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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>
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