Subversion Repositories HelenOS-doc

<?xml version="1.0" encoding="UTF-8"?>

<chapter id="mm">

  <?dbhtml filename="mm.html"?>

  <title>Memory management</title>

  <section><!-- VM -->

    <title>Virtual memory management</title>

    <section>

      <title>Address spaces</title>

      <para></para>

    </section>

    <section>

      <title>Virtual address translation</title>

      <para></para>

    </section>

  </section><!-- End of VM -->

  <section><!-- Phys mem -->

    <title>Physical memory management</title>

    <section id="zones_and_frames">

      <title>Zones and frames</title>

    <para>        <graphic fileref="images/mm2.png" /> </para>

      <para>On some architectures not whole physical memory is available for conventional usage. This limitations

      require from kernel to maintain a table of available and unavailable ranges of physical memory addresses.

      Main idea of zones is in creating memory zone entity, that is a continuous chunk of memory available for allocation.

      If some chunk is not available, we simply do not put it in any zone.

      </para>

      <para>

      Zone is also serves for informational purposes, containing information about number of free and busy frames. Physical memory

      allocation is also done inside the certain zone. Allocation of zone frame must be organized by the

      <link linkend="frame_allocator">frame allocator</link> associated with the zone.

      </para>

      <para>Some of the architectures (mips32, ppc32) have only one zone, that covers whole

      physical memory, and the others (like ia32) may have multiple zones.  Information about zones on current machine is stored

      in BIOS hardware tables or can be hardcoded into kernel during compile time.</para>

    </section>

    <section id="frame_allocator">

      <title>Frame allocator</title>

    <formalpara>

    <title>Overview</title>

        <para>Frame allocator provides physical memory allocation for the kernel. Because of zonal organization of physical memory,

    frame allocator is always working in context of some zone, thus making impossible to allocate a piece of memory, which lays in different zone, which

    cannot happen, because two adjacent zones can be merged into one. Frame allocator is also being responsible to update information on

    the number of free/busy frames in zone.

    Physical memory allocation inside one <link

        linkend="zones_and_frames">memory zone</link> is being handled by an

        instance of <link linkend="buddy_allocator">buddy allocator</link>

        tailored to allocate blocks of physical memory frames.

    </para>

    </formalpara>

    <formalpara>

    <title>Allocation / deallocation</title>

    <para>

    Upon allocation request, frame allocator tries to find first zone, that can satisfy the incoming request (has required amount of free frames to allocate).

    During deallocation, frame allocator needs to find zone, that contain deallocated frame.

    This approach could bring up two potential problems:

    <itemizedlist>

        <listitem>

            Linear search of zones does not any good to performance, but number of zones is not expected to be high. And if yes, list of zones can be replaced with more time-efficient B-tree.

        </listitem>

        <listitem>

            Quickly find out if zone contains required number of frames to allocate and if this chunk of memory is properly aligned. This issue is perfectly solved bu the buddy allocator.

        </listitem>

    </itemizedlist>

    </para>

    </formalpara>

      </section>

    </section>

    <section id="buddy_allocator">

      <title>Buddy allocator</title>

      <section>

        <title>Overview</title>

        <para>In buddy allocator, memory is broken down into power-of-two

        sized naturally aligned blocks. These blocks are organized in an array

        of lists in which list with index i contains all unallocated blocks of

        the size <mathphrase>2<superscript>i</superscript></mathphrase>. The

        index i is called the order of block. Should there be two adjacent

        equally sized blocks in list <mathphrase>i</mathphrase> (i.e.

