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
336 IF_DEBUG(sanity, checkFreeListSanity());
341 if (n_alloc_blocks > hw_alloc_blocks) hw_alloc_blocks = n_alloc_blocks;
345 while (ln < MAX_FREE_LIST && free_list[ln] == NULL) {
349 if (ln == MAX_FREE_LIST) {
351 if (((W_)mblocks_allocated * MBLOCK_SIZE_W - (W_)n_alloc_blocks * BLOCK_SIZE_W) > (1024*1024)/sizeof(W_)) {
352 debugBelch("Fragmentation, wanted %d blocks:", n);
353 RtsFlags.DebugFlags.block_alloc = 1;
354 checkFreeListSanity();
358 bd = alloc_mega_group(1);
360 initGroup(bd); // we know the group will fit
362 rem->blocks = BLOCKS_PER_MBLOCK-n;
363 initGroup(rem); // init the slop
364 n_alloc_blocks += rem->blocks;
365 freeGroup(rem); // add the slop on to the free list
366 IF_DEBUG(sanity, checkFreeListSanity());
372 if (bd->blocks == n) // exactly the right size!
374 dbl_link_remove(bd, &free_list[ln]);
376 else if (bd->blocks > n) // block too big...
378 bd = split_free_block(bd, n, ln);
382 barf("allocGroup: free list corrupted");
384 initGroup(bd); // initialise it
385 IF_DEBUG(sanity, checkFreeListSanity());
386 ASSERT(bd->blocks == n);
391 allocGroup_lock(nat n)
403 return allocGroup(1);
407 allocBlock_lock(void)
416 /* -----------------------------------------------------------------------------
418 -------------------------------------------------------------------------- */
420 STATIC_INLINE bdescr *
421 coalesce_mblocks (bdescr *p)
427 MBLOCK_ROUND_DOWN(q) ==
428 (StgWord8*)MBLOCK_ROUND_DOWN(p) +
429 BLOCKS_TO_MBLOCKS(p->blocks) * MBLOCK_SIZE) {
431 p->blocks = MBLOCK_GROUP_BLOCKS(BLOCKS_TO_MBLOCKS(p->blocks) +
432 BLOCKS_TO_MBLOCKS(q->blocks));
440 free_mega_group (bdescr *mg)
444 // Find the right place in the free list. free_mblock_list is
445 // sorted by *address*, not by size as the free_list is.
447 bd = free_mblock_list;
448 while (bd && bd->start < mg->start) {
453 // coalesce backwards
456 mg->link = prev->link;
458 mg = coalesce_mblocks(prev);
462 mg->link = free_mblock_list;
463 free_mblock_list = mg;
466 coalesce_mblocks(mg);
468 IF_DEBUG(sanity, checkFreeListSanity());
477 // Todo: not true in multithreaded GC
480 ASSERT(p->free != (P_)-1);
482 p->free = (void *)-1; /* indicates that this block is free */
485 /* fill the block group with garbage if sanity checking is on */
486 IF_DEBUG(sanity,memset(p->start, 0xaa, (W_)p->blocks * BLOCK_SIZE));
488 if (p->blocks == 0) barf("freeGroup: block size is zero");
490 if (p->blocks >= BLOCKS_PER_MBLOCK)
494 mblocks = BLOCKS_TO_MBLOCKS(p->blocks);
495 // If this is an mgroup, make sure it has the right number of blocks
496 ASSERT(p->blocks == MBLOCK_GROUP_BLOCKS(mblocks));
498 n_alloc_blocks -= mblocks * BLOCKS_PER_MBLOCK;
504 ASSERT(n_alloc_blocks >= p->blocks);
505 n_alloc_blocks -= p->blocks;
510 next = p + p->blocks;
