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<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" /-->

      <!--graphic fileref="images/buddy_alloc.svg" format="SVG" /-->

      </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>

      <section>

        <title>Introduction</title>

        <para>The majority of memory allocation requests in the kernel are for

        small, frequently used data structures. For this purpose the slab

        allocator is a perfect solution. The basic idea behind a slab

        allocator is to have lists of commonly used objects available packed

        into pages. This avoids the overhead of allocating and destroying

        commonly used types of objects such as inodes, threads, virtual memory

        structures etc.</para>

        <para>Original slab allocator locking mechanism has become a

        significant preformance bottleneck on SMP architectures. <termdef>Slab

        SMP perfromance bottleneck was resolved by introducing a per-CPU

        caching scheme called as <glossterm>magazine

        layer</glossterm></termdef>.</para>

      </section>

      <section>

        <title>Implementation details (needs revision)</title>

        <para>The SLAB allocator is closely modelled after <ulink

        url="http://www.usenix.org/events/usenix01/full_papers/bonwick/bonwick_html/">

        OpenSolaris SLAB allocator by Jeff Bonwick and Jonathan Adams </ulink>

        with the following exceptions: <itemizedlist>

            <listitem>

               empty SLABS are deallocated immediately (in Linux they are kept in linked list, in Solaris ???)

            </listitem>

            <listitem>

               empty magazines are deallocated when not needed (in Solaris they are held in linked list in slab cache)

            </listitem>

          </itemizedlist> Following features are not currently supported but

        would be easy to do: <itemizedlist>

            <listitem>

               - cache coloring

            </listitem>

            <listitem>

               - dynamic magazine growing (different magazine sizes are already supported, but we would need to adjust allocation strategy)

            </listitem>

          </itemizedlist></para>

        <para>The SLAB allocator supports per-CPU caches ('magazines') to

        facilitate good SMP scaling.</para>

        <para>When a new object is being allocated, it is first checked, if it

        is available in CPU-bound magazine. If it is not found there, it is

        allocated from CPU-shared SLAB - if partial full is found, it is used,

        otherwise a new one is allocated.</para>

        <para>When an object is being deallocated, it is put to CPU-bound

        magazine. If there is no such magazine, new one is allocated (if it

        fails, the object is deallocated into SLAB). If the magazine is full,

        it is put into cpu-shared list of magazines and new one is

        allocated.</para>

        <para>The CPU-bound magazine is actually a pair of magazines to avoid

        thrashing when somebody is allocating/deallocating 1 item at the

        magazine size boundary. LIFO order is enforced, which should avoid

        fragmentation as much as possible.</para>

        <para>Every cache contains list of full slabs and list of partialy

        full slabs. Empty SLABS are immediately freed (thrashing will be

        avoided because of magazines).</para>

        <para>The SLAB information structure is kept inside the data area, if

        possible. The cache can be marked that it should not use magazines.

        This is used only for SLAB related caches to avoid deadlocks and

        infinite recursion (the SLAB allocator uses itself for allocating all

        it's control structures).</para>

        <para>The SLAB allocator allocates lots of space and does not free it.

        When frame allocator fails to allocate the frame, it calls

        slab_reclaim(). It tries 'light reclaim' first, then brutal reclaim.

        The light reclaim releases slabs from cpu-shared magazine-list, until

        at least 1 slab is deallocated in each cache (this algorithm should

        probably change). The brutal reclaim removes all cached objects, even

        from CPU-bound magazines.</para>

        <para>TODO: <itemizedlist>

            <listitem>

               For better CPU-scaling the magazine allocation strategy should be extended. Currently, if the cache does not have magazine, it asks for non-cpu cached magazine cache to provide one. It might be feasible to add cpu-cached magazine cache (which would allocate it's magazines from non-cpu-cached mag. cache). This would provide a nice per-cpu buffer. The other possibility is to use the per-cache 'empty-magazine-list', which decreases competing for 1 per-system magazine cache.

            </listitem>

            <listitem>

               - it might be good to add granularity of locks even to slab level, we could then try_spinlock over all partial slabs and thus improve scalability even on slab level

            </listitem>

          </itemizedlist></para>

      </section>

    </section>

    <!-- End of Physmem -->

  </section>

  <section>

    <title>Memory sharing</title>

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

  </section>

</chapter>