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authorPeter D'Hoye <peter.dhoye@gmail.com>2009-05-24 21:28:16 +0000
committerPeter D'Hoye <peter.dhoye@gmail.com>2009-05-24 21:28:16 +0000
commit526b5580dabbfed7cfe5439dc3a90ec727f563c2 (patch)
tree22b1af92348785daad16714ee5e2b633017e0e48 /apps/plugins/pdbox/dbestfit-3.3/thoughts
parent4f2dfcc01b260d946044ef2b6af5fe36cb772c8d (diff)
downloadrockbox-526b5580dabbfed7cfe5439dc3a90ec727f563c2.tar.gz
rockbox-526b5580dabbfed7cfe5439dc3a90ec727f563c2.zip
Cut the files in half and it might work better (note to self: check your tree is really clean before patching)
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@21070 a1c6a512-1295-4272-9138-f99709370657
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@@ -168,173 +168,3 @@ MEMORY SYSTEM
168 168
169 * "A Memory Allocator" (Doug Lea) 169 * "A Memory Allocator" (Doug Lea)
170 http://g.oswego.edu/dl/html/malloc.html 170 http://g.oswego.edu/dl/html/malloc.html
171 =====================================
172 Memory Allocation Algorithm Theories.
173 =====================================
174
175GOAL
176 It is intended to be a 100% working memory allocation system. It should be
177 capable of replacing an ordinary Operating System's own routines. It should
178 work good in a multitasking, shared memory, non-virtual memory environment
179 without clogging the memory. Primary aimed for small machines, CPUs and
180 memory amounts.
181
182 I use a best-fit algorithm with a slight overhead in order to increase speed
183 a lot. It should remain scalable and work good with very large amount of
184 memory and free/used memory blocks too.
185
186TERMINOLOGY
187
188 FRAGMENT - small identically sized parts of a larger BLOCK, they are when
189 travered in lists etc _not_ allocated.
190 BLOCK - large memory area, if used for FRAGMENTS, they are linked in a
191 lists. One list for each FRAGMENT size supported.
192 TOP - head struct that holds information about and points to a chain
193 of BLOCKS for a particular FRAGMENT size.
194 CHUNK - a contiguous area of free memory
195
196MEMORY SYSTEM
197
198 We split the system in two parts. One part allocates small memory amounts
199 and one part allocates large memory amounts, but all allocations are done
200 "through" the small-part-system. There is an option to use only the small
201 system (and thus use the OS for large blocks) or the complete package.
202
203##############################################################################
204 SMALL SIZE ALLOCATIONS
205##############################################################################
206
207 Keywords for this system is 'Deferred Coalescing' and 'quick lists'.
208
209 ALLOC
210
211 * Small allocations are "aligned" upwards to a set of preset sizes. In the
212 current implementation I use 20, 28, 52, 116, 312, 580, 812, 2028 bytes.
213 Memory allocations of these sizes are refered to as FRAGMENTS.
214 (The reason for these specific sizes is the requirement that they must be
215 32-bit aligned and fit as good as possible within 4060 bytes.)
216
217 * Allocations larger than 2028 will get a BLOCK for that allocation only.
218
219 * Each of these sizes has it's own TOP. When a FRAGMENT is requested, a
220 larger BLOCK will be allocated and divided into many FRAGMENTS (all of the
221 same size). TOP points to a list with BLOCKS that contains FRAGMENTS of
222 the same size. Each BLOCK has a 'number of free FRAGMENTS' counter and so
223 has each TOP (for the entire chain).
224
225 * A BLOCK is around 4060 bytes plus the size of the information header. This
226 size is adjusted to make the allocation of the big block not require more
227 than 4096 bytes. (This might not be so easy to be sure of, if you don't
228 know how the big-block system works, but the BMALLOC system uses an
229 extra header of 12 bytes and the header for the FRAGMENT BLOCK is 20 bytes
230 in a general 32-bit unix environment.)
231
232 * In case the allocation of a BLOCK fails when a FRAGMENT is required, the
233 next size of FRAGMENTS will be checked for a free FRAGMENT. First when the
234 larger size lists have been tested without success it will fail for real.
235
236 FREE
237
238 * When FRAGMENTS are freed so that a BLOCK becomes non-used, it is returned
239 to the system.
