rts/sm/BlockAlloc.c

/* -----------------------------------------------------------------------------
 *
 * (c) The GHC Team 1998-2008
 *
 * The block allocator and free list manager.
 *
 * This is the architecture independent part of the block allocator.
 * It requires only the following support from the operating system:
 *
 *    void *getMBlocks(uint32_t n);
 *
 * returns the address of an n*MBLOCK_SIZE region of memory, aligned on
 * an MBLOCK_SIZE boundary.  There are no other restrictions on the
 * addresses of memory returned by getMBlocks().
 *
 * ---------------------------------------------------------------------------*/

#include "PosixSource.h"
#include "Rts.h"

#include "Storage.h"
#include "RtsUtils.h"
#include "BlockAlloc.h"
#include "OSMem.h"

#include <string.h>

static void  initMBlock(void *mblock, uint32_t node);

/* -----------------------------------------------------------------------------

  Implementation notes
  ~~~~~~~~~~~~~~~~~~~~

  Terminology:
    - bdescr = block descriptor
    - bgroup = block group (1 or more adjacent blocks)
    - mblock = mega block
    - mgroup = mega group (1 or more adjacent mblocks)

   Invariants on block descriptors
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   bd->start always points to the start of the block.

   bd->free is either:
      - zero for a non-group-head; bd->link points to the head
      - (-1) for the head of a free block group
      - or it points within the block (group)

   bd->blocks is either:
      - zero for a non-group-head; bd->link points to the head
      - number of blocks in this group otherwise

   bd->link either points to a block descriptor or is NULL

   The following fields are not used by the allocator:
     bd->flags
     bd->gen_no
     bd->gen
     bd->dest

  Exceptions: we don't maintain invariants for all the blocks within a
  group on the free list, because it is expensive to modify every
  bdescr in a group when coalescing.  Just the head and last bdescrs
  will be correct for a group on the free list.


  Free lists
  ~~~~~~~~~~

  Preliminaries:
    - most allocations are for a small number of blocks
    - sometimes the OS gives us new memory backwards in the address
      space, sometimes forwards, so we should not be biased towards
      any particular layout in the address space
    - We want to avoid fragmentation
    - We want allocation and freeing to be O(1) or close.

  Coalescing trick: when a bgroup is freed (freeGroup()), we can check
  whether it can be coalesced with other free bgroups by checking the
  bdescrs for the blocks on either side of it.  This means that we can
  coalesce in O(1) time.  Every free bgroup must have its head and tail
  bdescrs initialised, the rest don't matter.

  We keep the free list in buckets, using a heap-sort strategy.
  Bucket N contains blocks with sizes 2^N - 2^(N+1)-1.  The list of
  blocks in each bucket is doubly-linked, so that if a block is
  coalesced we can easily remove it from its current free list.

  To allocate a new block of size S, grab a block from bucket
  log2ceiling(S) (i.e. log2() rounded up), in which all blocks are at
  least as big as S, and split it if necessary.  If there are no
  blocks in that bucket, look at bigger buckets until a block is found
  Allocation is therefore O(logN) time.

  To free a block:
    - coalesce it with neighbours.
    - remove coalesced neighbour(s) from free list(s)
    - add the new (coalesced) block to the front of the appropriate
      bucket, given by log2(S) where S is the size of the block.

  Free is O(1).

  Megablocks
  ~~~~~~~~~~

  Separately from the free list of block groups, which are smaller than
  an mblock, we maintain a free list of mblock groups.  This is the unit
  of memory the operating system gives us, and we may either split mblocks
  into blocks or allocate them directly (when very large contiguous regions
  of memory).  mblocks have a different set of invariants than blocks:

  bd->start points to the start of the block IF the block is in the first mblock
  bd->blocks and bd->link are only valid IF this block is the first block
    of the first mblock
  No other fields are used (in particular, free is not used, meaning that
    space that is not used by the (single) object is wasted.

