Storage.c 31.4 KB
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/* -----------------------------------------------------------------------------
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 *
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 * (c) The GHC Team, 1998-2004
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 *
 * Storage manager front end
 *
 * ---------------------------------------------------------------------------*/

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#include "PosixSource.h"
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#include "Rts.h"
#include "RtsUtils.h"
#include "RtsFlags.h"
#include "Stats.h"
#include "Hooks.h"
#include "BlockAlloc.h"
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#include "MBlock.h"
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#include "Weak.h"
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#include "Sanity.h"
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#include "Arena.h"
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#include "OSThreads.h"
#include "Capability.h"
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#include "Storage.h"
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#include "Schedule.h"
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#include "RetainerProfile.h"	// for counting memory blocks (memInventory)

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#include <stdlib.h>
#include <string.h>

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/* 
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 * All these globals require sm_mutex to access in THREADED_RTS mode.
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 */
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StgClosure    *caf_list         = NULL;
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StgClosure    *revertible_caf_list = NULL;
rtsBool       keepCAFs;
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bdescr *small_alloc_list;	/* allocate()d small objects */
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bdescr *pinned_object_block;    /* allocate pinned objects into this block */
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nat alloc_blocks;		/* number of allocate()d blocks since GC */
nat alloc_blocks_lim;		/* approximate limit on alloc_blocks */

StgPtr alloc_Hp    = NULL;	/* next free byte in small_alloc_list */
StgPtr alloc_HpLim = NULL;	/* end of block at small_alloc_list   */

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generation *generations = NULL;	/* all the generations */
generation *g0		= NULL; /* generation 0, for convenience */
generation *oldest_gen  = NULL; /* oldest generation, for convenience */
step *g0s0 		= NULL; /* generation 0, step 0, for convenience */
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ullong total_allocated = 0;	/* total memory allocated during run */
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nat n_nurseries         = 0;    /* == RtsFlags.ParFlags.nNodes, convenience */
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step *nurseries         = NULL; /* array of nurseries, >1 only if THREADED_RTS */
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#ifdef THREADED_RTS
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/*
 * Storage manager mutex:  protects all the above state from
 * simultaneous access by two STG threads.
 */
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Mutex sm_mutex;
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/*
 * This mutex is used by atomicModifyMutVar# only
 */
Mutex atomic_modify_mutvar_mutex;
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#endif

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/*
 * Forward references
 */
static void *stgAllocForGMP   (size_t size_in_bytes);
static void *stgReallocForGMP (void *ptr, size_t old_size, size_t new_size);
static void  stgDeallocForGMP (void *ptr, size_t size);

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static void
initStep (step *stp, int g, int s)
{
    stp->no = s;
    stp->blocks = NULL;
    stp->n_blocks = 0;
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    stp->old_blocks = NULL;
    stp->n_old_blocks = 0;
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    stp->gen = &generations[g];
    stp->gen_no = g;
    stp->hp = NULL;
    stp->hpLim = NULL;
    stp->hp_bd = NULL;
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    stp->scavd_hp = NULL;
    stp->scavd_hpLim = NULL;
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    stp->scan = NULL;
    stp->scan_bd = NULL;
    stp->large_objects = NULL;
    stp->n_large_blocks = 0;
    stp->new_large_objects = NULL;
    stp->scavenged_large_objects = NULL;
    stp->n_scavenged_large_blocks = 0;
    stp->is_compacted = 0;
    stp->bitmap = NULL;
}

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void
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initStorage( void )
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{
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  nat g, s;
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  generation *gen;
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  if (generations != NULL) {
      // multi-init protection
      return;
  }

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  /* Sanity check to make sure the LOOKS_LIKE_ macros appear to be
   * doing something reasonable.
   */
  ASSERT(LOOKS_LIKE_INFO_PTR(&stg_BLACKHOLE_info));
  ASSERT(LOOKS_LIKE_CLOSURE_PTR(&stg_dummy_ret_closure));
  ASSERT(!HEAP_ALLOCED(&stg_dummy_ret_closure));
  
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  if (RtsFlags.GcFlags.maxHeapSize != 0 &&
      RtsFlags.GcFlags.heapSizeSuggestion > 
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      RtsFlags.GcFlags.maxHeapSize) {
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    RtsFlags.GcFlags.maxHeapSize = RtsFlags.GcFlags.heapSizeSuggestion;
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  }

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  if (RtsFlags.GcFlags.maxHeapSize != 0 &&
      RtsFlags.GcFlags.minAllocAreaSize > 
      RtsFlags.GcFlags.maxHeapSize) {
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      errorBelch("maximum heap size (-M) is smaller than minimum alloc area size (-A)");
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      exit(1);
  }

