1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team 1998-2008
5 * The block allocator and free list manager.
7 * This is the architecture independent part of the block allocator.
8 * It requires only the following support from the operating system:
10 * void *getMBlock(nat n);
12 * returns the address of an n*MBLOCK_SIZE region of memory, aligned on
13 * an MBLOCK_SIZE boundary. There are no other restrictions on the
14 * addresses of memory returned by getMBlock().
16 * ---------------------------------------------------------------------------*/
18 #include "PosixSource.h"
23 #include "BlockAlloc.h"
28 static void initMBlock(void *mblock);
30 /* -----------------------------------------------------------------------------
36 - bdescr = block descriptor
37 - bgroup = block group (1 or more adjacent blocks)
39 - mgroup = mega group (1 or more adjacent mblocks)
41 Invariants on block descriptors
42 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
43 bd->start always points to the start of the block.
46 - zero for a non-group-head; bd->link points to the head
47 - (-1) for the head of a free block group
48 - or it points within the block
51 - zero for a non-group-head; bd->link points to the head
52 - number of blocks in this group otherwise
54 bd->link either points to a block descriptor or is NULL
56 The following fields are not used by the allocator:
62 Exceptions: we don't maintain invariants for all the blocks within a
63 group on the free list, because it is expensive to modify every
64 bdescr in a group when coalescing. Just the head and last bdescrs
65 will be correct for a group on the free list.
72 - most allocations are for a small number of blocks
73 - sometimes the OS gives us new memory backwards in the address
74 space, sometimes forwards, so we should not be biased towards
75 any particular layout in the address space
76 - We want to avoid fragmentation
77 - We want allocation and freeing to be O(1) or close.
79 Coalescing trick: when a bgroup is freed (freeGroup()), we can check
80 whether it can be coalesced with other free bgroups by checking the
81 bdescrs for the blocks on either side of it. This means that we can
82 coalesce in O(1) time. Every free bgroup must have its head and tail
83 bdescrs initialised, the rest don't matter.
85 We keep the free list in buckets, using a heap-sort strategy.
86 Bucket N contains blocks with sizes 2^N - 2^(N+1)-1. The list of
87 blocks in each bucket is doubly-linked, so that if a block is
88 coalesced we can easily remove it from its current free list.
90 To allocate a new block of size S, grab a block from bucket
91 log2ceiling(S) (i.e. log2() rounded up), in which all blocks are at
92 least as big as S, and split it if necessary. If there are no
93 blocks in that bucket, look at bigger buckets until a block is found
94 Allocation is therefore O(logN) time.
97 - coalesce it with neighbours.
98 - remove coalesced neighbour(s) from free list(s)
99 - add the new (coalesced) block to the front of the appropriate
100 bucket, given by log2(S) where S is the size of the block.
104 We cannot play this coalescing trick with mblocks, because there is
105 no requirement that the bdescrs in the second and subsequent mblock
106 of an mgroup are initialised (the mgroup might be filled with a
107 large array, overwriting the bdescrs for example).
109 So there is a separate free list for megablocks, sorted in *address*
110 order, so that we can coalesce. Allocation in this list is best-fit
111 by traversing the whole list: we don't expect this list to be long,
112 and allocation/freeing of large blocks is rare; avoiding
113 fragmentation is more important than performance here.
115 freeGroup() might end up moving a block from free_list to
116 free_mblock_list, if after coalescing we end up with a full mblock.
118 checkFreeListSanity() checks all the invariants on the free lists.
120 --------------------------------------------------------------------------- */
122 /* ---------------------------------------------------------------------------
123 WATCH OUT FOR OVERFLOW
125 Be very careful with integer overflow here. If you have an
126 expression like (n_blocks * BLOCK_SIZE), and n_blocks is an int or
127 a nat, then it will very likely overflow on a 64-bit platform.
128 Always cast to StgWord (or W_ for short) first: ((W_)n_blocks * BLOCK_SIZE).
130 --------------------------------------------------------------------------- */
132 #define MAX_FREE_LIST 9
134 // In THREADED_RTS mode, the free list is protected by sm_mutex.
136 static bdescr *free_list[MAX_FREE_LIST];
137 static bdescr *free_mblock_list;
139 // free_list[i] contains blocks that are at least size 2^i, and at
140 // most size 2^(i+1) - 1.
142 // To find the free list in which to place a block, use log_2(size).
143 // To find a free block of the right size, use log_2_ceil(size).
