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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" /> </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>