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/* An object allocator for Python.
Here is an introduction to the layers of the Python memory architecture,
showing where the object allocator is actually used (layer +2), It is
called for every object allocation and deallocation (PyObject_New/Del),
unless the object-specific allocators implement a proprietary allocation
scheme (ex.: ints use a simple free list). This is also the place where
the cyclic garbage collector operates selectively on container objects.
Object-specific allocators
_____ ______ ______ ________
[ int ] [ dict ] [ list ] ... [ string ] Python core |
+3 | <----- Object-specific memory -----> | <-- Non-object memory --> |
_______________________________ | |
[ Python's object allocator ] | |
+2 | ####### Object memory ####### | <------ Internal buffers ------> |
______________________________________________________________ |
[ Python's raw memory allocator (PyMem_ API) ] |
+1 | <----- Python memory (under PyMem manager's control) ------> | |
__________________________________________________________________
[ Underlying general-purpose allocator (ex: C library malloc) ]
0 | <------ Virtual memory allocated for the python process -------> |
=========================================================================
_______________________________________________________________________
[ OS-specific Virtual Memory Manager (VMM) ]
-1 | <--- Kernel dynamic storage allocation & management (page-based) ---> |
__________________________________ __________________________________
[ ] [ ]
-2 | <-- Physical memory: ROM/RAM --> | | <-- Secondary storage (swap) --> |
*/
/*==========================================================================*/
/* A fast, special-purpose memory allocator for small blocks, to be used
on top of a general-purpose malloc -- heavily based on previous art. */
/* Vladimir Marangozov -- August 2000 */
/*
* "Memory management is where the rubber meets the road -- if we do the wrong
* thing at any level, the results will not be good. And if we don't make the
* levels work well together, we are in serious trouble." (1)
*
* (1) Paul R. Wilson, Mark S. Johnstone, Michael Neely, and David Boles,
* "Dynamic Storage Allocation: A Survey and Critical Review",
* in Proc. 1995 Int'l. Workshop on Memory Management, September 1995.
*/
/* #undef WITH_MEMORY_LIMITS */ /* disable mem limit checks */
#define WITH_MALLOC_HOOKS /* for profiling & debugging */
/*==========================================================================*/
/*
* Public functions exported by this allocator.
*
* -- Define and use these names in your code to obtain or release memory --
*/
#define _THIS_MALLOC PyCore_OBJECT_MALLOC_FUNC
#define _THIS_CALLOC /* unused */
#define _THIS_REALLOC PyCore_OBJECT_REALLOC_FUNC
#define _THIS_FREE PyCore_OBJECT_FREE_FUNC
/*
* Underlying allocator's functions called by this allocator.
* The underlying allocator is usually the one which comes with libc.
*
* -- Don't use these functions in your code (to avoid mixing allocators) --
*
* Redefine these __only__ if you are using a 3rd party general purpose
* allocator which exports functions with names _other_ than the standard
* malloc, calloc, realloc, free.
*/
#define _SYSTEM_MALLOC PyCore_MALLOC_FUNC
#define _SYSTEM_CALLOC /* unused */
#define _SYSTEM_REALLOC PyCore_REALLOC_FUNC
#define _SYSTEM_FREE PyCore_FREE_FUNC
/*
* If malloc hooks are needed, names of the hooks' set & fetch
* functions exported by this allocator.
*/
#ifdef WITH_MALLOC_HOOKS
#define _SET_HOOKS _PyCore_ObjectMalloc_SetHooks
#define _FETCH_HOOKS _PyCore_ObjectMalloc_FetchHooks
#endif
/*==========================================================================*/
/*
* Allocation strategy abstract:
*
* For small requests, the allocator sub-allocates <Big> blocks of memory.
* Requests greater than 256 bytes are routed to the system's allocator.
*
* Small requests are grouped in size classes spaced 8 bytes apart, due
* to the required valid alignment of the returned address. Requests of
* a particular size are serviced from memory pools of 4K (one VMM page).
