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    <para>Page tables. 4 level hierarchical and hash directly supported. B+

    Tree can be implemented.</para>

    <para>For paging there is an abstract layer</para>

    <para>TLB shootdown implementation (update TLB upon mapping

    update/remove). TLB shootdown ASID/ASID:PAGE/ALL. TLB shootdown requests

    can come in asynchroniously so there is a cache of TLB shootdown requests.

    Upon cache overflow TLB shootdown ALL is executed</para>

    <para>Address spaces. Address space area (B+ tree). Only for uspace. Set

    of syscalls (shrink/extend etc). Special address space area type - device

    - prohibits shrink/extend syscalls to call on it. Address space has link

    to mapping tables (hierarchical - per Address space, hash - global

    tables).</para>

        <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

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

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

        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

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

        and helps in allocating bigger continuous blocks of memory aligned to

        internal fragmentation of memory and is not suitable for general

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

        <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>

          <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>

      </section>

        <para><termdef><glossterm>Slab</glossterm> represents a contiguous

        piece of memory, usually made of several physically contiguous

        pages.</termdef> <termdef><glossterm>Slab cache</glossterm> consists

        of one or more slabs.</termdef></para>

          <para>Due to the extensive bottleneck on SMP architures, caused by

          global SLAB locking mechanism, making processing of all slab

          allocation requests serialized, a new layer was introduced to the

          classic slab allocator design. Slab allocator was extended to

          support per-CPU caches 'magazines' to achieve good SMP scaling.

          <termdef>Slab SMP perfromance bottleneck was resolved by introducing

          a per-CPU caching scheme called as <glossterm>magazine

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

          <para>Magazine is a N-element cache of objects, so each magazine can

          satisfy N allocations. Magazine behaves like a automatic weapon

          magazine (LIFO, stack), so the allocation/deallocation become simple

          push/pop pointer operation. Trick is that CPU does not access global

          slab allocator data during the allocation from its magazine, thus

          making possible parallel allocations between CPUs.</para>

          <para>Implementation also requires adding another feature as the

          CPU-bound magazine is actually a pair of magazines to avoid

          thrashing when during allocation/deallocatiion of 1 item at the

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

          fragmentation as much as possible.</para>

          <para>Another important entity of magazine layer is a full magazine

          depot, that stores full magazines which are used by any of the CPU

          magazine caches to reload active CPU magazine. Magazine depot can be

          pre-filled with full magazines during initialization, but in current

          implementation it is filled during object deallocation, when CPU

          magazine becomes full.</para>

          <para>Slab allocator control structures are allocated from special

          slabs, that are marked by special flag, indicating that it should

          not be used for slab magazine layer. This is done to avoid possible

          infinite recursions and deadlock during conventional slab allocaiton

          requests.</para>

        </section>

        <section>

          <title>Allocation/deallocation</title>

          <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 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>

            <title>Allocation</title>

            <para><emphasis>Step 1.</emphasis> When it comes to the allocation

            request, slab allocator first of all checks availability of memory

            in local CPU-bound magazine. If it is there, we would just "pop"

            the CPU magazine and return the pointer to object.</para>

            <para><emphasis>Step 2.</emphasis> If the CPU-bound magazine is

            empty, allocator will attempt to reload magazin, swapping it with

            second CPU magazine and returns to the first step.</para>

            <para><emphasis>Step 3.</emphasis> Now we are in the situation

            when both CPU-bound magazines are empty, which makes allocator to

            access shared full-magazines depot to reload CPU-bound magazines.

            If reload is succesful (meaning there are full magazines in depot)

            algoritm continues at Step 1.</para>

            <para><emphasis>Step 4.</emphasis> Final step of the allocation.

            and pointer is returned.</para>

          <formalpara>

            <title>Deallocation</title>

            <para><emphasis>Step 1.</emphasis> During deallocation request,

            slab allocator will check if the local CPU-bound magazine is not

            full. In this case we will just push the pointer to this

            <para><emphasis>Step 2.</emphasis> If the CPU-bound magazine is

            full, allocator will attempt to reload magazin, swapping it with

            second CPU magazine and returns to the first step.</para>

            <para><emphasis>Step 3.</emphasis> Now we are in the situation

            when both CPU-bound magazines are full, which makes allocator to

            access shared full-magazines depot to put one of the magazines to

            the depot and creating new empty magazine. Algoritm continues at

            Step 1.</para>

          </formalpara>

        </section>