  This has implications for the free list as well:
  We cannot play the coalescing trick with mblocks, because there is
  no requirement that the bdescrs in the second and subsequent mblock
  of an mgroup are initialised (the mgroup might be filled with a
  large array, overwriting the bdescrs for example).

  The separate free list for megablocks is thus sorted in *address*
  order, so that we can coalesce.  Allocation in this list is best-fit
  by traversing the whole list: we don't expect this list to be long,
  and allocation/freeing of large blocks is rare; avoiding
  fragmentation is more important than performance here.

  freeGroup() might end up moving a block from free_list to
  free_mblock_list, if after coalescing we end up with a full mblock.

  checkFreeListSanity() checks all the invariants on the free lists.

  --------------------------------------------------------------------------- */

/* ---------------------------------------------------------------------------
   WATCH OUT FOR OVERFLOW

   Be very careful with integer overflow here.  If you have an
   expression like (n_blocks * BLOCK_SIZE), and n_blocks is an int or
   a uint32_t, then it will very likely overflow on a 64-bit platform.
   Always cast to StgWord (or W_ for short) first: ((W_)n_blocks * BLOCK_SIZE).

  --------------------------------------------------------------------------- */

// free_list[i] contains blocks that are at least size 2^i, and at
// most size 2^(i+1) - 1.
//
// To find the free list in which to place a block, use log_2(size).
// To find a free block of the right size, use log_2_ceil(size).
//
// The largest free list (free_list[NUM_FREE_LISTS-1]) needs to contain sizes
// from half a megablock up to (but not including) a full megablock.

#define NUM_FREE_LISTS (MBLOCK_SHIFT-BLOCK_SHIFT)

// In THREADED_RTS mode, the free list is protected by sm_mutex.

static bdescr *free_list[MAX_NUMA_NODES][NUM_FREE_LISTS];
static bdescr *free_mblock_list[MAX_NUMA_NODES];

W_ n_alloc_blocks;   // currently allocated blocks
W_ hw_alloc_blocks;  // high-water allocated blocks

W_ n_alloc_blocks_by_node[MAX_NUMA_NODES];

/* -----------------------------------------------------------------------------
   Initialisation
   -------------------------------------------------------------------------- */

void initBlockAllocator(void)
{
    uint32_t i, node;
    for (node = 0; node < MAX_NUMA_NODES; node++) {
        for (i=0; i < NUM_FREE_LISTS; i++) {
            free_list[node][i] = NULL;
        }
        free_mblock_list[node] = NULL;
        n_alloc_blocks_by_node[node] = 0;
    }
    n_alloc_blocks = 0;
    hw_alloc_blocks = 0;
}

/* -----------------------------------------------------------------------------
   Accounting
   -------------------------------------------------------------------------- */

STATIC_INLINE
void recordAllocatedBlocks(uint32_t node, uint32_t n)
{
    n_alloc_blocks += n;
    n_alloc_blocks_by_node[node] += n;
    if (n > 0 && n_alloc_blocks > hw_alloc_blocks) {
        hw_alloc_blocks = n_alloc_blocks;
    }
}

STATIC_INLINE
void recordFreedBlocks(uint32_t node, uint32_t n)
{
    ASSERT(n_alloc_blocks >= n);
    n_alloc_blocks -= n;
    n_alloc_blocks_by_node[node] -= n;
}

/* -----------------------------------------------------------------------------
   Allocation
   -------------------------------------------------------------------------- */

STATIC_INLINE bdescr *
tail_of (bdescr *bd)
{
    return bd + bd->blocks - 1;
}

STATIC_INLINE void
initGroup(bdescr *head)
{
  head->free   = head->start;
  head->link   = NULL;

  // If this is a block group (but not a megablock group), we
  // make the last block of the group point to the head.  This is used
  // when coalescing blocks in freeGroup().  We don't do this for
  // megablock groups because blocks in the second and subsequent
  // mblocks don't have bdescrs; freeing these is handled in a
  // different way by free_mblock_group().