* Pools are fragmented on demand and contain free lists of blocks of one
* particular size class. In other words, there is a fixed-size allocator
* for each size class. Free pools are shared by the different allocators
* thus minimizing the space reserved for a particular size class.
*
* This allocation strategy is a variant of what is known as "simple
* segregated storage based on array of free lists". The main drawback of
* simple segregated storage is that we might end up with lot of reserved
* memory for the different free lists, which degenerate in time. To avoid
* this, we partition each free list in pools and we share dynamically the
* reserved space between all free lists. This technique is quite efficient
* for memory intensive programs which allocate mainly small-sized blocks.
*
* For small requests we have the following table:
*
* Request in bytes Size of allocated block Size class idx
* ----------------------------------------------------------------
* 1-8 8 0
* 9-16 16 1
* 17-24 24 2
* 25-32 32 3
* 33-40 40 4
* 41-48 48 5
* 49-56 56 6
* 57-64 64 7
* 65-72 72 8
* ... ... ...
* 241-248 248 30
* 249-256 256 31
*
* 0, 257 and up: routed to the underlying allocator.
*/
/*==========================================================================*/
/*
* -- Main tunable settings section --
*/
/*
* Alignment of addresses returned to the user. 8-bytes alignment works
* on most current architectures (with 32-bit or 64-bit address busses).
* The alignment value is also used for grouping small requests in size
* classes spaced ALIGNMENT bytes apart.
*
* You shouldn't change this unless you know what you are doing.
*/
#define ALIGNMENT 8 /* must be 2^N */
#define ALIGNMENT_SHIFT 3
#define ALIGNMENT_MASK (ALIGNMENT - 1)
/*
* Max size threshold below which malloc requests are considered to be
* small enough in order to use preallocated memory pools. You can tune
* this value according to your application behaviour and memory needs.
*
* The following invariants must hold:
* 1) ALIGNMENT <= SMALL_REQUEST_THRESHOLD <= 256
* 2) SMALL_REQUEST_THRESHOLD == N * ALIGNMENT
*
* Although not required, for better performance and space efficiency,
* it is recommended that SMALL_REQUEST_THRESHOLD is set to a power of 2.
*/
/*
* For Python compiled on systems with 32 bit pointers and integers,
* a value of 64 (= 8 * 8) is a reasonable speed/space tradeoff for
* the object allocator. To adjust automatically this threshold for
* systems with 64 bit pointers, we make this setting depend on a
* Python-specific slot size unit = sizeof(long) + sizeof(void *),
* which is expected to be 8, 12 or 16 bytes.
*/
#define _PYOBJECT_THRESHOLD ((SIZEOF_LONG + SIZEOF_VOID_P) * ALIGNMENT)
#define SMALL_REQUEST_THRESHOLD _PYOBJECT_THRESHOLD /* must be N * ALIGNMENT */
#define NB_SMALL_SIZE_CLASSES (SMALL_REQUEST_THRESHOLD / ALIGNMENT)
/*
* The system's VMM page size can be obtained on most unices with a
* getpagesize() call or deduced from various header files. To make
* things simpler, we assume that it is 4K, which is OK for most systems.
* It is probably better if this is the native page size, but it doesn't
* have to be.
*/
#define SYSTEM_PAGE_SIZE (4 * 1024)
#define SYSTEM_PAGE_SIZE_MASK (SYSTEM_PAGE_SIZE - 1)
/*
* Maximum amount of memory managed by the allocator for small requests.
*/
#ifdef WITH_MEMORY_LIMITS
#ifndef SMALL_MEMORY_LIMIT
#define SMALL_MEMORY_LIMIT (64 * 1024 * 1024) /* 64 MB -- more? */
#endif
#endif
/*
* The allocator sub-allocates <Big> blocks of memory (called arenas) aligned
* on a page boundary. This is a reserved virtual address space for the
* current process (obtained through a malloc call). In no way this means
* that the memory arenas will be used entirely. A malloc(<Big>) is usually
* an address range reservation for <Big> bytes, unless all pages within this
* space are referenced subsequently. So malloc'ing big blocks and not using
* them does not mean "wasting memory". It's an addressable range wastage...
