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1 ====================================
2 Memory Allocation Algorithm Theories
3 ====================================
4
5GOAL
6 It is intended to be a 100% working memory allocation system. It should be
7 capable of replacing an ordinary Operating System's own routines. It should
8 work good in a multitasking, shared memory, non-virtual memory environment
9 without clogging the memory. Primary aimed for small machines, CPUs and
10 memory amounts.
11
12 I use a best-fit algorithm with a slight overhead in order to increase speed
13 a lot. It should remain scalable and work good with very large amount of
14 memory and free/used memory blocks too.
15
16TERMINOLOGY
17
18 FRAGMENT - small identically sized parts of a larger BLOCK, they are _not_
19 allocated when traversed in lists etc
20 BLOCK - large memory area, if used for FRAGMENTS, they are linked in a
21 lists. One list for each FRAGMENT size supported.
22 TOP - head struct that holds information about and points to a chain
23 of BLOCKS for a particular FRAGMENT size.
24 CHUNK - a contiguous area of free memory
25
26MEMORY SYSTEM
27
28 We split the system in two parts. One part allocates small memory amounts
29 and one part allocates large memory amounts, but all allocations are done
30 "through" the small-part-system. There is an option to use only the small
31 system (and thus use the OS for large blocks) or the complete package.
32
33##############################################################################
34 SMALL SIZE ALLOCATIONS
35##############################################################################
36
37 Keywords for this system is 'Deferred Coalescing' and 'quick lists'.
38
39 ALLOC
40
41 * Small allocations are "aligned" upwards to a set of preset sizes. In the
42 current implementation I use 20, 28, 52, 116, 312, 580, 1016, 2032 bytes.
43 Memory allocations of these sizes are referred to as FRAGMENTS.
44 (The reason for these specific sizes is the requirement that they must be
45 32-bit aligned and fit as good as possible within 4064 bytes.)
46
47 * Allocations larger than 2032 will get a BLOCK for that allocation only.
48
49 * Each of these sizes has it's own TOP. When a FRAGMENT is requested, a
50 larger BLOCK will be allocated and divided into many FRAGMENTS (all of the
51 same size). TOP points to a list with BLOCKS that contains FRAGMENTS of
52 the same size. Each BLOCK has a 'number of free FRAGMENTS' counter and so
53 has each TOP (for the entire chain).
54
55 * A BLOCK is around 4064 bytes plus the size of the information header. This
56 size is adjusted to make the allocation of the big block not require more
57 than 4096 bytes. (This might not be so easy to be sure of, if you don't
58 know how the big-block system works, but the BMALLOC system uses an
59 extra header of 12 bytes and the header for the FRAGMENT BLOCK is 20 bytes
60 in a general 32-bit environment.)
61
62 * In case the allocation of a BLOCK fails when a FRAGMENT is required, the
63 next size of FRAGMENTS will be checked for a free FRAGMENT. First when the
64 larger size lists have been tested without success it will fail for real.
65
66 FREE
67
68 * When FRAGMENTS are freed so that a BLOCK becomes non-used, it is returned
69 to the system.
70
71 * FREEing a fragment adds the buffer in a LIFO-order. That means that the
72 next request for a fragment from the same list, the last freed buffer will
73 be returned first.
74
75 REALLOC
76
77 * REALLOCATION of a FRAGMENT does first check if the new size would fit
78 within the same FRAGMENT and if it would use the same FRAGMENT size. If it
79 does and would, the same pointer is returned.
80
81 OVERHEAD
82
83 Yes, there is an overhead on small allocations (internal fragmentation).
84 Yet, I do believe that small allocations more often than larger ones are
85 used dynamically. I believe that a large overhead is not a big problem if it
86 remains only for a while. The big gain is with the extreme speed we can GET
87 and RETURN small allocations. This has yet to be proven. I am open to other
88 systems of dealing with the small ones, but I don`t believe in using the
89 same system for all sizes of allocations.
90
91 IMPROVEMENT
92
93 An addition to the above described algorithm is the `save-empty-BLOCKS-a-
94 while-afterwards`. It will be used when the last used FRAGMENT within a
95 BLOCK is freed. The BLOCK will then not get returned to the system until "a
96 few more" FRAGMENTS have been freed in case the last [few] freed FRAGMENTS
97 are allocated yet again (and thus prevent the huge overhead of making
98 FRAGMENTS in a BLOCK). The "only" drawback of such a SEBAWA concept is
99 that it would mean an even bigger overhead...
100
101 HEADERS (in allocated data)
102
103 FRAGMENTS - 32-bit pointer to its parent BLOCK (lowest bit must be 0)
104 BLOCK - 32-bit size (lowest bit must be 1 to separate this from
105 FRAGMENTS)
106
107##############################################################################
108 LARGER ALLOCATIONS
109##############################################################################
110
111 If the requested size is larger than the largest FRAGMENT size supported,
112 the allocation will be made for this memory area alone, or if a BLOCK is
113 allocated to fit lots of FRAGMENTS a large block is also desired.
114
115 * We add memory to the "system" with the add_pool() function call. It
116 specifies the start and size of the new block of memory that will be
117 used in this memory allocation system. Several add_pool() calls are
118 supported and they may or may not add contiguous memory.
119
120 * Make all blocks get allocated aligned to BLOCKSIZE (sometimes referred to
121 as 'grain size'), 64 bytes in my implementation. Reports tell us there is
122 no real gain in increasing the size of the align.
123
124 * We link *all* pieces of memory (AREAS), free or not free. We keep the list
125 in address order and thus when a FREE() occurs we know instantly if there
126 are FREE CHUNKS wall-to-wall. No list "travels" needed. Requires some
127 extra space in every allocated BLOCK. Still needs to put the new CHUNK in
128 the right place in size-sorted list/tree. All memory areas, allocated or
129 not, contain the following header:
130 - size of this memory area (31 bits)
131 - FREE status (1 bit)
132 - pointer to the next AREA closest in memory (32 bits)
133 - pointer to the prev AREA closest in memory (32 bits)
134 (Totally 12 bytes)
135
136 * Sort all FREE CHUNKS in size-order. We use a SPLAY TREE algorithm for
137 maximum speed. Data/structs used for the size-sorting functions are kept
138 in an abstraction layer away from this since it is really not changing
139 anything (except executing speed).
140
141 ALLOC (RSIZE - requested size, aligned properly)
142
143 * Fetch a CHUNK that RSIZE fits within. If the found CHUNK is larger than
144 RSIZE, split it and return the RSIZE to the caller. Link the new CHUNK
145 into the list/tree.
146
147 FREE (AREA - piece of memory that is returned to the system)
148
149 * Since the allocated BLOCK has kept its link-pointers, we can without
150 checking any list instantly see if there are any FREE CHUNKS that are
151 wall-to-wall with the AREA (both sides). If the AREA *is* wall-to-wall
152 with one or two CHUNKS that or they are unlinked from the lists, enlarged
153 and re-linked into the lists.
154
155 REALLOC
156
157 * There IS NO realloc() of large blocks, they are performed in the previous
158 layer (dmalloc).
159
160
161##############################################################################
162 FURTHER READING
163##############################################################################
164
165 * "Dynamic Storage Allocation: A Survey and Critical Review" (Paul R. Wilson,
166 Mark S. Johnstone, Michael Neely, David Boles)
167 ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps
168
169 * "A Memory Allocator" (Doug Lea)
170 http://g.oswego.edu/dl/html/malloc.html