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  initBlockAllocator();
  
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#if defined(THREADED_RTS)
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  initMutex(&sm_mutex);
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  initMutex(&atomic_modify_mutvar_mutex);
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#endif

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  ACQUIRE_SM_LOCK;

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  /* allocate generation info array */
  generations = (generation *)stgMallocBytes(RtsFlags.GcFlags.generations 
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					     * sizeof(struct generation_),
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					     "initStorage: gens");

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  /* Initialise all generations */
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  for(g = 0; g < RtsFlags.GcFlags.generations; g++) {
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    gen = &generations[g];
    gen->no = g;
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    gen->mut_list = allocBlock();
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    gen->collections = 0;
    gen->failed_promotions = 0;
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    gen->max_blocks = 0;
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  }

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  /* A couple of convenience pointers */
  g0 = &generations[0];
  oldest_gen = &generations[RtsFlags.GcFlags.generations-1];

  /* Allocate step structures in each generation */
  if (RtsFlags.GcFlags.generations > 1) {
    /* Only for multiple-generations */

    /* Oldest generation: one step */
    oldest_gen->n_steps = 1;
    oldest_gen->steps = 
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      stgMallocBytes(1 * sizeof(struct step_), "initStorage: last step");
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    /* set up all except the oldest generation with 2 steps */
    for(g = 0; g < RtsFlags.GcFlags.generations-1; g++) {
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      generations[g].n_steps = RtsFlags.GcFlags.steps;
      generations[g].steps  = 
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	stgMallocBytes (RtsFlags.GcFlags.steps * sizeof(struct step_),
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			"initStorage: steps");
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    }
    
  } else {
    /* single generation, i.e. a two-space collector */
    g0->n_steps = 1;
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    g0->steps = stgMallocBytes (sizeof(struct step_), "initStorage: steps");
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  }

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#ifdef THREADED_RTS
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  n_nurseries = n_capabilities;
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  nurseries = stgMallocBytes (n_nurseries * sizeof(struct step_),
			      "initStorage: nurseries");
#else
  n_nurseries = 1;
  nurseries = g0->steps; // just share nurseries[0] with g0s0
#endif  

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  /* Initialise all steps */
  for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
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    for (s = 0; s < generations[g].n_steps; s++) {
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	initStep(&generations[g].steps[s], g, s);
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    }
  }
  
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#ifdef THREADED_RTS
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  for (s = 0; s < n_nurseries; s++) {
      initStep(&nurseries[s], 0, s);
  }
#endif
  
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  /* Set up the destination pointers in each younger gen. step */
  for (g = 0; g < RtsFlags.GcFlags.generations-1; g++) {
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    for (s = 0; s < generations[g].n_steps-1; s++) {
      generations[g].steps[s].to = &generations[g].steps[s+1];
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    }
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    generations[g].steps[s].to = &generations[g+1].steps[0];
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  }
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  oldest_gen->steps[0].to = &oldest_gen->steps[0];
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#ifdef THREADED_RTS
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  for (s = 0; s < n_nurseries; s++) {
      nurseries[s].to = generations[0].steps[0].to;
  }
#endif
  
  /* The oldest generation has one step. */
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  if (RtsFlags.GcFlags.compact) {
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      if (RtsFlags.GcFlags.generations == 1) {
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	  errorBelch("WARNING: compaction is incompatible with -G1; disabled");
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      } else {
	  oldest_gen->steps[0].is_compacted = 1;
      }
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  }
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#ifdef THREADED_RTS
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  if (RtsFlags.GcFlags.generations == 1) {
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      errorBelch("-G1 is incompatible with -threaded");
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      stg_exit(EXIT_FAILURE);
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  }
#endif
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  /* generation 0 is special: that's the nursery */
  generations[0].max_blocks = 0;

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  /* G0S0: the allocation area.  Policy: keep the allocation area
   * small to begin with, even if we have a large suggested heap
   * size.  Reason: we're going to do a major collection first, and we
   * don't want it to be a big one.  This vague idea is borne out by 
   * rigorous experimental evidence.
   */
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  g0s0 = &generations[0].steps[0];

  allocNurseries();
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  weak_ptr_list = NULL;
  caf_list = NULL;
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  revertible_caf_list = NULL;
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  /* initialise the allocate() interface */
  small_alloc_list = NULL;
  alloc_blocks = 0;
  alloc_blocks_lim = RtsFlags.GcFlags.minAllocAreaSize;