*
* Therefore, allocating arenas with malloc is not optimal, because there is
* some address space wastage, but this is the most portable way to request
* memory from the system accross various platforms.
*/
#define ARENA_SIZE (256 * 1024 - SYSTEM_PAGE_SIZE) /* 256k - 1p */
#ifdef WITH_MEMORY_LIMITS
#define MAX_ARENAS (SMALL_MEMORY_LIMIT / ARENA_SIZE)
#endif
/*
* Size of the pools used for small blocks. Should be a power of 2,
* between 1K and SYSTEM_PAGE_SIZE, that is: 1k, 2k, 4k, eventually 8k.
*/
#define POOL_SIZE SYSTEM_PAGE_SIZE /* must be 2^N */
#define POOL_SIZE_MASK SYSTEM_PAGE_SIZE_MASK
#define POOL_MAGIC 0x74D3A651 /* authentication id */
#define ARENA_NB_POOLS (ARENA_SIZE / POOL_SIZE)
#define ARENA_NB_PAGES (ARENA_SIZE / SYSTEM_PAGE_SIZE)
/*
* -- End of tunable settings section --
*/
/*==========================================================================*/
/*
* Locking
*
* To reduce lock contention, it would probably be better to refine the
* crude function locking with per size class locking. I'm not positive
* however, whether it's worth switching to such locking policy because
* of the performance penalty it might introduce.
*
* The following macros describe the simplest (should also be the fastest)
* lock object on a particular platform and the init/fini/lock/unlock
* operations on it. The locks defined here are not expected to be recursive
* because it is assumed that they will always be called in the order:
* INIT, [LOCK, UNLOCK]*, FINI.
*/
/*
* Python's threads are serialized, so object malloc locking is disabled.
*/
#define SIMPLELOCK_DECL(lock) /* simple lock declaration */
#define SIMPLELOCK_INIT(lock) /* allocate (if needed) and initialize */
#define SIMPLELOCK_FINI(lock) /* free/destroy an existing lock */
#define SIMPLELOCK_LOCK(lock) /* acquire released lock */
#define SIMPLELOCK_UNLOCK(lock) /* release acquired lock */
/*
* Basic types
* I don't care if these are defined in <sys/types.h> or elsewhere. Axiom.
*/
#undef uchar
#define uchar unsigned char /* assuming == 8 bits */
#undef ushort
#define ushort unsigned short /* assuming >= 16 bits */
#undef uint
#define uint unsigned int /* assuming >= 16 bits */
#undef ulong
#define ulong unsigned long /* assuming >= 32 bits */
#undef off_t
#define off_t uint /* 16 bits <= off_t <= 64 bits */
/* When you say memory, my mind reasons in terms of (pointers to) blocks */
typedef uchar block;
/* Pool for small blocks */
struct pool_header {
union { block *_padding;
uint count; } ref; /* number of allocated blocks */
block *freeblock; /* pool's free list head */
struct pool_header *nextpool; /* next pool of this size class */
struct pool_header *prevpool; /* previous pool "" */
struct pool_header *pooladdr; /* pool address (always aligned) */
uint magic; /* pool magic number */
uint szidx; /* block size class index */
uint capacity; /* pool capacity in # of blocks */
};
typedef struct pool_header *poolp;
#undef ROUNDUP
#define ROUNDUP(x) (((x) + ALIGNMENT_MASK) & ~ALIGNMENT_MASK)
#define POOL_OVERHEAD ROUNDUP(sizeof(struct pool_header))
#define DUMMY_SIZE_IDX 0xffff /* size class of newly cached pools */
/*==========================================================================*/
/*
* This malloc lock
*/
SIMPLELOCK_DECL(_malloc_lock);
#define LOCK() SIMPLELOCK_LOCK(_malloc_lock)
