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/design/trunk/src/ch_memory_management.xml
5,21 → 5,35
<title>Memory management</title>
 
<section>
<!-- VM -->
 
<title>Virtual memory management</title>
 
<section>
<title>Address spaces</title>
 
<para></para>
<para />
</section>
 
<section>
<title>Virtual address translation</title>
 
<para></para>
<para />
</section>
 
<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>
</section>
 
<!-- End of VM -->
32,13 → 46,7
<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><!--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
97,123 → 105,119
</itemizedlist></para>
</formalpara>
</section>
</section>
 
<section id="buddy_allocator">
<title>Buddy allocator</title>
<section id="buddy_allocator">
<title>Buddy allocator</title>
 
<section>
<title>Overview</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>
<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" /-->
<!--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>
<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>
<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>
<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>
<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>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>
<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>
<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>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>
 
<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>
 
<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>
 
<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>
 
<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>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>
<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">
220,25 → 224,25
<title>Slab allocator</title>
 
<section>
<title>Introduction</title>
<title>Overview</title>
 
<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>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 a perfect solution. The basic idea behind the 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>
commonly used types of objects such threads, virtual memory structures
etc. Also due to the exact allocated size matching, slab allocation
completely eliminates internal fragmentation issue.</para>
</section>
 
<section>
<title>Implementation details (needs revision)</title>
<title>Implementation</title>
 
<para>The SLAB allocator is closely modelled after <ulink
url="http://www.usenix.org/events/usenix01/full_papers/bonwick/bonwick_html/">
258,56 → 262,106
</listitem>
 
<listitem>
- dynamic magazine growing (different magazine sizes are already supported, but we would need to adjust allocation strategy)
- dynamic magazine grow (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>
<section>
<title>Magazine layer</title>
 
<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>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>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>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>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>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>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>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>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>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>
 
<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>
<section>
<title>Allocation/deallocation</title>
 
<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>
<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>
 
<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>
<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>
 
<formalpara>
<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.
In this step object is allocated from the conventional slab layer
and pointer is returned.</para>
</formalpara>
 
<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
magazine.</para>
 
<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>
</section>
</section>