  if (head->blocks > 1 && head->blocks <= BLOCKS_PER_MBLOCK) {
      bdescr *last = tail_of(head);
      last->blocks = 0;
      last->link = head;
  }
}

#if SIZEOF_VOID_P == SIZEOF_LONG
#define CLZW(n) (__builtin_clzl(n))
#else
#define CLZW(n) (__builtin_clzll(n))
#endif

// log base 2 (floor), needs to support up to (2^NUM_FREE_LISTS)-1
STATIC_INLINE uint32_t
log_2(W_ n)
{
    ASSERT(n > 0 && n < (1<<NUM_FREE_LISTS));
#if defined(__GNUC__)
    return CLZW(n) ^ (sizeof(StgWord)*8 - 1);
    // generates good code on x86.  __builtin_clz() compiles to bsr+xor, but
    // we want just bsr, so the xor here cancels out gcc's xor.
#else
    W_ i, x;
    x = n;
    for (i=0; i < NUM_FREE_LISTS; i++) {
        x = x >> 1;
        if (x == 0) return i;
    }
    return NUM_FREE_LISTS;
#endif
}

// log base 2 (ceiling), needs to support up to (2^NUM_FREE_LISTS)-1
STATIC_INLINE uint32_t
log_2_ceil(W_ n)
{
    ASSERT(n > 0 && n < (1<<NUM_FREE_LISTS));
#if defined(__GNUC__)
    uint32_t r = log_2(n);
    return (n & (n-1)) ? r+1 : r;
#else
    W_ i, x;
    x = 1;
    for (i=0; i < MAX_FREE_LIST; i++) {
        if (x >= n) return i;
        x = x << 1;
    }
    return MAX_FREE_LIST;
#endif
}

STATIC_INLINE void
free_list_insert (uint32_t node, bdescr *bd)
{
    uint32_t ln;

    ASSERT(bd->blocks < BLOCKS_PER_MBLOCK);
    ln = log_2(bd->blocks);

    dbl_link_onto(bd, &free_list[node][ln]);
}

// After splitting a group, the last block of each group must have a
// tail that points to the head block, to keep our invariants for
// coalescing.
STATIC_INLINE void
setup_tail (bdescr *bd)
{
    bdescr *tail;
    tail = tail_of(bd);
    if (tail != bd) {
        tail->blocks = 0;
        tail->free = 0;
        tail->link = bd;
    }
}


// Take a free block group bd, and split off a group of size n from
// it.  Adjust the free list as necessary, and return the new group.
static bdescr *
split_free_block (bdescr *bd, uint32_t node, W_ n, uint32_t ln)
{
    bdescr *fg; // free group

    ASSERT(bd->blocks > n);
    dbl_link_remove(bd, &free_list[node][ln]);
    fg = bd + bd->blocks - n; // take n blocks off the end
    fg->blocks = n;
    bd->blocks -= n;
    setup_tail(bd);
    ln = log_2(bd->blocks);
    dbl_link_onto(bd, &free_list[node][ln]);
    return fg;
}

/* Only initializes the start pointers on the first megablock and the
 * blocks field of the first bdescr; callers are responsible for calling
 * initGroup afterwards.
 */
static bdescr *
alloc_mega_group (uint32_t node, StgWord mblocks)
{
    bdescr *best, *bd, *prev;
    StgWord n;

    n = MBLOCK_GROUP_BLOCKS(mblocks);

    best = NULL;
    prev = NULL;
    for (bd = free_mblock_list[node]; bd != NULL; prev = bd, bd = bd->link)
    {
        if (bd->blocks == n)
        {
            if (prev) {
                prev->link = bd->link;
            } else {
                free_mblock_list[node] = bd->link;
            }
            return bd;
        }
        else if (bd->blocks > n)
        {
            if (!best || bd->blocks < best->blocks)
            {
                best = bd;
            }
        }
    }

    if (best)
    {
        // we take our chunk off the end here.
