obmalloc.c

python s60 1.4.5版本的源代码

    It's not linked to from anything then anymore, and its nextpool and
    prevpool members are meaningless until it transitions back to used.
    A free of a block in a full pool puts the pool back in the used state.
    Then it's linked in at the front of the appropriate usedpools[] list, so
    that the next allocation for its size class will reuse the freed block.

empty == all the pool's blocks are currently available for allocation
    On transition to empty, a pool is unlinked from its usedpools[] list,
    and linked to the front of its arena_object's singly-linked freepools list,
    via its nextpool member.  The prevpool member has no meaning in this case.
    Empty pools have no inherent size class:  the next time a malloc finds
    an empty list in usedpools[], it takes the first pool off of freepools.
    If the size class needed happens to be the same as the size class the pool
    last had, some pool initialization can be skipped.


Block Management

Blocks within pools are again carved out as needed.  pool->freeblock points to
the start of a singly-linked list of free blocks within the pool.  When a
block is freed, it's inserted at the front of its pool's freeblock list.  Note
that the available blocks in a pool are *not* linked all together when a pool
is initialized.  Instead only "the first two" (lowest addresses) blocks are
set up, returning the first such block, and setting pool->freeblock to a
one-block list holding the second such block.  This is consistent with that
pymalloc strives at all levels (arena, pool, and block) never to touch a piece
of memory until it's actually needed.

So long as a pool is in the used state, we're certain there *is* a block
available for allocating, and pool->freeblock is not NULL.  If pool->freeblock
points to the end of the free list before we've carved the entire pool into
blocks, that means we simply haven't yet gotten to one of the higher-address
blocks.  The offset from the pool_header to the start of "the next" virgin
block is stored in the pool_header nextoffset member, and the largest value
of nextoffset that makes sense is stored in the maxnextoffset member when a
pool is initialized.  All the blocks in a pool have been passed out at least
once when and only when nextoffset > maxnextoffset.


Major obscurity:  While the usedpools vector is declared to have poolp
entries, it doesn't really.  It really contains two pointers per (conceptual)
poolp entry, the nextpool and prevpool members of a pool_header.  The
excruciating initialization code below fools C so that

    usedpool[i+i]

"acts like" a genuine poolp, but only so long as you only reference its
nextpool and prevpool members.  The "- 2*sizeof(block *)" gibberish is
compensating for that a pool_header's nextpool and prevpool members
immediately follow a pool_header's first two members:

	union { block *_padding;
		uint count; } ref;
	block *freeblock;

each of which consume sizeof(block *) bytes.  So what usedpools[i+i] really
contains is a fudged-up pointer p such that *if* C believes it's a poolp
pointer, then p->nextpool and p->prevpool are both p (meaning that the headed
circular list is empty).

It's unclear why the usedpools setup is so convoluted.  It could be to
minimize the amount of cache required to hold this heavily-referenced table
(which only *needs* the two interpool pointer members of a pool_header). OTOH,
referencing code has to remember to "double the index" and doing so isn't
free, usedpools[0] isn't a strictly legal pointer, and we're crucially relying
on that C doesn't insert any padding anywhere in a pool_header at or before
the prevpool member.
**************************************************************************** */


#define PTA(x)	((poolp )((uchar *)&(usedpools[2*(x)]) - 2*sizeof(block *)))
#define PT(x)	PTA(x), PTA(x)

