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Code Editor : memory-map-what.page

<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>
 Difficult conceptual topic. Explain how to use memory maps to do
 whatever the hell you're supposed to use them for.
 </comment>

<comment>
 <cite date="2014-01-28" href="mailto:kittykat3756@gmail.com">Kat</cite>
 TODO: check gui tags.
 </comment>

<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 more than the
 existing physical memory by <gui>mapping</gui> locations in physical memory
 to files on disk.

When a program is launched, the system assigns it a unique process ID
 (PID) and its instructions and data are loaded into memory
 in pages. 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.

If multiple copies of a program are running, the instructions (or
 text) 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.

When the system needs more pages of memory than are available, some of the
 existing pages will be paged out 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 dirty, 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.

</page>

${this.title}

Code Editor : memory-map-what.page