  /* Tell GNU multi-precision pkg about our custom alloc functions */
  mp_set_memory_functions(stgAllocForGMP, stgReallocForGMP, stgDeallocForGMP);
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  IF_DEBUG(gc, statDescribeGens());
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  RELEASE_SM_LOCK;
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}

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void
exitStorage (void)
{
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    stat_exit(calcAllocated());
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}

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/* -----------------------------------------------------------------------------
   CAF management.
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   The entry code for every CAF does the following:
     
      - builds a CAF_BLACKHOLE in the heap
      - pushes an update frame pointing to the CAF_BLACKHOLE
      - invokes UPD_CAF(), which:
          - calls newCaf, below
	  - updates the CAF with a static indirection to the CAF_BLACKHOLE
      
   Why do we build a BLACKHOLE in the heap rather than just updating
   the thunk directly?  It's so that we only need one kind of update
   frame - otherwise we'd need a static version of the update frame too.

   newCaf() does the following:
       
      - it puts the CAF on the oldest generation's mut-once list.
        This is so that we can treat the CAF as a root when collecting
	younger generations.

   For GHCI, we have additional requirements when dealing with CAFs:

      - we must *retain* all dynamically-loaded CAFs ever entered,
        just in case we need them again.
      - we must be able to *revert* CAFs that have been evaluated, to
        their pre-evaluated form.

      To do this, we use an additional CAF list.  When newCaf() is
      called on a dynamically-loaded CAF, we add it to the CAF list
      instead of the old-generation mutable list, and save away its
      old info pointer (in caf->saved_info) for later reversion.

      To revert all the CAFs, we traverse the CAF list and reset the
      info pointer to caf->saved_info, then throw away the CAF list.
      (see GC.c:revertCAFs()).

      -- SDM 29/1/01

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

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void
newCAF(StgClosure* caf)
{
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  ACQUIRE_SM_LOCK;
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  if(keepCAFs)
  {
    // HACK:
    // If we are in GHCi _and_ we are using dynamic libraries,
    // then we can't redirect newCAF calls to newDynCAF (see below),
    // so we make newCAF behave almost like newDynCAF.
    // The dynamic libraries might be used by both the interpreted
    // program and GHCi itself, so they must not be reverted.
    // This also means that in GHCi with dynamic libraries, CAFs are not
    // garbage collected. If this turns out to be a problem, we could
    // do another hack here and do an address range test on caf to figure
    // out whether it is from a dynamic library.
    ((StgIndStatic *)caf)->saved_info  = (StgInfoTable *)caf->header.info;
    ((StgIndStatic *)caf)->static_link = caf_list;
    caf_list = caf;
  }
  else
  {
    /* Put this CAF on the mutable list for the old generation.
    * This is a HACK - the IND_STATIC closure doesn't really have
    * a mut_link field, but we pretend it has - in fact we re-use
    * the STATIC_LINK field for the time being, because when we
    * come to do a major GC we won't need the mut_link field
    * any more and can use it as a STATIC_LINK.
    */
    ((StgIndStatic *)caf)->saved_info = NULL;
    recordMutableGen(caf, oldest_gen);
  }
  
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  RELEASE_SM_LOCK;
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#ifdef PAR
  /* If we are PAR or DIST then  we never forget a CAF */
  { globalAddr *newGA;
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    //debugBelch("<##> Globalising CAF %08x %s",caf,info_type(caf));
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    newGA=makeGlobal(caf,rtsTrue); /*given full weight*/
    ASSERT(newGA);
  } 
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#endif /* PAR */
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}

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// An alternate version of newCaf which is used for dynamically loaded
// object code in GHCi.  In this case we want to retain *all* CAFs in
// the object code, because they might be demanded at any time from an
// expression evaluated on the command line.
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// Also, GHCi might want to revert CAFs, so we add these to the
// revertible_caf_list.
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//
// The linker hackily arranges that references to newCaf from dynamic
// code end up pointing to newDynCAF.
void
newDynCAF(StgClosure *caf)
{
    ACQUIRE_SM_LOCK;

    ((StgIndStatic *)caf)->saved_info  = (StgInfoTable *)caf->header.info;
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    ((StgIndStatic *)caf)->static_link = revertible_caf_list;
    revertible_caf_list = caf;
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    RELEASE_SM_LOCK;
}

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/* -----------------------------------------------------------------------------
   Nursery management.
   -------------------------------------------------------------------------- */

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static bdescr *
allocNursery (step *stp, bdescr *tail, nat blocks)
{
    bdescr *bd;
    nat i;