511 if (next <= LAST_BDESCR(MBLOCK_ROUND_DOWN(p)) && next->free == (P_)-1)
513 p->blocks += next->blocks;
514 ln = log_2(next->blocks);
515 dbl_link_remove(next, &free_list[ln]);
516 if (p->blocks == BLOCKS_PER_MBLOCK)
525 // coalesce backwards
526 if (p != FIRST_BDESCR(MBLOCK_ROUND_DOWN(p)))
530 if (prev->blocks == 0) prev = prev->link; // find the head
532 if (prev->free == (P_)-1)
534 ln = log_2(prev->blocks);
535 dbl_link_remove(prev, &free_list[ln]);
536 prev->blocks += p->blocks;
537 if (prev->blocks >= BLOCKS_PER_MBLOCK)
539 free_mega_group(prev);
549 IF_DEBUG(sanity, checkFreeListSanity());
553 freeGroup_lock(bdescr *p)
561 freeChain(bdescr *bd)
572 freeChain_lock(bdescr *bd)
579 // splitBlockGroup(bd,B) splits bd in two. Afterward, bd will have B
580 // blocks, and a new block descriptor pointing to the remainder is
583 splitBlockGroup (bdescr *bd, nat blocks)
587 if (bd->blocks <= blocks) {
588 barf("splitLargeBlock: too small");
591 if (bd->blocks > BLOCKS_PER_MBLOCK) {
592 nat low_mblocks, high_mblocks;
594 if ((blocks - BLOCKS_PER_MBLOCK) % (MBLOCK_SIZE / BLOCK_SIZE) != 0) {
595 barf("splitLargeBlock: not a multiple of a megablock");
597 low_mblocks = 1 + (blocks - BLOCKS_PER_MBLOCK) / (MBLOCK_SIZE / BLOCK_SIZE);
598 high_mblocks = (bd->blocks - blocks) / (MBLOCK_SIZE / BLOCK_SIZE);
600 new_mblock = (void *) ((P_)MBLOCK_ROUND_DOWN(bd) + (W_)low_mblocks * MBLOCK_SIZE_W);
601 initMBlock(new_mblock);
602 new_bd = FIRST_BDESCR(new_mblock);
603 new_bd->blocks = MBLOCK_GROUP_BLOCKS(high_mblocks);
605 ASSERT(blocks + new_bd->blocks ==
606 bd->blocks + BLOCKS_PER_MBLOCK - MBLOCK_SIZE/BLOCK_SIZE);
610 // NB. we're not updating all the bdescrs in the split groups to
611 // point to the new heads, so this can only be used for large
612 // objects which do not start in the non-head block.
613 new_bd = bd + blocks;
614 new_bd->blocks = bd->blocks - blocks;
622 initMBlock(void *mblock)
627 /* the first few Bdescr's in a block are unused, so we don't want to
628 * put them all on the free list.
630 block = FIRST_BLOCK(mblock);
631 bd = FIRST_BDESCR(mblock);
633 /* Initialise the start field of each block descriptor
635 for (; block <= (StgWord8*)LAST_BLOCK(mblock); bd += 1,
636 block += BLOCK_SIZE) {
637 bd->start = (void*)block;
641 /* -----------------------------------------------------------------------------
643 -------------------------------------------------------------------------- */
646 countBlocks(bdescr *bd)
649 for (n=0; bd != NULL; bd=bd->link) {
655 // (*1) Just like countBlocks, except that we adjust the count for a
656 // megablock group so that it doesn't include the extra few blocks
657 // that would be taken up by block descriptors in the second and
658 // subsequent megablock. This is so we can tally the count with the
659 // number of blocks allocated in the system, for memInventory().
