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<page xmlns="http://projectmallard.org/1.0/"
xmlns:ui="http://projectmallard.org/ui/1.0/"
type="topic" style="task"
id="memory-map-use">
<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>View the memory map of a process.</desc>
</info>
<title>Using memory maps</title>
<comment>
<cite date="2011-06-18" href="mailto:philbull@gmail.com">Phil Bull</cite>
<p>Explain how to use the Memory Map feature.</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. 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 swap space.</p>
<p>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>To display the <link xref="memory-map-what">memory map</link> of a
process:</p>
<steps>
<item><p>Click the <gui>Processes</gui> tab.</p></item>
<item><p>Right click the desired process in the <gui>process list</gui>.</p></item>
<item><p>Click <gui>Memory Maps</gui>.</p></item>
</steps>
<section id="read">
<title>Reading the memory map</title>
<list>
<item>
<p>Addresses are displayed in hexadecimal (base 16).</p>
</item>
<item>
<p>Sizes are displayed in <link xref="units">IEC binary
prefixes</link>.</p>
</item>
<item>
<p>At runtime the process can allocate more memory dynamically into an
area called the <em>heap</em>, and store arguments and variables into
another area called the <em>stack</em>.</p>
</item>
<item>
<p>The program itself and each of the shared libraries has three entries
each, one for the read-execute text segment, one for the read-write data
segment and one for a read-only data segment. Both data segments need to
be paged out at swap time.</p>
</item>
</list>
<table shade="rows" ui:expanded="false">
<title>Properties</title>
<tr>
<td><p>Filename</p></td>
<td><p>The location of a shared library that is currently used by the
process. If this field is blank, the memory information in this row
describes memory that is owned by the process whose name is displayed above
the memory-map table.</p></td>
</tr>
<tr>
<td><p>VM Start</p></td>
<td><p>The address at which the memory segment begins. VM Start, VM End and
VM Offset together specify the location on disk to which the shared library
is mapped.</p></td>
</tr>
<tr>
<td><p>VM End</p></td>
<td><p>The address at which the memory segment ends.</p></td>
</tr>
<tr>
<td><p>VM Size</p></td>
<td><p>The size of the memory segment.</p></td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>The following flags describe the different types of memory-segment
access that the process can have:</p>
<terms>
<item>
<title><gui>p</gui></title>
<p>The memory segment is private to the process, and is not accessible
to other processes.</p>
</item>
<item>
<title><gui>r</gui></title>
<p>The process has permission to read from the memory segment.</p>
</item>
<item>
<title><gui>s</gui></title>
<p>The memory segment is shared with other processes.</p>
</item>
<item>
<title><gui>w</gui></title>
<p>The process has permission to write into the memory segment.</p>
</item>
<item>
<title><gui>x</gui></title>
<p>The process has permission to execute instructions that are
contained within the memory segment.</p>
</item>
</terms>
</td>
</tr>
<tr>
<td><p>VM Offset</p></td>
<td><p>The location of the address within the memory segment,
measured from VM Start.</p></td>
</tr>
<tr>
<td><p>Private, Shared, Clean, Dirty</p></td>
<!-- <td><p>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.</p></td>
-->
<td><list><item><p><em>private</em> pages are accessed by one
process</p></item>
<item><p><em>shared</em> pages can be accessed by more than
one process</p></item>
<item><p><em>clean</em> pages have not yet been modified
while in memory and can be discarded when designated to be swapped
out</p></item>
<item><p><em>dirty</em> pages have been modified while in
memory and must be written to disk when designated to be swapped
out</p></item></list></td>
</tr>
<tr>
<td><p>Device</p></td>
<td><p>The major and minor numbers of the device on which the shared
library filename is located. Together these specify a partition on the
system.</p></td>
</tr>
<tr>
<td><p>Inode</p></td>
<td><p>The inode on the device from which the shared library location
is loaded into memory. An inode is the structure the filesystem uses to
store a file, and the number assigned to it is unique.</p></td>
</tr>
</table>
</section>
</page>
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