    // Allocate a nursery: we allocate fresh blocks one at a time and
    // cons them on to the front of the list, not forgetting to update
    // the back pointer on the tail of the list to point to the new block.
    for (i=0; i < blocks; i++) {
	// @LDV profiling
	/*
	  processNursery() in LdvProfile.c assumes that every block group in
	  the nursery contains only a single block. So, if a block group is
	  given multiple blocks, change processNursery() accordingly.
	*/
	bd = allocBlock();
	bd->link = tail;
	// double-link the nursery: we might need to insert blocks
	if (tail != NULL) {
	    tail->u.back = bd;
	}
	bd->step = stp;
	bd->gen_no = 0;
	bd->flags = 0;
	bd->free = bd->start;
	tail = bd;
    }
    tail->u.back = NULL;
    return tail;
}

static void
assignNurseriesToCapabilities (void)
{
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#ifdef THREADED_RTS
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    nat i;
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    for (i = 0; i < n_nurseries; i++) {
	capabilities[i].r.rNursery        = &nurseries[i];
	capabilities[i].r.rCurrentNursery = nurseries[i].blocks;
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	capabilities[i].r.rCurrentAlloc   = NULL;
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    }
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#else /* THREADED_RTS */
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    MainCapability.r.rNursery        = &nurseries[0];
    MainCapability.r.rCurrentNursery = nurseries[0].blocks;
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    MainCapability.r.rCurrentAlloc   = NULL;
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#endif
}
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void
allocNurseries( void )
{ 
    nat i;

    for (i = 0; i < n_nurseries; i++) {
	nurseries[i].blocks = 
	    allocNursery(&nurseries[i], NULL, 
			 RtsFlags.GcFlags.minAllocAreaSize);
	nurseries[i].n_blocks    = RtsFlags.GcFlags.minAllocAreaSize;
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	nurseries[i].old_blocks   = NULL;
	nurseries[i].n_old_blocks = 0;
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	/* hp, hpLim, hp_bd, to_space etc. aren't used in the nursery */
    }
    assignNurseriesToCapabilities();
}
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void
resetNurseries( void )
{
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    nat i;
    bdescr *bd;
    step *stp;
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    for (i = 0; i < n_nurseries; i++) {
	stp = &nurseries[i];
	for (bd = stp->blocks; bd; bd = bd->link) {
	    bd->free = bd->start;
	    ASSERT(bd->gen_no == 0);
	    ASSERT(bd->step == stp);
	    IF_DEBUG(sanity,memset(bd->start, 0xaa, BLOCK_SIZE));
	}
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    }
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    assignNurseriesToCapabilities();
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}

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lnat
countNurseryBlocks (void)
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{
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    nat i;
    lnat blocks = 0;
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    for (i = 0; i < n_nurseries; i++) {
	blocks += nurseries[i].n_blocks;
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    }
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    return blocks;
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}

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static void
resizeNursery ( step *stp, nat blocks )
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{
  bdescr *bd;
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  nat nursery_blocks;
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  nursery_blocks = stp->n_blocks;
  if (nursery_blocks == blocks) return;
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  if (nursery_blocks < blocks) {
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    IF_DEBUG(gc, debugBelch("Increasing size of nursery to %d blocks\n", 
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			 blocks));
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    stp->blocks = allocNursery(stp, stp->blocks, blocks-nursery_blocks);
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  } 
  else {
    bdescr *next_bd;
    
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    IF_DEBUG(gc, debugBelch("Decreasing size of nursery to %d blocks\n", 
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			 blocks));
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    bd = stp->blocks;
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    while (nursery_blocks > blocks) {
	next_bd = bd->link;
	next_bd->u.back = NULL;
	nursery_blocks -= bd->blocks; // might be a large block
	freeGroup(bd);
	bd = next_bd;
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    }
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    stp->blocks = bd;
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    // might have gone just under, by freeing a large block, so make
    // up the difference.
    if (nursery_blocks < blocks) {
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	stp->blocks = allocNursery(stp, stp->blocks, blocks-nursery_blocks);
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    }
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  }
  
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  stp->n_blocks = blocks;
  ASSERT(countBlocks(stp->blocks) == stp->n_blocks);
}
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// 
// Resize each of the nurseries to the specified size.
//
void
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resizeNurseriesFixed (nat blocks)
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{
    nat i;
    for (i = 0; i < n_nurseries; i++) {
	resizeNursery(&nurseries[i], blocks);
    }
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}

