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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-592B6D20-4ABA-5C79-9734-D18E2CE4A86C" xml:lang="en"><title>Writable

    13 Data Paging Overview</title><shortdesc>Overview of data paging, the next phase in Symbian's implementation

    14 of demand paging. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    15 <section id="GUID-B1D8D3AD-4438-4C51-B900-1DD2B55202BC"><title>Purpose</title> <p>Writable

    16 data paging allows the demand paging of any user-side data (for example thread

    17 stacks and heaps). Writable data paging makes use of a fixed size backing

    18 store to page out data. The backing store may use a NAND flash partition or

    19 an embedded MMC (eMMC). </p> <p>The aim of data paging is to enable more memory-hungry

    20 use cases without increasing the amount of physical RAM present in the device. </p> <p>For

    21 example, data paging enables: </p> <ul>

    22 <li id="GUID-F3B36258-F661-5562-A641-1875D33600B7"><p>running applications

    23 that are not designed with the memory constraints of embedded devices in mind,

    24 for example applications ported from other environments with writable data

    25 paging (such as a PC). </p> </li>

    26 <li id="GUID-7BB35E5C-6E38-50E6-B27E-094948F0F6A8"><p>running multiple applications

    27 at the same time where previously there would not have been enough memory. </p> </li>

    28 </ul> <p> <note> Data paging offers no performance increase, unlike code paging

    29 which can speed up application loading by reading only the necessary pages

    30 into RAM. </note></p> <p> </p> <p>However, writable data paging does have

    31 the following limitations: </p> <ul>

    32 <li id="GUID-EB2514D7-CE57-52F2-91D5-E790A41D12B2"><p>It is only possible

    33 to page the following types of memory: </p> <ul>

    34 <li id="GUID-9C9002DF-BF9B-5656-A57F-84C442E855B7"><p>user heaps, </p> </li>

    35 <li id="GUID-8F6AC93B-F1BE-5CA6-899E-8433E2055013"><p>user thread stacks, </p> </li>

    36 <li id="GUID-0EC7E799-24CC-590E-A67D-DAA014825A1E"><p>private chunks, </p> </li>

    37 <li id="GUID-9789A61F-CB01-5AD8-8886-41CA74EA564F"><p>global chunks, </p> </li>

    38 <li id="GUID-1C8479BE-B4E6-5E6D-83C0-D23E8DC6F9CF"><p>static data. </p> </li>

    39 </ul> </li>

    40 <li id="GUID-E11A66FB-C2DF-5DE7-B074-0CBDFC7EBE6D"><p>A single memory object

    41 (e.g. a heap, stack, DLL or chunk) is the smallest granularity at which paging

    42 can be configured </p> </li>

    43 <li id="GUID-06F9012E-85CE-587C-B0E9-6BF7EE37A39F"><p>No attempt will be made

    44 to page kernel-side data </p> </li>

    45 <li id="GUID-88EE2287-EF50-5B7C-B63E-C2D172A1EA80"><p>No attempt will be made

    46 to implement memory mapped files (e.g. posix's mmap) </p> </li>

    47 <li id="GUID-C79316F5-BFF4-569E-8D44-AF5BDCBD0811"><p>No attempt will be made

    48 to implement any kind of paging on the emulator. </p> </li>

    49 </ul> </section>

    50 <section id="GUID-9E27B27E-898B-43DA-8614-4AAEEE0A294E"><title>Required background</title> <p>This

    51 document assumes that the reader is familiar with the concept of <xref href="GUID-F123D574-44DE-528A-806C-DB64229BCEA2.dita">Demand

    52 Paging</xref>, and the existing implementation of ROM and code paging in Symbian

    53 platform. </p> </section>

    54 <section id="GUID-1028A9E3-C9D7-4A4B-9DD4-B719475D09E5"><title>Key concepts

    55 and terms</title> <dl>

    56 <dlentry>

    57 <dt>Page</dt>

    58 <dd><p>Memory is managed in fixed size units called pages. The size of a page

    59 is usually determined by the hardware, and for ARM architectures this is 4K. </p> </dd>

    60 </dlentry>

    61 <dlentry>

    62 <dt>Paged in or Paged out</dt>

    63 <dd><p>If a given page of memory is present in RAM it is said to be <i>paged

    64 in</i> (or just 'present'), as opposed to <i>paged out</i>. </p> </dd>

    65 </dlentry>

    66 <dlentry>

    67 <dt>Paging in</dt>

    68 <dd><p>The process of moving a page of memory from being paged out to being

    69 paged in. </p> </dd>

    70 </dlentry>

    71 <dlentry>

    72 <dt>Paging out</dt>

    73 <dd><p>The process of moving a page of memory from being paged in to being

    74 paged out. </p> </dd>

    75 </dlentry>

    76 <dlentry>

    77 <dt>backing store</dt>

    78 <dd><p>The external media used to hold <i>paged out</i> pages is referred

    79 to as the <i>swap area</i>. This area may be much larger than physical RAM,

    80 although a factor of around twice as large is considered normal in existing

    81 systems. </p> </dd>

    82 </dlentry>

    83 <dlentry>

    84 <dt>Working set</dt>

    85 <dd><p>The term <i>working set</i> is defined as the memory accessed during

    86 a given time period. </p> </dd>

    87 </dlentry>

    88 <dlentry>

    89 <dt>Pinning</dt>

    90 <dd><p> <i>Pinning</i> pages refers to paging in demand-paged memory and forcing

