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+<page xmlns="http://projectmallard.org/1.0/"
+ type="topic" style="task"
+ id="memory-map-what">
+
+ <info>
+ <revision pkgversion="3.11" date="2014-01-28" status="candidate"/>
+ <link type="guide" xref="index#memory" group="memory" />
+
+ <include href="legal.xml" xmlns="http://www.w3.org/2001/XInclude"/>
+
+ <credit type="author copyright">
+ <name>Phil Bull</name>
+ <email>philbull@gmail.com</email>
+ <years>2011</years>
+ </credit>
+
+ <credit type="author copyright">
+ <name>Michael Hill</name>
+ <email>mdhillca@gmail.com</email>
+ <years>2011, 2014</years>
+ </credit>
+
+ <desc>The memory map of a process can help diagnose certain memory
+ issues.</desc>
+ </info>
+
+ <title>What is a memory map?</title>
+
+ <comment>
+ <cite date="2011-06-18" href="mailto:philbull@gmail.com">Phil Bull</cite>
+ <p>Difficult conceptual topic. Explain how to use memory maps to do
+ whatever the hell you're supposed to use them for.</p>
+ </comment>
+
+ <comment>
+ <cite date="2014-01-28" href="mailto:kittykat3756@gmail.com">Kat</cite>
+ <p>TODO: check gui tags.</p>
+ </comment>
+
+ <p><gui>Virtual memory</gui> is a representation of the combined
+ <gui>physical memory</gui> and <link xref="mem-swap">swap space</link> in a
+ system. It enables running processes to access <em>more</em> than the
+ existing physical memory by <gui>mapping</gui> locations in physical memory
+ to files on disk.</p>
+
+ <p>When a program is launched, the system assigns it a unique process ID
+ (PID) and its <em>instructions</em> and <em>data</em> are loaded into memory
+ in <em>pages</em>. The page table of each process maps the correspondence
+ between its pages in virtual memory and their location in physical memory.
+ The <gui>memory map</gui> displays the total virtual memory use of the
+ process, and can be used to determine the memory cost of running a single or
+ multiple instances of the program, to ensure the use of the correct shared
+ libraries, to see the results of adjusting various performance tuning
+ parameters the program may have, or to diagnose issues such as memory
+ leaks.</p>
+
+ <p>If multiple copies of a program are running, the instructions (or
+ <em>text</em>) of the program only need to be loaded once into physical
+ memory. Each time a new instance of the program is launched (as a unique
+ process with its own virtual memory pages), its page table maps the location
+ of the text in virtual memory to those instructions in the original real
+ memory location. In addition, any dynamic shared libraries used by the
+ process appear as distinct in virtual memory, but are shared references to a
+ single copy of the library's text segment in real memory. When accounting for
+ the total memory use of a process, it is important to note that the cost of
+ shared libraries loaded into memory can be spread across all processes
+ currently using them.</p>
+
+ <p>When the system needs more pages of memory than are available, some of the
+ existing pages will be <em>paged out</em> or written to the <gui>swap
+ space</gui>. Text pages are flagged read-execute in memory and don't need to
+ be written to swap since they can be re-loaded from their original location
+ on disk. Data pages have read-write permissions, and if modified when in
+ memory, they are labeled <em>dirty</em>, and when designated for swapping,
+ must be paged out. When a page in swap space is required by a running
+ process, it needs to be swapped back in before use, perhaps causing another
+ page to be paged out.</p>
+
+</page>