#ifndef SYMBIAN
static poolp usedpools[2 * ((NB_SMALL_SIZE_CLASSES + 7) / 8) * 8] = {
	PT(0), PT(1), PT(2), PT(3), PT(4), PT(5), PT(6), PT(7)
#if NB_SMALL_SIZE_CLASSES > 8
	, PT(8), PT(9), PT(10), PT(11), PT(12), PT(13), PT(14), PT(15)
#if NB_SMALL_SIZE_CLASSES > 16
	, PT(16), PT(17), PT(18), PT(19), PT(20), PT(21), PT(22), PT(23)
#if NB_SMALL_SIZE_CLASSES > 24
	, PT(24), PT(25), PT(26), PT(27), PT(28), PT(29), PT(30), PT(31)
#if NB_SMALL_SIZE_CLASSES > 32
	, PT(32), PT(33), PT(34), PT(35), PT(36), PT(37), PT(38), PT(39)
#if NB_SMALL_SIZE_CLASSES > 40
	, PT(40), PT(41), PT(42), PT(43), PT(44), PT(45), PT(46), PT(47)
#if NB_SMALL_SIZE_CLASSES > 48
	, PT(48), PT(49), PT(50), PT(51), PT(52), PT(53), PT(54), PT(55)
#if NB_SMALL_SIZE_CLASSES > 56
	, PT(56), PT(57), PT(58), PT(59), PT(60), PT(61), PT(62), PT(63)
#endif /* NB_SMALL_SIZE_CLASSES > 56 */
#endif /* NB_SMALL_SIZE_CLASSES > 48 */
#endif /* NB_SMALL_SIZE_CLASSES > 40 */
#endif /* NB_SMALL_SIZE_CLASSES > 32 */
#endif /* NB_SMALL_SIZE_CLASSES > 24 */
#endif /* NB_SMALL_SIZE_CLASSES > 16 */
#endif /* NB_SMALL_SIZE_CLASSES >  8 */
};
#else
/* XXX remove WSD */

/*==========================================================================
Arena management.

`arenas` is a vector of arena_objects.  It contains maxarenas entries, some of
which may not be currently used (== they're arena_objects that aren't
currently associated with an allocated arena).  Note that arenas proper are
separately malloc'ed.

Prior to Python 2.5, arenas were never free()'ed.  Starting with Python 2.5,
we do try to free() arenas, and use some mild heuristic strategies to increase
the likelihood that arenas eventually can be freed.

unused_arena_objects

    This is a singly-linked list of the arena_objects that are currently not
    being used (no arena is associated with them).  Objects are taken off the
    head of the list in new_arena(), and are pushed on the head of the list in
    PyObject_Free() when the arena is empty.  Key invariant:  an arena_object
    is on this list if and only if its .address member is 0.

usable_arenas

    This is a doubly-linked list of the arena_objects associated with arenas
    that have pools available.  These pools are either waiting to be reused,
    or have not been used before.  The list is sorted to have the most-
    allocated arenas first (ascending order based on the nfreepools member).
    This means that the next allocation will come from a heavily used arena,
    which gives the nearly empty arenas a chance to be returned to the system.
    In my unscientific tests this dramatically improved the number of arenas
    that could be freed.

Note that an arena_object associated with an arena all of whose pools are
currently in use isn't on either list.
*/

/* Array of objects used to track chunks of memory (arenas). */
static struct arena_object* arenas = NULL;
/* Number of slots currently allocated in the `arenas` vector. */
static uint maxarenas = 0;

/* The head of the singly-linked, NULL-terminated list of available
 * arena_objects.
 */
static struct arena_object* unused_arena_objects = NULL;

/* The head of the doubly-linked, NULL-terminated at each end, list of
 * arena_objects associated with arenas that have pools available.
 */
static struct arena_object* usable_arenas = NULL;

/* How many arena_objects do we initially allocate?
 * 16 = can allocate 16 arenas = 16 * ARENA_SIZE = 4MB before growing the
 * `arenas` vector.
 */
#define INITIAL_ARENA_OBJECTS 16