        StgWord best_mblocks  = BLOCKS_TO_MBLOCKS(best->blocks);
        bd = FIRST_BDESCR((StgWord8*)MBLOCK_ROUND_DOWN(best) +
                          (best_mblocks-mblocks)*MBLOCK_SIZE);

        best->blocks = MBLOCK_GROUP_BLOCKS(best_mblocks - mblocks);
        initMBlock(MBLOCK_ROUND_DOWN(bd), node);
    }
    else
    {
        void *mblock;
        if (RtsFlags.GcFlags.numa) {
            mblock = getMBlocksOnNode(node, mblocks);
        } else {
            mblock = getMBlocks(mblocks);
        }
        initMBlock(mblock, node); // only need to init the 1st one
        bd = FIRST_BDESCR(mblock);
    }
    bd->blocks = MBLOCK_GROUP_BLOCKS(mblocks);
    return bd;
}

bdescr *
allocGroupOnNode (uint32_t node, W_ n)
{
    bdescr *bd, *rem;
    StgWord ln;

    if (n == 0) barf("allocGroup: requested zero blocks");

    if (n >= BLOCKS_PER_MBLOCK)
    {
        StgWord mblocks;

        mblocks = BLOCKS_TO_MBLOCKS(n);

        // n_alloc_blocks doesn't count the extra blocks we get in a
        // megablock group.
        recordAllocatedBlocks(node, mblocks * BLOCKS_PER_MBLOCK);

        bd = alloc_mega_group(node, mblocks);
        // only the bdescrs of the first MB are required to be initialised
        initGroup(bd);
        goto finish;
    }

    recordAllocatedBlocks(node, n);

    ln = log_2_ceil(n);

    while (ln < NUM_FREE_LISTS && free_list[node][ln] == NULL) {
        ln++;
    }

    if (ln == NUM_FREE_LISTS) {
#if 0  /* useful for debugging fragmentation */
        if ((W_)mblocks_allocated * BLOCKS_PER_MBLOCK * BLOCK_SIZE_W
             - (W_)((n_alloc_blocks - n) * BLOCK_SIZE_W) > (2*1024*1024)/sizeof(W_)) {
            debugBelch("Fragmentation, wanted %d blocks, %ld MB free\n", n, ((mblocks_allocated * BLOCKS_PER_MBLOCK) - n_alloc_blocks) / BLOCKS_PER_MBLOCK);
            RtsFlags.DebugFlags.block_alloc = 1;
            checkFreeListSanity();
        }
#endif

        bd = alloc_mega_group(node,1);
        bd->blocks = n;
        initGroup(bd);                   // we know the group will fit
        rem = bd + n;
        rem->blocks = BLOCKS_PER_MBLOCK-n;
        initGroup(rem);                  // init the slop
        recordAllocatedBlocks(node,rem->blocks);
        freeGroup(rem);                  // add the slop on to the free list
        goto finish;
    }

    bd = free_list[node][ln];

    if (bd->blocks == n)                // exactly the right size!
    {
        dbl_link_remove(bd, &free_list[node][ln]);
        initGroup(bd);
    }
    else if (bd->blocks >  n)            // block too big...
    {
        bd = split_free_block(bd, node, n, ln);
        ASSERT(bd->blocks == n);
        initGroup(bd);
    }
    else
    {
        barf("allocGroup: free list corrupted");
    }

finish:
    IF_DEBUG(sanity, memset(bd->start, 0xaa, bd->blocks * BLOCK_SIZE));
    IF_DEBUG(sanity, checkFreeListSanity());
    return bd;
}

STATIC_INLINE
uint32_t nodeWithLeastBlocks (void)
{
    uint32_t node = 0, i;
    uint32_t min_blocks = n_alloc_blocks_by_node[0];
    for (i = 1; i < n_numa_nodes; i++) {
        if (n_alloc_blocks_by_node[i] < min_blocks) {
            min_blocks = n_alloc_blocks_by_node[i];
            node = i;
        }
    }
    return node;
}

bdescr* allocGroup (W_ n)
{
    return allocGroupOnNode(nodeWithLeastBlocks(),n);
}


//
// Allocate a chunk of blocks that is at least min and at most max
// blocks in size. This API is used by the nursery allocator that
// wants contiguous memory preferably, but doesn't require it.  When
// memory is fragmented we might have lots of chunks that are
// less than a full megablock, so allowing the nursery allocator to
// use these reduces fragmentation considerably.  e.g. on a GHC build
// with +RTS -H, I saw fragmentation go from 17MB down to 3MB on a
// single compile.