145 lnat n_alloc_blocks; // currently allocated blocks
146 lnat hw_alloc_blocks; // high-water allocated blocks
148 /* -----------------------------------------------------------------------------
150 -------------------------------------------------------------------------- */
152 void initBlockAllocator(void)
155 for (i=0; i < MAX_FREE_LIST; i++) {
158 free_mblock_list = NULL;
163 /* -----------------------------------------------------------------------------
165 -------------------------------------------------------------------------- */
168 initGroup(bdescr *head)
174 head->free = head->start;
176 for (i=1, bd = head+1; i < n; i++, bd++) {
183 // There are quicker non-loopy ways to do log_2, but we expect n to be
184 // usually small, and MAX_FREE_LIST is also small, so the loop version
185 // might well be the best choice here.
191 for (i=0; i < MAX_FREE_LIST; i++) {
192 if (x >= n) return i;
195 return MAX_FREE_LIST;
203 for (i=0; i < MAX_FREE_LIST; i++) {
205 if (x == 0) return i;
207 return MAX_FREE_LIST;
211 free_list_insert (bdescr *bd)
215 ASSERT(bd->blocks < BLOCKS_PER_MBLOCK);
216 ln = log_2(bd->blocks);
218 dbl_link_onto(bd, &free_list[ln]);
222 STATIC_INLINE bdescr *
225 return bd + bd->blocks - 1;
228 // After splitting a group, the last block of each group must have a
229 // tail that points to the head block, to keep our invariants for
232 setup_tail (bdescr *bd)
244 // Take a free block group bd, and split off a group of size n from
245 // it. Adjust the free list as necessary, and return the new group.
247 split_free_block (bdescr *bd, nat n, nat ln)
249 bdescr *fg; // free group
251 ASSERT(bd->blocks > n);
252 dbl_link_remove(bd, &free_list[ln]);
253 fg = bd + bd->blocks - n; // take n blocks off the end
257 ln = log_2(bd->blocks);
258 dbl_link_onto(bd, &free_list[ln]);
263 alloc_mega_group (nat mblocks)
265 bdescr *best, *bd, *prev;
268 n = MBLOCK_GROUP_BLOCKS(mblocks);
272 for (bd = free_mblock_list; bd != NULL; prev = bd, bd = bd->link)
277 prev->link = bd->link;
279 free_mblock_list = bd->link;
284 else if (bd->blocks > n)
286 if (!best || bd->blocks < best->blocks)
295 // we take our chunk off the end here.
296 StgWord best_mblocks = BLOCKS_TO_MBLOCKS(best->blocks);
297 bd = FIRST_BDESCR((StgWord8*)MBLOCK_ROUND_DOWN(best) +
298 (best_mblocks-mblocks)*MBLOCK_SIZE);
300 best->blocks = MBLOCK_GROUP_BLOCKS(best_mblocks - mblocks);
301 initMBlock(MBLOCK_ROUND_DOWN(bd));
305 void *mblock = getMBlocks(mblocks);
306 initMBlock(mblock); // only need to init the 1st one
307 bd = FIRST_BDESCR(mblock);
309 bd->blocks = MBLOCK_GROUP_BLOCKS(mblocks);
319 if (n == 0) barf("allocGroup: requested zero blocks");
321 if (n >= BLOCKS_PER_MBLOCK)
325 mblocks = BLOCKS_TO_MBLOCKS(n);
327 // n_alloc_blocks doesn't count the extra blocks we get in a
329 n_alloc_blocks += mblocks * BLOCKS_PER_MBLOCK;
330 if (n_alloc_blocks > hw_alloc_blocks) hw_alloc_blocks = n_alloc_blocks;
332 bd = alloc_mega_group(mblocks);
333 // only the bdescrs of the first MB are required to be initialised
339 if (n_alloc_blocks > hw_alloc_blocks) hw_alloc_blocks = n_alloc_blocks;
343 while (ln < MAX_FREE_LIST && free_list[ln] == NULL) {
347 if (ln == MAX_FREE_LIST) {
348 #if 0 /* useful for debugging fragmentation */
349 if ((W_)mblocks_allocated * BLOCKS_PER_MBLOCK * BLOCK_SIZE_W
350 - (W_)((n_alloc_blocks - n) * BLOCK_SIZE_W) > (2*1024*1024)/sizeof(W_)) {
351 debugBelch("Fragmentation, wanted %d blocks:", n);
352 RtsFlags.DebugFlags.block_alloc = 1;
353 checkFreeListSanity();
357 bd = alloc_mega_group(1);
359 initGroup(bd); // we know the group will fit
361 rem->blocks = BLOCKS_PER_MBLOCK-n;
362 initGroup(rem); // init the slop
363 n_alloc_blocks += rem->blocks;
364 freeGroup(rem); // add the slop on to the free list
370 if (bd->blocks == n) // exactly the right size!
372 dbl_link_remove(bd, &free_list[ln]);
375 else if (bd->blocks > n) // block too big...
