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1 <?xml version="1.0" encoding="utf-8"?> |
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2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. --> |
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3 <!-- This component and the accompanying materials are made available under the terms of the License |
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4 "Eclipse Public License v1.0" which accompanies this distribution, |
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5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". --> |
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6 <!-- Initial Contributors: |
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7 Nokia Corporation - initial contribution. |
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8 Contributors: |
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9 --> |
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10 <!DOCTYPE concept |
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11 PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
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12 <concept id="GUID-7A50630B-2B44-5D27-AA18-3BEEE1453020" xml:lang="en"><title> The |
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13 Paging Algorithm Technology Guide</title><shortdesc>Describes the paging algorithm used in demand paging.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody> |
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14 <section id="GUID-0F07152A-5469-463B-8BEE-71170A920B19"><title>Purpose</title> <p>Kernel side developers will sometimes need |
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15 to know the algorithm used to move data in and out of paged memory. </p> <p><b>Intended Audience:</b> </p> <p>This document is intended to be read by kernel |
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16 side developers. </p> </section> |
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17 <section id="GUID-55C2239D-C351-4081-96F8-CAF1538B4BE3"><title>The algorithm</title> <p><b>Terminology</b> </p> <p>Paged |
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18 memory (virtual addresses and their contents) is called either: </p> <ul> |
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19 <li id="GUID-8533664E-057B-57EF-8A24-4BA7D3B4EC75"><p>'live', meaning present |
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20 in the cache of physical RAM, or </p> </li> |
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21 <li id="GUID-057D65C6-27D2-5BDD-B5CD-081808CCF44D"><p>'dead', meaning stored |
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22 elsewhere in the backing store. </p> </li> |
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23 </ul> <p>Physical RAM not being used to hold paged memory is called the free |
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24 pool. </p> <p>Items of paged memory can be reassigned from 'live' to 'dead' |
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25 and vice versa. 'Live' items are classed as either 'old' or 'young' for this |
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26 purpose. </p> <p><b>Paging |
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27 out memory</b> </p> <p>When more RAM is needed and the pool of free memory |
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28 is empty then RAM is freed up. This means changing an item of paged memory |
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29 from live to dead. The process is called paging out and it involves these |
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30 tasks. </p> <ol id="GUID-20C35CC3-0845-51A8-B3C0-0F542E0570DF"> |
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31 <li id="GUID-42AC538A-A676-5A7D-9DE3-FDE880E4A508"><p>The oldest virtual page |
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32 is removed from the cache and physically stored elsewhere </p> </li> |
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33 <li id="GUID-1FBD9975-97D5-50CA-8A2F-864B688C9AE4"><p>The MMU marks the virtual |
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34 page as inaccessible. </p> </li> |
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35 <li id="GUID-F8CDC9C1-C779-5D25-A16C-8BFC4B82B605"><p>The associated page |
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36 of RAM cache is returned to the free pool. </p> </li> |
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37 </ol> <p><b>Paging |
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38 in memory</b> </p> <p>When a program attempts to access dead paged memory, |
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39 the MMU generates a page fault and the executing thread is diverted to the |
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40 Symbian OS exception handler. This performs the following tasks. </p> <ol id="GUID-3C52A214-F7FC-55CF-9346-16A4D7E8E37A"> |
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41 <li id="GUID-B10392F3-D0CA-5C31-8A88-59E151661821"><p>Obtain a page of RAM |
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42 from the free pool. If the free pool is empty, free up some memory by paging |
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43 out the oldest live page. </p> </li> |
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44 <li id="GUID-EA4E0F16-68D2-564D-B46E-5CB4CB515855"><p>Read the contents of |
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45 the dead paged memory from its actual location (e.g. NAND flash) and write |
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46 them to the page of RAM. </p> </li> |
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47 <li id="GUID-5F9C3CEE-5836-5243-B84D-BA7795D0F056"><p>Update the live list |
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48 described in the next section. </p> </li> |
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49 <li id="GUID-A2A188F0-87EA-5403-8281-652AC52D41C3"><p>The MMU maps the linear |
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50 address of the dead paged memory on to the page of RAM. </p> </li> |
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51 <li id="GUID-F7A50A62-EBAD-50DD-B8F8-DAD42E598D02"><p>Resume program execution. </p> </li> |
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52 </ol> <p><b>The |
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53 paging cache</b> </p> <p>The paging cache is only useful if it is used to |
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54 hold the pages most likely to be required. The paging subsystem provides for |
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55 this by selecting the oldest pages to be paged out making space for new ones |
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56 to be paged in. </p> <p>All live pages are stored in a list called the 'live |
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57 page list'. Live pages are labelled 'young' or 'old' and are stored in two |
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58 sub-lists, one for each type: the item at the start of the young list is the |
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59 youngest item and the one at the end of the old list is the oldest item. The |
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60 MMU makes young pages accessible to programs and old pages inaccessible. However, |
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61 old pages are different from dead pages because their contents are still held |
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62 in RAM. </p> <p>The young and old lists are maintained in accordance with |
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63 these four rules. </p> <ul> |
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64 <li id="GUID-ACB20E61-3467-56B4-8697-6DD6C79203FA"><p>When a dead page is |
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65 paged in (made live) it is added to the start of the young list, becoming |
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66 the youngest item. </p> </li> |
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67 <li id="GUID-6B56324C-3A75-57C4-90E3-527994DAADFF"><p>When a live page must |
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68 be paged out (made dead) to free up RAM, the oldest page is selected. </p> </li> |
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69 <li id="GUID-EB5C43F4-1AC9-541F-8235-4DED9714B6F3"><p>If an old page is accessed |
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70 by a program a page fault results because old pages are marked inaccessible. |
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71 The paging system handles the fault by turning the page into a young page |
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72 ('rejuvenating' it). To compensate, the oldest young page is then turned into |
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73 an old page ('aging' it). </p> </li> |
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74 <li id="GUID-366367C8-BBF3-59ED-9122-38112C6EA6FE"><p>Efficient operation |
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75 requires a stable ratio of young to old pages. If the number of young pages |
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76 exceeds a specified ratio of old pages, the oldest young page is turned into |
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77 the youngest old page ('aging' it). </p> </li> |
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78 </ul> </section> |
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79 </conbody></concept> |