/* Number of arenas allocated that haven't been free()'d. */
static size_t narenas_currently_allocated = 0;

extern int obmalloc_globals_init()
{
    SPy_Python_globals* pyglobals = PYTHON_GLOBALS;
    int sz = (2 * ((NB_SMALL_SIZE_CLASSES + 7) / 8) * 8);
    int i, array_index;

    poolp* usedpoolarray = PyMem_Malloc(sz*sizeof(poolp));
    if (!usedpoolarray)
      return (-1);

    for (i = 0, array_index = 0; i < (sz/2); i++) {
      usedpoolarray[array_index++] = ((poolp )((uchar *)&(usedpoolarray[2*(i)]) - 2*sizeof(block *)));
      usedpoolarray[array_index++] = ((poolp )((uchar *)&(usedpoolarray[2*(i)]) - 2*sizeof(block *)));
    } 
    
    pyglobals->usedpools = usedpoolarray;
    pyglobals->freepools = NULL;

    arenas = NULL;
    maxarenas = 0;
    unused_arena_objects = NULL;
    usable_arenas = NULL;
    narenas_currently_allocated = 0;

#ifdef WITH_DLC
    if (dlc_init() != 0)
        return -1;
#endif
    
    return 0;
}


#define OBMALLOC_PYTHON_GLOBALS PYTHON_GLOBALS
#define usedpools ((poolp*)(OBMALLOC_PYTHON_GLOBALS->usedpools))
//#define freepools ((poolp)(OBMALLOC_PYTHON_GLOBALS->freepools))

extern int obmalloc_globals_fini()
{
#ifdef WITH_DLC
    dlc_fini();
#endif
    PyMem_Free(usedpools);
    return 0;
}

#endif 

#ifdef PYMALLOC_DEBUG
/* Total number of times malloc() called to allocate an arena. */
static size_t ntimes_arena_allocated = 0;
/* High water mark (max value ever seen) for narenas_currently_allocated. */
static size_t narenas_highwater = 0;
#endif

/* Allocate a new arena.  If we run out of memory, return NULL.  Else
 * allocate a new arena, and return the address of an arena_object
 * describing the new arena.  It's expected that the caller will set
 * `usable_arenas` to the return value.
 */
static struct arena_object*
new_arena(void)
{
	struct arena_object* arenaobj;
	uint excess;	/* number of bytes above pool alignment */

#ifdef PYMALLOC_DEBUG
	if (Py_GETENV("PYTHONMALLOCSTATS"))
		_PyObject_DebugMallocStats();
#endif
	if (unused_arena_objects == NULL) {
		uint i;
		uint numarenas;
		size_t nbytes;

		/* Double the number of arena objects on each allocation.
		 * Note that it's possible for `numarenas` to overflow.
		 */
		numarenas = maxarenas ? maxarenas << 1 : INITIAL_ARENA_OBJECTS;
		if (numarenas <= maxarenas)
			return NULL;	/* overflow */
		nbytes = numarenas * sizeof(*arenas);
		if (nbytes / sizeof(*arenas) != numarenas)
			return NULL;	/* overflow */
		arenaobj = (struct arena_object *)_SYSTEM_REALLOC(arenas, nbytes);
		if (arenaobj == NULL)
			return NULL;
		arenas = arenaobj;

		/* We might need to fix pointers that were copied.  However,
		 * new_arena only gets called when all the pages in the
		 * previous arenas are full.  Thus, there are *no* pointers
		 * into the old array. Thus, we don't have to worry about
		 * invalid pointers.  Just to be sure, some asserts:
		 */
		assert(usable_arenas == NULL);
		assert(unused_arena_objects == NULL);

		/* Put the new arenas on the unused_arena_objects list. */
		for (i = maxarenas; i < numarenas; ++i) {
			arenas[i].address = 0;	/* mark as unassociated */
			arenas[i].nextarena = i < numarenas - 1 ?
					       &arenas[i+1] : NULL;
		}

		/* Update globals. */
		unused_arena_objects = &arenas[maxarenas];
		maxarenas = numarenas;
	}

	/* Take the next available arena object off the head of the list. */
	assert(unused_arena_objects != NULL);
	arenaobj = unused_arena_objects;
	unused_arena_objects = arenaobj->nextarena;
	assert(arenaobj->address == 0);
#ifdef WITH_DLC    
    arenaobj->address = (uptr)dlc_alloc(ARENA_SIZE);
#else
    arenaobj->address = (uptr)_SYSTEM_MALLOC(ARENA_SIZE);
#endif
	if (arenaobj->address == 0) {
		/* The allocation failed: return NULL after putting the
		 * arenaobj back.
		 */
		arenaobj->nextarena = unused_arena_objects;
		unused_arena_objects = arenaobj;
		return NULL;
	}

