Subversion Repositories HelenOS-doc

<?xml version="1.0" encoding="UTF-8"?>
<chapter id="mm">
  <?dbhtml filename="mm.html"?>

  <title>Memory management</title>

  <section>
    <title>Virtual memory management</title>

    <section>
      <title>Address spaces</title>

      <para></para>
    </section>

    <section>
      <title>Virtual address translation</title>

      <para></para>
    </section>
  </section>

  <section>
    <title>Physical memory management</title>

    <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/buddy_alloc.eps" 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>
      </section>
    </section>

    <section id="zones_and_frames">
      <title>Zones and frames</title>

      <para>Physical memory is divided into zones. Each zone represents
      continuous area of physical memory frames. Allocation of frames is
      handled by the <link linkend="frame_allocator">frame allocator</link>
      associated with the zone. Zone also contains information about free and
      occupied frames and its base addresss in the memory. Some of the
      architectures (mips32, ppc32) have only one zone, that covers whole
      physical memory. Other architectures (ia32) have multiple zones.</para>
    </section>

    <section id="frame_allocator">
      <title>Frame allocator</title>

      <section>
        <title>Overview</title>

        <para>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>

        <graphic fileref="images/mm1.png" />
      </section>

      <section>
        <title>Implementation</title>

        <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>

    <section id="slab">
      <title>Slab allocator</title>

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

    <section>
      <title>Memory sharing</title>

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