//
// Further to this: in #7257 there is a program that creates serious
// fragmentation such that the heap is full of tiny <4 block chains.
// The nursery allocator therefore has to use single blocks to avoid
// fragmentation, but we make sure that we allocate large blocks
// preferably if there are any.
//
bdescr* allocLargeChunkOnNode (uint32_t node, W_ min, W_ max)
{
    bdescr *bd;
    StgWord ln, lnmax;

    if (min >= BLOCKS_PER_MBLOCK) {
        return allocGroupOnNode(node,max);
    }

    ln = log_2_ceil(min);
    lnmax = log_2_ceil(max);

    while (ln < NUM_FREE_LISTS && ln < lnmax && free_list[node][ln] == NULL) {
        ln++;
    }
    if (ln == NUM_FREE_LISTS || ln == lnmax) {
        return allocGroupOnNode(node,max);
    }
    bd = free_list[node][ln];

    if (bd->blocks <= max)              // exactly the right size!
    {
        dbl_link_remove(bd, &free_list[node][ln]);
        initGroup(bd);
    }
    else   // block too big...
    {
        bd = split_free_block(bd, node, max, ln);
        ASSERT(bd->blocks == max);
        initGroup(bd);
    }

    recordAllocatedBlocks(node, bd->blocks);

    IF_DEBUG(sanity, memset(bd->start, 0xaa, bd->blocks * BLOCK_SIZE));
    IF_DEBUG(sanity, checkFreeListSanity());
    return bd;
}

bdescr* allocLargeChunk (W_ min, W_ max)
{
    return allocLargeChunkOnNode(nodeWithLeastBlocks(), min, max);
}

bdescr *
allocGroup_lock(W_ n)
{
    bdescr *bd;
    ACQUIRE_SM_LOCK;
    bd = allocGroup(n);
    RELEASE_SM_LOCK;
    return bd;
}

bdescr *
allocBlock_lock(void)
{
    bdescr *bd;
    ACQUIRE_SM_LOCK;
    bd = allocBlock();
    RELEASE_SM_LOCK;
    return bd;
}

bdescr *
allocGroupOnNode_lock(uint32_t node, W_ n)
{
    bdescr *bd;
    ACQUIRE_SM_LOCK;
    bd = allocGroupOnNode(node,n);
    RELEASE_SM_LOCK;
    return bd;
}

bdescr *
allocBlockOnNode_lock(uint32_t node)
{
    bdescr *bd;
    ACQUIRE_SM_LOCK;
    bd = allocBlockOnNode(node);
    RELEASE_SM_LOCK;
    return bd;
}

/* -----------------------------------------------------------------------------
   De-Allocation
   -------------------------------------------------------------------------- */

STATIC_INLINE bdescr *
coalesce_mblocks (bdescr *p)
{
    bdescr *q;

    q = p->link;
    if (q != NULL &&
        MBLOCK_ROUND_DOWN(q) ==
        (StgWord8*)MBLOCK_ROUND_DOWN(p) +
        BLOCKS_TO_MBLOCKS(p->blocks) * MBLOCK_SIZE) {
        // can coalesce
        p->blocks  = MBLOCK_GROUP_BLOCKS(BLOCKS_TO_MBLOCKS(p->blocks) +
                                         BLOCKS_TO_MBLOCKS(q->blocks));
        p->link = q->link;
        return p;
    }
    return q;
}

static void
free_mega_group (bdescr *mg)
{
    bdescr *bd, *prev;
    uint32_t node;

    // Find the right place in the free list.  free_mblock_list is
    // sorted by *address*, not by size as the free_list is.
