gnome-system-monitor/help/C/memory-map-use.page

<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>