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// 
// Resize the nurseries to the total specified size.
//
void
resizeNurseries (nat blocks)
{
    // If there are multiple nurseries, then we just divide the number
    // of available blocks between them.
    resizeNurseriesFixed(blocks / n_nurseries);
}

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/* -----------------------------------------------------------------------------
   The allocate() interface

   allocate(n) always succeeds, and returns a chunk of memory n words
   long.  n can be larger than the size of a block if necessary, in
   which case a contiguous block group will be allocated.
   -------------------------------------------------------------------------- */

StgPtr
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allocate( nat n )
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{
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    bdescr *bd;
    StgPtr p;
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    ACQUIRE_SM_LOCK;
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    TICK_ALLOC_HEAP_NOCTR(n);
    CCS_ALLOC(CCCS,n);
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    /* big allocation (>LARGE_OBJECT_THRESHOLD) */
    /* ToDo: allocate directly into generation 1 */
    if (n >= LARGE_OBJECT_THRESHOLD/sizeof(W_)) {
	nat req_blocks =  (lnat)BLOCK_ROUND_UP(n*sizeof(W_)) / BLOCK_SIZE;
	bd = allocGroup(req_blocks);
	dbl_link_onto(bd, &g0s0->large_objects);
	g0s0->n_large_blocks += req_blocks;
	bd->gen_no  = 0;
	bd->step = g0s0;
	bd->flags = BF_LARGE;
	bd->free = bd->start + n;
	alloc_blocks += req_blocks;
	RELEASE_SM_LOCK;
	return bd->start;
	
	/* small allocation (<LARGE_OBJECT_THRESHOLD) */
    } else if (small_alloc_list == NULL || alloc_Hp + n > alloc_HpLim) {
	if (small_alloc_list) {
	    small_alloc_list->free = alloc_Hp;
	}
	bd = allocBlock();
	bd->link = small_alloc_list;
	small_alloc_list = bd;
	bd->gen_no = 0;
	bd->step = g0s0;
	bd->flags = 0;
	alloc_Hp = bd->start;
	alloc_HpLim = bd->start + BLOCK_SIZE_W;
	alloc_blocks++;
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    }
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    p = alloc_Hp;
    alloc_Hp += n;
    RELEASE_SM_LOCK;
    return p;
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}

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lnat
allocated_bytes( void )
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{
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    lnat allocated;

    allocated = alloc_blocks * BLOCK_SIZE_W - (alloc_HpLim - alloc_Hp);
    if (pinned_object_block != NULL) {
	allocated -= (pinned_object_block->start + BLOCK_SIZE_W) - 
	    pinned_object_block->free;
    }
	
    return allocated;
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}

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void
tidyAllocateLists (void)
{
    if (small_alloc_list != NULL) {
	ASSERT(alloc_Hp >= small_alloc_list->start && 
	       alloc_Hp <= small_alloc_list->start + BLOCK_SIZE);
	small_alloc_list->free = alloc_Hp;
    }
}

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/* -----------------------------------------------------------------------------
   allocateLocal()

   This allocates memory in the current thread - it is intended for
   use primarily from STG-land where we have a Capability.  It is
   better than allocate() because it doesn't require taking the
   sm_mutex lock in the common case.

   Memory is allocated directly from the nursery if possible (but not
   from the current nursery block, so as not to interfere with
   Hp/HpLim).
   -------------------------------------------------------------------------- */

StgPtr
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allocateLocal (Capability *cap, nat n)
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{
    bdescr *bd;
    StgPtr p;

    TICK_ALLOC_HEAP_NOCTR(n);
    CCS_ALLOC(CCCS,n);
    
    /* big allocation (>LARGE_OBJECT_THRESHOLD) */
    /* ToDo: allocate directly into generation 1 */
    if (n >= LARGE_OBJECT_THRESHOLD/sizeof(W_)) {
	nat req_blocks =  (lnat)BLOCK_ROUND_UP(n*sizeof(W_)) / BLOCK_SIZE;
	ACQUIRE_SM_LOCK;
	bd = allocGroup(req_blocks);
	dbl_link_onto(bd, &g0s0->large_objects);
	g0s0->n_large_blocks += req_blocks;
	bd->gen_no  = 0;
	bd->step = g0s0;
	bd->flags = BF_LARGE;
	bd->free = bd->start + n;
	alloc_blocks += req_blocks;
	RELEASE_SM_LOCK;
	return bd->start;
	
	/* small allocation (<LARGE_OBJECT_THRESHOLD) */
    } else {

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	bd = cap->r.rCurrentAlloc;
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	if (bd == NULL || bd->free + n > bd->start + BLOCK_SIZE_W) {