    prev = NULL;
    node = mg->node;
    bd = free_mblock_list[node];
    while (bd && bd->start < mg->start) {
        prev = bd;
        bd = bd->link;
    }

    // coalesce backwards
    if (prev)
    {
        mg->link = prev->link;
        prev->link = mg;
        mg = coalesce_mblocks(prev);
    }
    else
    {
        mg->link = free_mblock_list[node];
        free_mblock_list[node] = mg;
    }
    // coalesce forwards
    coalesce_mblocks(mg);

    IF_DEBUG(sanity, checkFreeListSanity());
}


void
freeGroup(bdescr *p)
{
  StgWord ln;
  uint32_t node;

  // not true in multithreaded GC:
  // ASSERT_SM_LOCK();

  ASSERT(p->free != (P_)-1);

  node = p->node;

  p->free = (void *)-1;  /* indicates that this block is free */
  p->gen = NULL;
  p->gen_no = 0;
  /* fill the block group with garbage if sanity checking is on */
  IF_DEBUG(sanity,memset(p->start, 0xaa, (W_)p->blocks * BLOCK_SIZE));

  if (p->blocks == 0) barf("freeGroup: block size is zero");

  if (p->blocks >= BLOCKS_PER_MBLOCK)
  {
      StgWord mblocks;

      mblocks = BLOCKS_TO_MBLOCKS(p->blocks);
      // If this is an mgroup, make sure it has the right number of blocks
      ASSERT(p->blocks == MBLOCK_GROUP_BLOCKS(mblocks));

      recordFreedBlocks(node, mblocks * BLOCKS_PER_MBLOCK);

      free_mega_group(p);
      return;
  }

  recordFreedBlocks(node, p->blocks);

  // coalesce forwards
  {
      bdescr *next;
      next = p + p->blocks;
      if (next <= LAST_BDESCR(MBLOCK_ROUND_DOWN(p)) && next->free == (P_)-1)
      {
          p->blocks += next->blocks;
          ln = log_2(next->blocks);
          dbl_link_remove(next, &free_list[node][ln]);
          if (p->blocks == BLOCKS_PER_MBLOCK)
          {
              free_mega_group(p);
              return;
          }
          setup_tail(p);
      }
  }

  // coalesce backwards
  if (p != FIRST_BDESCR(MBLOCK_ROUND_DOWN(p)))
  {
      bdescr *prev;
      prev = p - 1;
      if (prev->blocks == 0) prev = prev->link; // find the head

      if (prev->free == (P_)-1)
      {
          ln = log_2(prev->blocks);
          dbl_link_remove(prev, &free_list[node][ln]);
          prev->blocks += p->blocks;
          if (prev->blocks >= BLOCKS_PER_MBLOCK)
          {
              free_mega_group(prev);
              return;
          }
          p = prev;
      }
  }

  setup_tail(p);
  free_list_insert(node,p);

  IF_DEBUG(sanity, checkFreeListSanity());
}

void
freeGroup_lock(bdescr *p)
{
    ACQUIRE_SM_LOCK;
    freeGroup(p);
    RELEASE_SM_LOCK;
}

void
freeChain(bdescr *bd)
{
  bdescr *next_bd;
  while (bd != NULL) {
    next_bd = bd->link;
    freeGroup(bd);
    bd = next_bd;
  }
}

void
freeChain_lock(bdescr *bd)
{
    ACQUIRE_SM_LOCK;
    freeChain(bd);
    RELEASE_SM_LOCK;
}

static void
initMBlock(void *mblock, uint32_t node)
{
    bdescr *bd;
    StgWord8 *block;

    /* the first few Bdescr's in a block are unused, so we don't want to
     * put them all on the free list.