        buddies), the buddy allocator would coalesce them and put the

        resulting block in list <mathphrase>i + 1</mathphrase>, provided that

        the resulting block would be naturally aligned. Similarily, when the

        allocator is asked to allocate a block of size

        <mathphrase>2<superscript>i</superscript></mathphrase>, it first tries

        to satisfy the request from list with index i. If the request cannot

        be satisfied (i.e. the list i is empty), the buddy allocator will try

        to allocate and split larger block from list with index i + 1. Both of

        these algorithms are recursive. The recursion ends either when there

        are no blocks to coalesce in the former case or when there are no

        blocks that can be split in the latter case.</para>

        <graphic fileref="images/mm1.png" format="EPS" />

        <para>This approach greatly reduces external fragmentation of memory

        and helps in allocating bigger continuous blocks of memory aligned to

        their size. On the other hand, the buddy allocator suffers increased

        internal fragmentation of memory and is not suitable for general

        kernel allocations. This purpose is better addressed by the <link

        linkend="slab">slab allocator</link>.</para>

      </section>

      <section>

        <title>Implementation</title>

        <para>The buddy allocator is, in fact, an abstract framework wich can

        be easily specialized to serve one particular task. It knows nothing

        about the nature of memory it helps to allocate. In order to beat the

        lack of this knowledge, the buddy allocator exports an interface that

        each of its clients is required to implement. When supplied an

        implementation of this interface, the buddy allocator can use

        specialized external functions to find buddy for a block, split and

        coalesce blocks, manipulate block order and mark blocks busy or

        available. For precize documentation of this interface, refer to <link

        linkend="???">HelenOS Generic Kernel Reference Manual</link>.</para>

        <formalpara>

          <title>Data organization</title>

          <para>Each entity allocable by the buddy allocator is required to

          contain space for storing block order number and a link variable

          used to interconnect blocks within the same order.</para>

          <para>Whatever entities are allocated by the buddy allocator, the

          first entity within a block is used to represent the entire block.

          The first entity keeps the order of the whole block. Other entities

          within the block are assigned the magic value

          <constant>BUDDY_INNER_BLOCK</constant>. This is especially important

          for effective identification of buddies in one-dimensional array

          because the entity that represents a potential buddy cannot be

          associated with <constant>BUDDY_INNER_BLOCK</constant> (i.e. if it

          is associated with <constant>BUDDY_INNER_BLOCK</constant> then it is

          not a buddy).</para>

        </formalpara>

        <formalpara>

          <title>Data organization</title>

          <para>Buddy allocator always uses first frame to represent frame

          block. This frame contains <varname>buddy_order</varname> variable

          to provide information about the block size it actually represents (

          <mathphrase>2<superscript>buddy_order</superscript></mathphrase>

          frames block). Other frames in block have this value set to magic

          <constant>BUDDY_INNER_BLOCK</constant> that is much greater than

          buddy <varname>max_order</varname> value.</para>

          <para>Each <varname>frame_t</varname> also contains pointer member

          to hold frame structure in the linked list inside one order.</para>

        </formalpara>

        <formalpara>

          <title>Allocation algorithm</title>

          <para>Upon <mathphrase>2<superscript>i</superscript></mathphrase>

          frames block allocation request, allocator checks if there are any

          blocks available at the order list <varname>i</varname>. If yes,

          removes block from order list and returns its address. If no,

          recursively allocates

          <mathphrase>2<superscript>i+1</superscript></mathphrase> frame

          block, splits it into two

          <mathphrase>2<superscript>i</superscript></mathphrase> frame blocks.

          Then adds one of the blocks to the <varname>i</varname> order list

          and returns address of another.</para>

        </formalpara>

        <formalpara>

          <title>Deallocation algorithm</title>

          <para>Check if block has so called buddy (another free

          <mathphrase>2<superscript>i</superscript></mathphrase> frame block

          that can be linked with freed block into the

          <mathphrase>2<superscript>i+1</superscript></mathphrase> block).

          Technically, buddy is a odd/even block for even/odd block

          respectively. Plus we can put an extra requirement, that resulting

          block must be aligned to its size. This requirement guarantees

          natural block alignment for the blocks coming out the allocation

          system.</para>

          <para>Using direct pointer arithmetics,

          <varname>frame_t::ref_count</varname> and

          <varname>frame_t::buddy_order</varname> variables, finding buddy is

          done at constant time.</para>

        </formalpara>

      </section>

    <section id="slab">

      <title>Slab allocator</title>

      <para>Kernel memory allocation is handled by slab.</para>

    </section><!-- End of Physmem -->

  </section>

    <section>

      <title>Memory sharing</title>

      <para>Not implemented yet(?)</para>

    </section>

</chapter>