	    // The CurrentAlloc block is full, we need to find another
	    // one.  First, we try taking the next block from the
	    // nursery:
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	    bd = cap->r.rCurrentNursery->link;
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	    if (bd == NULL || bd->free + n > bd->start + BLOCK_SIZE_W) {
		// The nursery is empty, or the next block is already
		// full: allocate a fresh block (we can't fail here).
		ACQUIRE_SM_LOCK;
		bd = allocBlock();
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		cap->r.rNursery->n_blocks++;
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		RELEASE_SM_LOCK;
		bd->gen_no = 0;
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		bd->step = cap->r.rNursery;
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		bd->flags = 0;
	    } else {
		// we have a block in the nursery: take it and put
		// it at the *front* of the nursery list, and use it
		// to allocate() from.
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		cap->r.rCurrentNursery->link = bd->link;
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		if (bd->link != NULL) {
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		    bd->link->u.back = cap->r.rCurrentNursery;
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		}
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	    }
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	    dbl_link_onto(bd, &cap->r.rNursery->blocks);
	    cap->r.rCurrentAlloc = bd;
	    IF_DEBUG(sanity, checkNurserySanity(cap->r.rNursery));
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	}
    }
    p = bd->free;
    bd->free += n;
    return p;
}

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/* ---------------------------------------------------------------------------
   Allocate a fixed/pinned object.

   We allocate small pinned objects into a single block, allocating a
   new block when the current one overflows.  The block is chained
   onto the large_object_list of generation 0 step 0.

   NOTE: The GC can't in general handle pinned objects.  This
   interface is only safe to use for ByteArrays, which have no
   pointers and don't require scavenging.  It works because the
   block's descriptor has the BF_LARGE flag set, so the block is
   treated as a large object and chained onto various lists, rather
   than the individual objects being copied.  However, when it comes
   to scavenge the block, the GC will only scavenge the first object.
   The reason is that the GC can't linearly scan a block of pinned
   objects at the moment (doing so would require using the
   mostly-copying techniques).  But since we're restricting ourselves
   to pinned ByteArrays, not scavenging is ok.

   This function is called by newPinnedByteArray# which immediately
   fills the allocated memory with a MutableByteArray#.
   ------------------------------------------------------------------------- */

StgPtr
allocatePinned( nat n )
{
    StgPtr p;
    bdescr *bd = pinned_object_block;

    // If the request is for a large object, then allocate()
    // will give us a pinned object anyway.
    if (n >= LARGE_OBJECT_THRESHOLD/sizeof(W_)) {
	return allocate(n);
    }

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    ACQUIRE_SM_LOCK;
    
    TICK_ALLOC_HEAP_NOCTR(n);
    CCS_ALLOC(CCCS,n);

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    // we always return 8-byte aligned memory.  bd->free must be
    // 8-byte aligned to begin with, so we just round up n to
    // the nearest multiple of 8 bytes.
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    if (sizeof(StgWord) == 4) {
	n = (n+1) & ~1;
    }
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    // If we don't have a block of pinned objects yet, or the current
    // one isn't large enough to hold the new object, allocate a new one.
    if (bd == NULL || (bd->free + n) > (bd->start + BLOCK_SIZE_W)) {
	pinned_object_block = bd = allocBlock();
	dbl_link_onto(bd, &g0s0->large_objects);
	bd->gen_no = 0;
	bd->step   = g0s0;
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	bd->flags  = BF_PINNED | BF_LARGE;
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	bd->free   = bd->start;
	alloc_blocks++;
    }

    p = bd->free;
    bd->free += n;
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    RELEASE_SM_LOCK;
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    return p;
}

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/* -----------------------------------------------------------------------------
   This is the write barrier for MUT_VARs, a.k.a. IORefs.  A
   MUT_VAR_CLEAN object is not on the mutable list; a MUT_VAR_DIRTY
   is.  When written to, a MUT_VAR_CLEAN turns into a MUT_VAR_DIRTY
   and is put on the mutable list.
   -------------------------------------------------------------------------- */

void
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dirty_MUT_VAR(StgRegTable *reg, StgClosure *p)
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{
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    Capability *cap = regTableToCapability(reg);
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    bdescr *bd;
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    if (p->header.info == &stg_MUT_VAR_CLEAN_info) {
	p->header.info = &stg_MUT_VAR_DIRTY_info;
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	bd = Bdescr((StgPtr)p);
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	if (bd->gen_no > 0) recordMutableCap(p,cap,bd->gen_no);
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    }
}

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/* -----------------------------------------------------------------------------
   Allocation functions for GMP.

