gigabytes (GB) of memory. As a result, each application has its own private address space from which it can use the lower 2 GB—the system reserves the upper 2 GB of every process's address space for its own use.	UNIX systems use the system function sysconf(), as illustrated in the C programming language.The Unix kernel divides the memory into manageable chunks called pages. A single page of memory is usually 4096 or 8192 bytes (4 or 8KB). Memory pages are laid down contiguously across the physical and virtual memory.
Thrashing  The total number of pages is being used by this particular instruction is eight, and all eight pages must be present in memory at the same time. If the operating system will allocate less than eight pages of actual memory in this example, when it attempts to swap out some part of the instruction or data to bring in the remainder, the instruction will again page fault, and it will thrash on every attempt to restart the failing instruction. To resolve thrashing due to excessive paging, a user can do any of the following. 1. Increase the amount of RAM in the computer 2. Decrease the number of programs being run on the computer.

 

 

Resourse:
• http://en.wikipedia.org/wiki/Page_size
• http://members.shaw.ca/bsanders/WindowsGeneralWeb/RAMVirtualMemoryPageFileEtc.htm 1.1.
• http://www.cs.uml.edu/~cgould/#Memory%20Management
• http://www.michaelhorowitz.com/Linux.vs.Windows.html