     */
    block = FIRST_BLOCK(mblock);
    bd    = FIRST_BDESCR(mblock);

    /* Initialise the start field of each block descriptor
     */
    for (; block <= (StgWord8*)LAST_BLOCK(mblock); bd += 1,
             block += BLOCK_SIZE) {
        bd->start = (void*)block;
        bd->node = node;
    }
}

/* -----------------------------------------------------------------------------
   Stats / metrics
   -------------------------------------------------------------------------- */

W_
countBlocks(bdescr *bd)
{
    W_ n;
    for (n=0; bd != NULL; bd=bd->link) {
        n += bd->blocks;
    }
    return n;
}

// (*1) Just like countBlocks, except that we adjust the count for a
// megablock group so that it doesn't include the extra few blocks
// that would be taken up by block descriptors in the second and
// subsequent megablock.  This is so we can tally the count with the
// number of blocks allocated in the system, for memInventory().
W_
countAllocdBlocks(bdescr *bd)
{
    W_ n;
    for (n=0; bd != NULL; bd=bd->link) {
        n += bd->blocks;

        // hack for megablock groups: see (*1) above
        if (bd->blocks > BLOCKS_PER_MBLOCK) {
            n -= (MBLOCK_SIZE / BLOCK_SIZE - BLOCKS_PER_MBLOCK)
                * (bd->blocks/(MBLOCK_SIZE/BLOCK_SIZE));
        }
    }
    return n;
}

void returnMemoryToOS(uint32_t n /* megablocks */)
{
    bdescr *bd;
    uint32_t node;
    StgWord size;

    // ToDo: not fair, we free all the memory starting with node 0.
    for (node = 0; n > 0 && node < n_numa_nodes; node++) {
        bd = free_mblock_list[node];
        while ((n > 0) && (bd != NULL)) {
            size = BLOCKS_TO_MBLOCKS(bd->blocks);
            if (size > n) {
                StgWord newSize = size - n;
                char *freeAddr = MBLOCK_ROUND_DOWN(bd->start);
                freeAddr += newSize * MBLOCK_SIZE;
                bd->blocks = MBLOCK_GROUP_BLOCKS(newSize);
                freeMBlocks(freeAddr, n);
                n = 0;
            }
            else {
                char *freeAddr = MBLOCK_ROUND_DOWN(bd->start);
                n -= size;
                bd = bd->link;
                freeMBlocks(freeAddr, size);
            }
        }
        free_mblock_list[node] = bd;
    }

    // Ask the OS to release any address space portion
    // that was associated with the just released MBlocks
    //
    // Historically, we used to ask the OS directly (via
    // osReleaseFreeMemory()) - now the MBlock layer might
    // have a reason to preserve the address space range,
    // so we keep it
    releaseFreeMemory();

    IF_DEBUG(gc,
        if (n != 0) {
            debugBelch("Wanted to free %d more MBlocks than are freeable\n",
                       n);
        }
    );
}

/* -----------------------------------------------------------------------------
   Debugging
   -------------------------------------------------------------------------- */

#if defined(DEBUG)
static void
check_tail (bdescr *bd)
{
    bdescr *tail = tail_of(bd);

    if (tail != bd)
    {
        ASSERT(tail->blocks == 0);
        ASSERT(tail->free == 0);
        ASSERT(tail->link == bd);
    }
}

void
checkFreeListSanity(void)
{
    bdescr *bd, *prev;
    StgWord ln, min;
    uint32_t node;

    for (node = 0; node < n_numa_nodes; node++) {
        min = 1;
        for (ln = 0; ln < NUM_FREE_LISTS; ln++) {
            IF_DEBUG(block_alloc,
                     debugBelch("free block list [%" FMT_Word "]:\n", ln));

            prev = NULL;
            for (bd = free_list[node][ln]; bd != NULL; prev = bd, bd = bd->link)