   These all use the allocate() interface - we can't have any garbage
   collection going on during a gmp operation, so we use allocate()
   which always succeeds.  The gmp operations which might need to
   allocate will ask the storage manager (via doYouWantToGC()) whether
   a garbage collection is required, in case we get into a loop doing
   only allocate() style allocation.
   -------------------------------------------------------------------------- */

static void *
stgAllocForGMP (size_t size_in_bytes)
{
  StgArrWords* arr;
  nat data_size_in_words, total_size_in_words;
  
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  /* round up to a whole number of words */
  data_size_in_words  = (size_in_bytes + sizeof(W_) + 1) / sizeof(W_);
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  total_size_in_words = sizeofW(StgArrWords) + data_size_in_words;
  
  /* allocate and fill it in. */
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#if defined(THREADED_RTS)
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  arr = (StgArrWords *)allocateLocal(myTask()->cap, total_size_in_words);
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#else
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  arr = (StgArrWords *)allocateLocal(&MainCapability, total_size_in_words);
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#endif
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  SET_ARR_HDR(arr, &stg_ARR_WORDS_info, CCCS, data_size_in_words);
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  /* and return a ptr to the goods inside the array */
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  return arr->payload;
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}

static void *
stgReallocForGMP (void *ptr, size_t old_size, size_t new_size)
{
    void *new_stuff_ptr = stgAllocForGMP(new_size);
    nat i = 0;
    char *p = (char *) ptr;
    char *q = (char *) new_stuff_ptr;

    for (; i < old_size; i++, p++, q++) {
	*q = *p;
    }

    return(new_stuff_ptr);
}

static void
stgDeallocForGMP (void *ptr STG_UNUSED, 
		  size_t size STG_UNUSED)
{
    /* easy for us: the garbage collector does the dealloc'n */
}
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/* -----------------------------------------------------------------------------
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 * Stats and stuff
 * -------------------------------------------------------------------------- */

/* -----------------------------------------------------------------------------
 * calcAllocated()
 *
 * Approximate how much we've allocated: number of blocks in the
 * nursery + blocks allocated via allocate() - unused nusery blocks.
 * This leaves a little slop at the end of each block, and doesn't
 * take into account large objects (ToDo).
 * -------------------------------------------------------------------------- */

lnat
calcAllocated( void )
{
  nat allocated;
  bdescr *bd;

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  allocated = allocated_bytes();
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  allocated += countNurseryBlocks() * BLOCK_SIZE_W;
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  {
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#ifdef THREADED_RTS
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  nat i;
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  for (i = 0; i < n_nurseries; i++) {
      Capability *cap;
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      for ( bd = capabilities[i].r.rCurrentNursery->link; 
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	    bd != NULL; bd = bd->link ) {
	  allocated -= BLOCK_SIZE_W;
      }
      cap = &capabilities[i];
      if (cap->r.rCurrentNursery->free < 
	  cap->r.rCurrentNursery->start + BLOCK_SIZE_W) {
	  allocated -= (cap->r.rCurrentNursery->start + BLOCK_SIZE_W)
	      - cap->r.rCurrentNursery->free;
      }
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  }
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#else
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  bdescr *current_nursery = MainCapability.r.rCurrentNursery;
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  for ( bd = current_nursery->link; bd != NULL; bd = bd->link ) {
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      allocated -= BLOCK_SIZE_W;
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  }
  if (current_nursery->free < current_nursery->start + BLOCK_SIZE_W) {
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      allocated -= (current_nursery->start + BLOCK_SIZE_W)
	  - current_nursery->free;
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  }
#endif
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  }
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  total_allocated += allocated;
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  return allocated;
}  
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/* Approximate the amount of live data in the heap.  To be called just
 * after garbage collection (see GarbageCollect()).
 */
extern lnat 
calcLive(void)
{
  nat g, s;
  lnat live = 0;
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  step *stp;
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  if (RtsFlags.GcFlags.generations == 1) {
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    live = (g0s0->n_blocks - 1) * BLOCK_SIZE_W + 
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      ((lnat)g0s0->hp_bd->free - (lnat)g0s0->hp_bd->start) / sizeof(W_);
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    return live;
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  }

  for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
    for (s = 0; s < generations[g].n_steps; s++) {
      /* approximate amount of live data (doesn't take into account slop
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       * at end of each block).
       */
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      if (g == 0 && s == 0) { 
	  continue; 
      }
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      stp = &generations[g].steps[s];
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      live += (stp->n_large_blocks + stp->n_blocks - 1) * BLOCK_SIZE_W;
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      if (stp->hp_bd != NULL) {
	  live += ((lnat)stp->hp_bd->free - (lnat)stp->hp_bd->start) 
	      / sizeof(W_);
      }
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      if (stp->scavd_hp != NULL) {
	  live -= (P_)(BLOCK_ROUND_UP(stp->scavd_hp)) - stp->scavd_hp;
      }
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    }
  }
  return live;
}