377 bd = split_free_block(bd, n, ln);
378 ASSERT(bd->blocks == n);
383 barf("allocGroup: free list corrupted");
387 IF_DEBUG(sanity, memset(bd->start, 0xaa, bd->blocks * BLOCK_SIZE));
388 IF_DEBUG(sanity, checkFreeListSanity());
393 allocGroup_lock(nat n)
405 return allocGroup(1);
409 allocBlock_lock(void)
418 /* -----------------------------------------------------------------------------
420 -------------------------------------------------------------------------- */
422 STATIC_INLINE bdescr *
423 coalesce_mblocks (bdescr *p)
429 MBLOCK_ROUND_DOWN(q) ==
430 (StgWord8*)MBLOCK_ROUND_DOWN(p) +
431 BLOCKS_TO_MBLOCKS(p->blocks) * MBLOCK_SIZE) {
433 p->blocks = MBLOCK_GROUP_BLOCKS(BLOCKS_TO_MBLOCKS(p->blocks) +
434 BLOCKS_TO_MBLOCKS(q->blocks));
442 free_mega_group (bdescr *mg)
446 // Find the right place in the free list. free_mblock_list is
447 // sorted by *address*, not by size as the free_list is.
449 bd = free_mblock_list;
450 while (bd && bd->start < mg->start) {
455 // coalesce backwards
458 mg->link = prev->link;
460 mg = coalesce_mblocks(prev);
464 mg->link = free_mblock_list;
465 free_mblock_list = mg;
468 coalesce_mblocks(mg);
470 IF_DEBUG(sanity, checkFreeListSanity());
479 // Todo: not true in multithreaded GC
482 ASSERT(p->free != (P_)-1);
484 p->free = (void *)-1; /* indicates that this block is free */
487 /* fill the block group with garbage if sanity checking is on */
488 IF_DEBUG(sanity,memset(p->start, 0xaa, (W_)p->blocks * BLOCK_SIZE));
490 if (p->blocks == 0) barf("freeGroup: block size is zero");
492 if (p->blocks >= BLOCKS_PER_MBLOCK)
496 mblocks = BLOCKS_TO_MBLOCKS(p->blocks);
497 // If this is an mgroup, make sure it has the right number of blocks
498 ASSERT(p->blocks == MBLOCK_GROUP_BLOCKS(mblocks));
500 n_alloc_blocks -= mblocks * BLOCKS_PER_MBLOCK;
506 ASSERT(n_alloc_blocks >= p->blocks);
507 n_alloc_blocks -= p->blocks;
512 next = p + p->blocks;
513 if (next <= LAST_BDESCR(MBLOCK_ROUND_DOWN(p)) && next->free == (P_)-1)
515 p->blocks += next->blocks;
516 ln = log_2(next->blocks);
517 dbl_link_remove(next, &free_list[ln]);
518 if (p->blocks == BLOCKS_PER_MBLOCK)
527 // coalesce backwards
528 if (p != FIRST_BDESCR(MBLOCK_ROUND_DOWN(p)))
532 if (prev->blocks == 0) prev = prev->link; // find the head
534 if (prev->free == (P_)-1)
536 ln = log_2(prev->blocks);
537 dbl_link_remove(prev, &free_list[ln]);
538 prev->blocks += p->blocks;
539 if (prev->blocks >= BLOCKS_PER_MBLOCK)
541 free_mega_group(prev);
551 IF_DEBUG(sanity, checkFreeListSanity());
555 freeGroup_lock(bdescr *p)
563 freeChain(bdescr *bd)
574 freeChain_lock(bdescr *bd)
582 initMBlock(void *mblock)
587 /* the first few Bdescr's in a block are unused, so we don't want to
588 * put them all on the free list.
590 block = FIRST_BLOCK(mblock);
591 bd = FIRST_BDESCR(mblock);
593 /* Initialise the start field of each block descriptor
595 for (; block <= (StgWord8*)LAST_BLOCK(mblock); bd += 1,
596 block += BLOCK_SIZE) {
597 bd->start = (void*)block;
601 /* -----------------------------------------------------------------------------
603 -------------------------------------------------------------------------- */
606 countBlocks(bdescr *bd)
609 for (n=0; bd != NULL; bd=bd->link) {
615 // (*1) Just like countBlocks, except that we adjust the count for a
616 // megablock group so that it doesn't include the extra few blocks
617 // that would be taken up by block descriptors in the second and
618 // subsequent megablock. This is so we can tally the count with the
619 // number of blocks allocated in the system, for memInventory().