            {
                IF_DEBUG(block_alloc,
                         debugBelch("group at %p, length %ld blocks\n",
                                    bd->start, (long)bd->blocks));
                ASSERT(bd->free == (P_)-1);
                ASSERT(bd->blocks > 0 && bd->blocks < BLOCKS_PER_MBLOCK);
                ASSERT(bd->blocks >= min && bd->blocks <= (min*2 - 1));
                ASSERT(bd->link != bd); // catch easy loops
                ASSERT(bd->node == node);

                check_tail(bd);

                if (prev)
                    ASSERT(bd->u.back == prev);
                else
                    ASSERT(bd->u.back == NULL);

                {
                    bdescr *next;
                    next = bd + bd->blocks;
                    if (next <= LAST_BDESCR(MBLOCK_ROUND_DOWN(bd)))
                    {
                        ASSERT(next->free != (P_)-1);
                    }
                }
            }
            min = min << 1;
        }

        prev = NULL;
        for (bd = free_mblock_list[node]; bd != NULL; prev = bd, bd = bd->link)
        {
            IF_DEBUG(block_alloc,
                     debugBelch("mega group at %p, length %ld blocks\n",
                                bd->start, (long)bd->blocks));

            ASSERT(bd->link != bd); // catch easy loops

            if (bd->link != NULL)
            {
                // make sure the list is sorted
                ASSERT(bd->start < bd->link->start);
            }

            ASSERT(bd->blocks >= BLOCKS_PER_MBLOCK);
            ASSERT(MBLOCK_GROUP_BLOCKS(BLOCKS_TO_MBLOCKS(bd->blocks))
                   == bd->blocks);

            // make sure we're fully coalesced
            if (bd->link != NULL)
            {
                ASSERT(MBLOCK_ROUND_DOWN(bd->link) !=
                       (StgWord8*)MBLOCK_ROUND_DOWN(bd) +
                       BLOCKS_TO_MBLOCKS(bd->blocks) * MBLOCK_SIZE);
            }
        }
    }
}

W_ /* BLOCKS */
countFreeList(void)
{
  bdescr *bd;
  W_ total_blocks = 0;
  StgWord ln;
  uint32_t node;

  for (node = 0; node < n_numa_nodes; node++) {
      for (ln=0; ln < NUM_FREE_LISTS; ln++) {
          for (bd = free_list[node][ln]; bd != NULL; bd = bd->link) {
              total_blocks += bd->blocks;
          }
      }
      for (bd = free_mblock_list[node]; bd != NULL; bd = bd->link) {
          total_blocks += BLOCKS_PER_MBLOCK * BLOCKS_TO_MBLOCKS(bd->blocks);
          // The caller of this function, memInventory(), expects to match
          // the total number of blocks in the system against mblocks *
          // BLOCKS_PER_MBLOCK, so we must subtract the space for the
          // block descriptors from *every* mblock.
      }
  }
  return total_blocks;
}

void
markBlocks (bdescr *bd)
{
    for (; bd != NULL; bd = bd->link) {
        bd->flags |= BF_KNOWN;
    }
}

void
reportUnmarkedBlocks (void)
{
    void *mblock;
    void *state;
    bdescr *bd;

    debugBelch("Unreachable blocks:\n");
    for (mblock = getFirstMBlock(&state); mblock != NULL;
         mblock = getNextMBlock(&state, mblock)) {
        for (bd = FIRST_BDESCR(mblock); bd <= LAST_BDESCR(mblock); ) {
            if (!(bd->flags & BF_KNOWN) && bd->free != (P_)-1) {
                debugBelch("  %p\n",bd);
            }
            if (bd->blocks >= BLOCKS_PER_MBLOCK) {
                mblock = (StgWord8*)mblock +
                    (BLOCKS_TO_MBLOCKS(bd->blocks) - 1) * MBLOCK_SIZE;
                break;
            } else {
                bd += bd->blocks;
            }
        }
    }
}

#endif