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Full Discussion: virtual memory
Top Forums UNIX for Dummies Questions & Answers virtual memory Post 85608 by RTM on Thursday 6th of October 2005 09:36:02 AM
Old 10-06-2005
Virtual memory: The use of a disk partition or a file on disk to provide the same facilities usually provided by RAM. A way of using disk storage space to make the computer work as if it had more memory. When a file or program is too big for the computer to work with in its memory, part of the data is stored on disk. This virtual storage is divided into segments called pages; each page is correlated with a location in physical memory, or RAM. When an address is referenced, the page is swapped into memory; it is sent back to disk when other pages must be called. The program runs as if all the data is in memory.

RAM: Random Access Memory. The working memory of the computer. RAM is the memory used for storing data temporarily while working on it, running application programs, etc. "Random access" refers to the fact that any area of RAM can be accessed directly and immediately, in contrast to other media such as a magnetic tape where the tape must be wound to the point where the data is. RAM is called volatile memory; information in RAM will disappear if the power is switched off before it is saved to disk.
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MEMSTAT(1)						     Linux Programmer's Manual							MEMSTAT(1)

NAME
memstat - Identify what's using up virtual memory. SYNOPSIS
memstat [-w][-p PID] DESCRIPTION
memstat lists all accessible processes, executables, and shared libraries that are using up virtual memory. To get a complete list memstat has to be run as root to be able to access the data of all running processes. First, the processes are listed. An amount of memory is shown along with a process ID and the name of the executable which the process is running. The amount of memory shown does not include shared memory: it only includes memory which is private to that process. So, if a process is using a shared library like libc, the memory used to hold that library is not included. The memory used to hold the exe- cutable's text-segment is also not included, since that too is shareable. After the processes, the shared objects are listed. The amount of memory is shown along with the filename of the shared object, followed by a list of the processes using the shared object. The memory is listed as the total amount of memory allocated to this object throughout the whole namespace. In brackets also the amount that is really shared is listed. Finally, a grand total is shown. Note that this program shows the amount of virtual (not real) memory used by the various items. memstat gets its input from the /proc filesystem. This must be compiled into your kernel and mounted for memstat to work. The pathnames shown next to the shared objects are determined by scanning the disk. memstat uses a configuration file, /etc/memstat.conf, to determine which directories to scan. This file should include all the major bin and lib directories in your system, as well as the /dev directory. If you run an executable which is not in one of these directories, it will be listed by memstat as ``[0dev]:<inode>''. Options The -w switch causes a wide printout: lines are not truncated at 80 columns. The -p switch causes memstat to only print data gathered from looking at the process with the gicen PID. NOTES
These reports are intended to help identify programs that are using an excessive amount of memory, and to reduce overall memory waste. FILES
/etc/memstat.conf /proc/*/maps SEE ALSO
ps(1), top(1), free(1), vmstat(8), lsof(8), /usr/share/doc/memstat/memstat-tutorial.txt.gz BUGS
memstat ignores all devices that just map main memory, though this may cause memstat to ignore some memory usage. Memory used by the kernel itself is not listed. AUTHOR
Originally written by Joshua Yelon <jyelon@uiuc.edu> and patched by Bernd Eckenfels <ecki@debian.org>. Taken over and rewritten by Michael Meskes <meskes@debian.org>. Debian 01 November 1998 MEMSTAT(1)
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