/* Approximate the number of blocks that will be needed at the next
 * garbage collection.
 *
 * Assume: all data currently live will remain live.  Steps that will
 * be collected next time will therefore need twice as many blocks
 * since all the data will be copied.
 */
extern lnat 
calcNeeded(void)
{
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    lnat needed = 0;
    nat g, s;
    step *stp;
    
    for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
	for (s = 0; s < generations[g].n_steps; s++) {
	    if (g == 0 && s == 0) { continue; }
	    stp = &generations[g].steps[s];
	    if (generations[g].steps[0].n_blocks +
		generations[g].steps[0].n_large_blocks 
		> generations[g].max_blocks
		&& stp->is_compacted == 0) {
		needed += 2 * stp->n_blocks;
	    } else {
		needed += stp->n_blocks;
	    }
	}
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    }
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    return needed;
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}

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

   memInventory() checks for memory leaks by counting up all the
   blocks we know about and comparing that to the number of blocks
   allegedly floating around in the system.
   -------------------------------------------------------------------------- */

#ifdef DEBUG

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static lnat
stepBlocks (step *stp)
{
    lnat total_blocks;
    bdescr *bd;

    total_blocks = stp->n_blocks;    
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    total_blocks += stp->n_old_blocks;
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    for (bd = stp->large_objects; bd; bd = bd->link) {
	total_blocks += bd->blocks;
	/* hack for megablock groups: they have an extra block or two in
	   the second and subsequent megablocks where the block
	   descriptors would normally go.
	*/
	if (bd->blocks > BLOCKS_PER_MBLOCK) {
	    total_blocks -= (MBLOCK_SIZE / BLOCK_SIZE - BLOCKS_PER_MBLOCK)
		* (bd->blocks/(MBLOCK_SIZE/BLOCK_SIZE));
	}
    }
    return total_blocks;
}

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void
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memInventory(void)
{
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  nat g, s, i;
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  step *stp;
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  bdescr *bd;
  lnat total_blocks = 0, free_blocks = 0;

  /* count the blocks we current have */
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  for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
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      for (i = 0; i < n_capabilities; i++) {
	  for (bd = capabilities[i].mut_lists[g]; bd != NULL; bd = bd->link) {
	      total_blocks += bd->blocks;
	  }
      }	  
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      for (bd = generations[g].mut_list; bd != NULL; bd = bd->link) {
	  total_blocks += bd->blocks;
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      }
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      for (s = 0; s < generations[g].n_steps; s++) {
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	  if (g==0 && s==0) continue;
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	  stp = &generations[g].steps[s];
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	  total_blocks += stepBlocks(stp);
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      }
  }

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  for (i = 0; i < n_nurseries; i++) {
      total_blocks += stepBlocks(&nurseries[i]);
  }
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#ifdef THREADED_RTS
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  // We put pinned object blocks in g0s0, so better count blocks there too.
  total_blocks += stepBlocks(g0s0);
#endif
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  /* any blocks held by allocate() */
  for (bd = small_alloc_list; bd; bd = bd->link) {
    total_blocks += bd->blocks;
  }
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#ifdef PROFILING
  if (RtsFlags.ProfFlags.doHeapProfile == HEAP_BY_RETAINER) {
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      total_blocks += retainerStackBlocks();
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  }
#endif

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  // count the blocks allocated by the arena allocator
  total_blocks += arenaBlocks();

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  /* count the blocks on the free list */
  free_blocks = countFreeList();

  if (total_blocks + free_blocks != mblocks_allocated *
      BLOCKS_PER_MBLOCK) {
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    debugBelch("Blocks: %ld live + %ld free  = %ld total (%ld around)\n",
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	    total_blocks, free_blocks, total_blocks + free_blocks,
	    mblocks_allocated * BLOCKS_PER_MBLOCK);
  }

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  ASSERT(total_blocks + free_blocks == mblocks_allocated * BLOCKS_PER_MBLOCK);
}
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nat
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countBlocks(bdescr *bd)