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/design/trunk/src/ch_memory_management.xml
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<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="buddy_allocator">buddy allocator</link>
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 (Mips, PPC) have only one zone, that covers whole physical
memory. Other architectures (IA32) have multiple zones.</para>
architectures (mips32, ppc32) have only one zone, that covers whole
physical memory. Other architectures (ia32) have multiple zones.</para>
</section>
 
<section id="buddy_allocator">
<title>Buddy allocator</title>
<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
buddy allocation system. This approach greatly reduces possibility of
outer memory fragmentation and helps in allocating bigger continious
blocks of physical memory aligned to their size. Problem of inner
memory fragmentation is being solved by <link linkend="slab">SLAB
allocation system.</link></para>
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" />
 
<para>Frames are grouped into bigger blocks and blocks of the size
<mathphrase>2<superscript>i</superscript></mathphrase> are stored in
the list indexed with <varname>i</varname> (so called order index). If
list contains 2 ajacent blocks (of a same size of cause) they can be
merged into the bigger one and moved into the list with higher order
index, thus making possible allocation of a bigger block.</para>
</section>
 
<section>
103,13 → 162,12
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>
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>