621 countAllocdBlocks(bdescr *bd)
624 for (n=0; bd != NULL; bd=bd->link) {
626 // hack for megablock groups: see (*1) above
627 if (bd->blocks > BLOCKS_PER_MBLOCK) {
628 n -= (MBLOCK_SIZE / BLOCK_SIZE - BLOCKS_PER_MBLOCK)
629 * (bd->blocks/(MBLOCK_SIZE/BLOCK_SIZE));
635 void returnMemoryToOS(nat n /* megablocks */)
640 bd = free_mblock_list;
641 while ((n > 0) && (bd != NULL)) {
642 size = BLOCKS_TO_MBLOCKS(bd->blocks);
644 nat newSize = size - n;
645 char *freeAddr = MBLOCK_ROUND_DOWN(bd->start);
646 freeAddr += newSize * MBLOCK_SIZE;
647 bd->blocks = MBLOCK_GROUP_BLOCKS(newSize);
648 freeMBlocks(freeAddr, n);
652 char *freeAddr = MBLOCK_ROUND_DOWN(bd->start);
655 freeMBlocks(freeAddr, size);
658 free_mblock_list = bd;
660 osReleaseFreeMemory();
664 debugBelch("Wanted to free %d more MBlocks than are freeable\n",
670 /* -----------------------------------------------------------------------------
672 -------------------------------------------------------------------------- */
676 check_tail (bdescr *bd)
678 bdescr *tail = tail_of(bd);
682 ASSERT(tail->blocks == 0);
683 ASSERT(tail->free == 0);
684 ASSERT(tail->link == bd);
689 checkFreeListSanity(void)
696 for (ln = 0; ln < MAX_FREE_LIST; ln++) {
697 IF_DEBUG(block_alloc, debugBelch("free block list [%d]:\n", ln));
700 for (bd = free_list[ln]; bd != NULL; prev = bd, bd = bd->link)
702 IF_DEBUG(block_alloc,
703 debugBelch("group at %p, length %ld blocks\n",
704 bd->start, (long)bd->blocks));
705 ASSERT(bd->free == (P_)-1);
706 ASSERT(bd->blocks > 0 && bd->blocks < BLOCKS_PER_MBLOCK);
707 ASSERT(bd->blocks >= min && bd->blocks <= (min*2 - 1));
708 ASSERT(bd->link != bd); // catch easy loops
713 ASSERT(bd->u.back == prev);
715 ASSERT(bd->u.back == NULL);
719 next = bd + bd->blocks;
720 if (next <= LAST_BDESCR(MBLOCK_ROUND_DOWN(bd)))
722 ASSERT(next->free != (P_)-1);
730 for (bd = free_mblock_list; bd != NULL; prev = bd, bd = bd->link)
732 IF_DEBUG(block_alloc,
733 debugBelch("mega group at %p, length %ld blocks\n",
734 bd->start, (long)bd->blocks));
736 ASSERT(bd->link != bd); // catch easy loops
738 if (bd->link != NULL)
740 // make sure the list is sorted
741 ASSERT(bd->start < bd->link->start);
744 ASSERT(bd->blocks >= BLOCKS_PER_MBLOCK);
745 ASSERT(MBLOCK_GROUP_BLOCKS(BLOCKS_TO_MBLOCKS(bd->blocks))
748 // make sure we're fully coalesced
749 if (bd->link != NULL)
751 ASSERT (MBLOCK_ROUND_DOWN(bd->link) !=
752 (StgWord8*)MBLOCK_ROUND_DOWN(bd) +
753 BLOCKS_TO_MBLOCKS(bd->blocks) * MBLOCK_SIZE);
762 lnat total_blocks = 0;
765 for (ln=0; ln < MAX_FREE_LIST; ln++) {
766 for (bd = free_list[ln]; bd != NULL; bd = bd->link) {
767 total_blocks += bd->blocks;
770 for (bd = free_mblock_list; bd != NULL; bd = bd->link) {
771 total_blocks += BLOCKS_PER_MBLOCK * BLOCKS_TO_MBLOCKS(bd->blocks);
772 // The caller of this function, memInventory(), expects to match
773 // the total number of blocks in the system against mblocks *
774 // BLOCKS_PER_MBLOCK, so we must subtract the space for the
775 // block descriptors from *every* mblock.
781 markBlocks (bdescr *bd)
783 for (; bd != NULL; bd = bd->link) {
784 bd->flags |= BF_KNOWN;
789 reportUnmarkedBlocks (void)
794 debugBelch("Unreachable blocks:\n");
795 for (mblock = getFirstMBlock(); mblock != NULL;
796 mblock = getNextMBlock(mblock)) {
797 for (bd = FIRST_BDESCR(mblock); bd <= LAST_BDESCR(mblock); ) {
798 if (!(bd->flags & BF_KNOWN) && bd->free != (P_)-1) {
799 debugBelch(" %p\n",bd);
801 if (bd->blocks >= BLOCKS_PER_MBLOCK) {
802 mblock = (StgWord8*)mblock +
803 (BLOCKS_TO_MBLOCKS(bd->blocks) - 1) * MBLOCK_SIZE;