661 countAllocdBlocks(bdescr *bd)
664 for (n=0; bd != NULL; bd=bd->link) {
666 // hack for megablock groups: see (*1) above
667 if (bd->blocks > BLOCKS_PER_MBLOCK) {
668 n -= (MBLOCK_SIZE / BLOCK_SIZE - BLOCKS_PER_MBLOCK)
669 * (bd->blocks/(MBLOCK_SIZE/BLOCK_SIZE));
675 void returnMemoryToOS(nat n /* megablocks */)
680 bd = free_mblock_list;
681 while ((n > 0) && (bd != NULL)) {
682 size = BLOCKS_TO_MBLOCKS(bd->blocks);
684 nat newSize = size - n;
685 char *freeAddr = MBLOCK_ROUND_DOWN(bd->start);
686 freeAddr += newSize * MBLOCK_SIZE;
687 bd->blocks = MBLOCK_GROUP_BLOCKS(newSize);
688 freeMBlocks(freeAddr, n);
692 char *freeAddr = MBLOCK_ROUND_DOWN(bd->start);
695 freeMBlocks(freeAddr, size);
698 free_mblock_list = bd;
702 debugBelch("Wanted to free %d more MBlocks than are freeable\n",
708 /* -----------------------------------------------------------------------------
710 -------------------------------------------------------------------------- */
714 check_tail (bdescr *bd)
716 bdescr *tail = tail_of(bd);
720 ASSERT(tail->blocks == 0);
721 ASSERT(tail->free == 0);
722 ASSERT(tail->link == bd);
727 checkFreeListSanity(void)
734 for (ln = 0; ln < MAX_FREE_LIST; ln++) {
735 IF_DEBUG(block_alloc, debugBelch("free block list [%d]:\n", ln));
738 for (bd = free_list[ln]; bd != NULL; prev = bd, bd = bd->link)
740 IF_DEBUG(block_alloc,
741 debugBelch("group at %p, length %ld blocks\n",
742 bd->start, (long)bd->blocks));
743 ASSERT(bd->free == (P_)-1);
744 ASSERT(bd->blocks > 0 && bd->blocks < BLOCKS_PER_MBLOCK);
745 ASSERT(bd->blocks >= min && bd->blocks <= (min*2 - 1));
746 ASSERT(bd->link != bd); // catch easy loops
751 ASSERT(bd->u.back == prev);
753 ASSERT(bd->u.back == NULL);
757 next = bd + bd->blocks;
758 if (next <= LAST_BDESCR(MBLOCK_ROUND_DOWN(bd)))
760 ASSERT(next->free != (P_)-1);
768 for (bd = free_mblock_list; bd != NULL; prev = bd, bd = bd->link)
770 IF_DEBUG(block_alloc,
771 debugBelch("mega group at %p, length %ld blocks\n",
772 bd->start, (long)bd->blocks));
774 ASSERT(bd->link != bd); // catch easy loops
776 if (bd->link != NULL)
778 // make sure the list is sorted
779 ASSERT(bd->start < bd->link->start);
782 ASSERT(bd->blocks >= BLOCKS_PER_MBLOCK);
783 ASSERT(MBLOCK_GROUP_BLOCKS(BLOCKS_TO_MBLOCKS(bd->blocks))
786 // make sure we're fully coalesced
787 if (bd->link != NULL)
789 ASSERT (MBLOCK_ROUND_DOWN(bd->link) !=
790 (StgWord8*)MBLOCK_ROUND_DOWN(bd) +
791 BLOCKS_TO_MBLOCKS(bd->blocks) * MBLOCK_SIZE);
800 lnat total_blocks = 0;
803 for (ln=0; ln < MAX_FREE_LIST; ln++) {
804 for (bd = free_list[ln]; bd != NULL; bd = bd->link) {
805 total_blocks += bd->blocks;
808 for (bd = free_mblock_list; bd != NULL; bd = bd->link) {
809 total_blocks += BLOCKS_PER_MBLOCK * BLOCKS_TO_MBLOCKS(bd->blocks);
810 // The caller of this function, memInventory(), expects to match
811 // the total number of blocks in the system against mblocks *
812 // BLOCKS_PER_MBLOCK, so we must subtract the space for the
813 // block descriptors from *every* mblock.
819 markBlocks (bdescr *bd)
821 for (; bd != NULL; bd = bd->link) {
822 bd->flags |= BF_KNOWN;
827 reportUnmarkedBlocks (void)
832 debugBelch("Unreachable blocks:\n");
833 for (mblock = getFirstMBlock(); mblock != NULL;
834 mblock = getNextMBlock(mblock)) {
835 for (bd = FIRST_BDESCR(mblock); bd <= LAST_BDESCR(mblock); ) {
836 if (!(bd->flags & BF_KNOWN) && bd->free != (P_)-1) {
837 debugBelch(" %p\n",bd);
839 if (bd->blocks >= BLOCKS_PER_MBLOCK) {
840 mblock = (StgWord8*)mblock +
841 (BLOCKS_TO_MBLOCKS(bd->blocks) - 1) * MBLOCK_SIZE;