	++narenas_currently_allocated;
#ifdef PYMALLOC_DEBUG
	++ntimes_arena_allocated;
	if (narenas_currently_allocated > narenas_highwater)
		narenas_highwater = narenas_currently_allocated;
#endif
	arenaobj->freepools = NULL;
	/* pool_address <- first pool-aligned address in the arena
	   nfreepools <- number of whole pools that fit after alignment */
	arenaobj->pool_address = (block*)arenaobj->address;
	arenaobj->nfreepools = ARENA_SIZE / POOL_SIZE;
	assert(POOL_SIZE * arenaobj->nfreepools == ARENA_SIZE);
	excess = (uint)(arenaobj->address & POOL_SIZE_MASK);
	if (excess != 0) {
		--arenaobj->nfreepools;
		arenaobj->pool_address += POOL_SIZE - excess;
	}
	arenaobj->ntotalpools = arenaobj->nfreepools;

	return arenaobj;
}

/*
Py_ADDRESS_IN_RANGE(P, POOL)

Return true if and only if P is an address that was allocated by pymalloc.
POOL must be the pool address associated with P, i.e., POOL = POOL_ADDR(P)
(the caller is asked to compute this because the macro expands POOL more than
once, and for efficiency it's best for the caller to assign POOL_ADDR(P) to a
variable and pass the latter to the macro; because Py_ADDRESS_IN_RANGE is
called on every alloc/realloc/free, micro-efficiency is important here).

Tricky:  Let B be the arena base address associated with the pool, B =
arenas[(POOL)->arenaindex].address.  Then P belongs to the arena if and only if

	B <= P < B + ARENA_SIZE

Subtracting B throughout, this is true iff

	0 <= P-B < ARENA_SIZE

By using unsigned arithmetic, the "0 <=" half of the test can be skipped.

Obscure:  A PyMem "free memory" function can call the pymalloc free or realloc
before the first arena has been allocated.  `arenas` is still NULL in that
case.  We're relying on that maxarenas is also 0 in that case, so that
(POOL)->arenaindex < maxarenas  must be false, saving us from trying to index
into a NULL arenas.

Details:  given P and POOL, the arena_object corresponding to P is AO =
arenas[(POOL)->arenaindex].  Suppose obmalloc controls P.  Then (barring wild
stores, etc), POOL is the correct address of P's pool, AO.address is the
correct base address of the pool's arena, and P must be within ARENA_SIZE of
AO.address.  In addition, AO.address is not 0 (no arena can start at address 0
(NULL)).  Therefore Py_ADDRESS_IN_RANGE correctly reports that obmalloc
controls P.

Now suppose obmalloc does not control P (e.g., P was obtained via a direct
call to the system malloc() or realloc()).  (POOL)->arenaindex may be anything
in this case -- it may even be uninitialized trash.  If the trash arenaindex
is >= maxarenas, the macro correctly concludes at once that obmalloc doesn't
control P.

Else arenaindex is < maxarena, and AO is read up.  If AO corresponds to an
allocated arena, obmalloc controls all the memory in slice AO.address :
AO.address+ARENA_SIZE.  By case assumption, P is not controlled by obmalloc,
so P doesn't lie in that slice, so the macro correctly reports that P is not
controlled by obmalloc.

Finally, if P is not controlled by obmalloc and AO corresponds to an unused
arena_object (one not currently associated with an allocated arena),
AO.address is 0, and the second test in the macro reduces to:

	P < ARENA_SIZE

If P >= ARENA_SIZE (extremely likely), the macro again correctly concludes
that P is not controlled by obmalloc.  However, if P < ARENA_SIZE, this part
of the test still passes, and the third clause (AO.address != 0) is necessary