    91 it to remain in RAM until it is <i>unpinned</i> at a later time. </p> </dd>

    92 </dlentry>

    93 </dl> </section>

    94 <section id="GUID-AED67EBB-6176-4357-BC22-4A0B4E518265"><title>Classes</title> <p>The

    95 following classes are affected by the use of writable data demand paging: </p> <p>The

    96 following classes are involved in the use of threads and processes: </p> <table id="GUID-24779260-DE34-5184-9A58-560F06F0DE9D">

    97 <tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>

    98 <tbody>

    99 <row>

   100 <entry><p>Class </p> </entry>

   101 <entry><p>Description </p> </entry>

   102 </row>

   103 <row>

   104 <entry><p> <xref href="GUID-B0E661BC-4058-3256-B9C3-5A4FD52F6DE5.dita"><apiname>RThread</apiname></xref>  </p> </entry>

   105 <entry><p>This class is used to handle threads. </p> </entry>

   106 </row>

   107 <row>

   108 <entry><p> <xref href="GUID-9DD1EA2B-DC59-315C-8E9C-CE6D9461B695.dita"><apiname>RProcess</apiname></xref>  </p> </entry>

   109 <entry><p>This class is used to handle processes. </p> </entry>

   110 </row>

   111 <row>

   112 <entry><p> <xref href="GUID-1D0D14AD-43CF-3B52-AD8D-641F75B8098C.dita"><apiname>TThreadCreateInfo</apiname></xref>  </p> </entry>

   113 <entry><p>Used to specify the attributes of a new thread. </p> </entry>

   114 </row>

   115 </tbody>

   116 </tgroup>

   117 </table> <p>These classes are used in handling memory: </p> <table id="GUID-ACCA6C7E-92B5-51BE-B86E-D94E9FB0FF9F">

   118 <tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>

   119 <tbody>

   120 <row>

   121 <entry><p>Class </p> </entry>

   122 <entry><p>Description </p> </entry>

   123 </row>

   124 <row>

   125 <entry><p> <xref href="GUID-326A2F4D-0E99-31C0-A35D-E8BF45913F07.dita"><apiname>RChunk</apiname></xref>  </p> </entry>

   126 <entry><p>Handle a chunk. </p> </entry>

   127 </row>

   128 <row>

   129 <entry><p> <xref href="GUID-51F7DBCF-BFB6-31F9-8882-5D263A1AD4B4.dita"><apiname>TChunkCreateInfo</apiname></xref>  </p> </entry>

   130 <entry><p>A structure that specifies the type and properties of the chunk

   131 that is to be created. </p> </entry>

   132 </row>

   133 <row>

   134 <entry><p> <xref href="GUID-3DCB92FB-9C74-3B73-B229-BF7944087EE9.dita"><apiname>TChunkHeapCreateInfo</apiname></xref>  </p> </entry>

   135 <entry><p>A structure that specifies the type and properties of a chunk with

   136 a heap within it. </p> </entry>

   137 </row>

   138 <row>

   139 <entry><p> <xref href="GUID-C5D0C7E7-061F-3BA5-AE24-B83237684B01.dita"><apiname>UserHeap</apiname></xref>  </p> </entry>

   140 <entry><p>A set of functions that are used to create heaps. </p> </entry>

   141 </row>

   142 </tbody>

   143 </tgroup>

   144 </table> <p>The following classes are used in client/server communication: </p> <table id="GUID-E823BA5C-620C-5B58-9945-8135EF7D39D1">

   145 <tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>

   146 <tbody>

   147 <row>

   148 <entry><p>Class </p> </entry>

   149 <entry><p>Description </p> </entry>

   150 </row>

   151 <row>

   152 <entry><p> <xref href="GUID-8E316AC4-4676-301A-9A23-659E83AA1D1C.dita"><apiname>CServer2</apiname></xref>  </p> </entry>

   153 <entry><p>The abstract base class for servers. </p> </entry>

   154 </row>

   155 <row>

   156 <entry><p> <xref href="GUID-4AD02F14-1142-372F-9D11-224595932034.dita"><apiname>TIpcArgs</apiname></xref>  </p> </entry>

   157 <entry><p>A client/server class that clients use to package arguments that

   158 are to be sent to a sever. </p> </entry>

   159 </row>

   160 </tbody>

   161 </tgroup>

   162 </table> </section>

   163 </conbody><related-links>

   164 <link href="GUID-9D66A168-AF11-51D9-903C-4A6C6D31DA3D.dita"><linktext>Flexible

   165 Memory Model Overview</linktext></link>

   166 </related-links></concept>