240
241 * FREEing a fragment adds the buffer in a LIFO-order. That means that the
242 next request for a fragment from the same list, the last freed buffer will
243 be returned first.
244
245 REALLOC
246
247 * REALLOCATION of a FRAGMENT does first check if the new size would fit
248 within the same FRAGMENT and if it would use the same FRAGMENT size. If it
249 does and would, the same pointer is returned.
250
251 OVERHEAD
252
253 Yes, there is an overhead on small allocations (internal fragmentation).
254 Yet, I do believe that small allocations more often than larger ones are
255 used dynamically. I believe that a large overhead is not a big problem if it
256 remains only for a while. The big gain is with the extreme speed we can GET
257 and RETURN small allocations. This has yet to be proven. I am open to other
258 systems of dealing with the small ones, but I don`t believe in using the
259 same system for all sizes of allocations.
260
261 IMPROVEMENT
262
263 An addition to the above described algorithm is the `save-empty-BLOCKS-a-
264 while-afterwards`. It will be used when the last used FRAGMENT within a
265 BLOCK is freed. The BLOCK will then not get returned to the system until "a
266 few more" FRAGMENTS have been freed in case the last [few] freed FRAGMENTS
267 are allocated yet again (and thus prevent the huge overhead of making
268 FRAGMENTS in a BLOCK). The "only" drawback of such a SEBAWA concept is
269 that it would mean an even bigger overhead...
270
271 HEADERS (in allocated data)
272
273 FRAGMENTS - 32-bit pointer to its parent BLOCK (lowest bit must be 0)
274 BLOCK - 32-bit size (lowest bit must be 1 to separate this from
275 FRAGMENTS)
276
277##############################################################################
278 LARGER ALLOCATIONS
279##############################################################################
280
281 If the requested size is larger than the largest FRAGMENT size supported,
282 the allocation will be made for this memory area alone, or if a BLOCK is
283 allocated to fit lots of FRAGMENTS a large block is also desired.
284
285 * We add memory to the "system" with the add_pool() function call. It
286 specifies the start and size of the new block of memory that will be
287 used in this memory allocation system. Several add_pool() calls are
288 supported and they may or may not add contiguous memory.
289
290 * Make all blocks get allocated aligned to BLOCKSIZE (sometimes referred to
291 as 'grain size'), 64 bytes in my implementation. Reports tell us there is
292 no real gain in increasing the size of the align.
293
294 * We link *all* pieces of memory (AREAS), free or not free. We keep the list
295 in address order and thus when a FREE() occurs we know instanstly if there
296 are FREE CHUNKS wall-to-wall. No list "travels" needed. Requires some
297 extra space in every allocated BLOCK. Still needs to put the new CHUNK in
298 the right place in size-sorted list/tree. All memory areas, allocated or
299 not, contain the following header:
300 - size of this memory area
301 - FREE status
302 - pointer to the next AREA closest in memory
303 - pointer to the prev AREA closest in memory
304 (12 bytes)
305
306 * Sort all FREE CHUNKS in size-order. We use a SPLAY TREE algorithm for
307 maximum speed. Data/structs used for the size-sorting functions are kept
308 in an abstraction layer away from this since it is really not changing
309 anything (except executing speed).
310
311 ALLOC (RSIZE - requested size, aligned properly)
312
313 * Fetch a CHUNK that RSIZE fits within. If the found CHUNK is larger than
314 RSIZE, split it and return the RSIZE to the caller. Link the new CHUNK
315 into the list/tree.
316
317 FREE (AREA - piece of memory that is returned to the system)
318
319 * Since the allocated BLOCK has kept its link-pointers, we can without
320 checking any list instantly see if there are any FREE CHUNKS that are
321 wall-to-wall with the AREA (both sides). If the AREA *is* wall-to-wall
322 with one or two CHUNKS that or they are unlinked from the lists, enlarged
323 and re-linked into the lists.
324
325 REALLOC
326
327 * There IS NO realloc() of large blocks, they are performed in the higher
328 layer (dmalloc).
329
330
331##############################################################################
332 FURTHER READING
333##############################################################################
334
335 * "Dynamic Storage Allocation: A Survey and Critical Review" (Paul R. Wilson,
336 Mark S. Johnstone, Michael Neely, David Boles)
337 ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps
338
339 * "A Memory Allocator" (Doug Lea)
340 http://g.oswego.edu/dl/html/malloc.html