#define UNLOCK() SIMPLELOCK_UNLOCK(_malloc_lock)
#define LOCK_INIT() SIMPLELOCK_INIT(_malloc_lock)
#define LOCK_FINI() SIMPLELOCK_FINI(_malloc_lock)
/*
* Pool table -- doubly linked lists of partially used pools
*/
#define PTA(x) ((poolp )((uchar *)&(usedpools[2*(x)]) - 2*sizeof(block *)))
#define PT(x) PTA(x), PTA(x)
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 */
};
/*
* Free (cached) pools
*/
static poolp freepools = NULL; /* free list for cached pools */
/*
* Arenas
*/
static uint arenacnt = 0; /* number of allocated arenas */
static uint watermark = ARENA_NB_POOLS; /* number of pools allocated from
the current arena */
static block *arenalist = NULL; /* list of allocated arenas */
static block *arenabase = NULL; /* free space start address in
current arena */
/*
* Hooks
*/
#ifdef WITH_MALLOC_HOOKS
static void *(*malloc_hook)(size_t) = NULL;
static void *(*calloc_hook)(size_t, size_t) = NULL;
static void *(*realloc_hook)(void *, size_t) = NULL;
static void (*free_hook)(void *) = NULL;
#endif /* !WITH_MALLOC_HOOKS */
/*==========================================================================*/
/* malloc */
/*
* The basic blocks are ordered by decreasing execution frequency,
* which minimizes the number of jumps in the most common cases,
* improves branching prediction and instruction scheduling (small
* block allocations typically result in a couple of instructions).
* Unless the optimizer reorders everything, being too smart...
*/
void *
_THIS_MALLOC(size_t nbytes)
{
block *bp;
poolp pool;
poolp next;
uint size;
#ifdef WITH_MALLOC_HOOKS
if (malloc_hook != NULL)
return (*malloc_hook)(nbytes);
#endif
/*
* This implicitly redirects malloc(0)
*/
if ((nbytes - 1) < SMALL_REQUEST_THRESHOLD) {
LOCK();
/*
* Most frequent paths first
*/
size = (uint )(nbytes - 1) >> ALIGNMENT_SHIFT;
pool = usedpools[size + size];
if (pool != pool->nextpool) {
/*
* There is a used pool for this size class.
* Pick up the head block of its free list.
*/
++pool->ref.count;
bp = pool->freeblock;
if ((pool->freeblock = *(block **)bp) != NULL) {
UNLOCK();
return (void *)bp;
}
/*
* Reached the end of the free list, try to extend it
*/
if (pool->ref.count < pool->capacity) {
/*
* There is room for another block
*/
size++;
size <<= ALIGNMENT_SHIFT; /* block size */
pool->freeblock = (block *)pool + \
POOL_OVERHEAD + \
pool->ref.count * size;
*(block **)(pool->freeblock) = NULL;
UNLOCK();
return (void *)bp;
}
/*
* Pool is full, unlink from used pools
*/
next = pool->nextpool;
pool = pool->prevpool;
next->prevpool = pool;
pool->nextpool = next;
UNLOCK();
return (void *)bp;
}
/*
* Try to get a cached free pool
*/
pool = freepools;
if (pool != NULL) {
/*
* Unlink from cached pools
*/
freepools = pool->nextpool;
init_pool:
/*
* Frontlink to used pools
*/
next = usedpools[size + size]; /* == prev */
pool->nextpool = next;
pool->prevpool = next;
next->nextpool = pool;
next->prevpool = pool;
pool->ref.count = 1;
if (pool->szidx == size) {
/*
* Luckily, this pool last contained blocks
* of the same size class, so its header
* and free list are already initialized.
*/
bp = pool->freeblock;
pool->freeblock = *(block **)bp;
UNLOCK();
return (void *)bp;
}
/*
* Initialize the pool header and free list
* then return the first block.
*/
pool->szidx = size;
size++;
size <<= ALIGNMENT_SHIFT; /* block size */
bp = (block *)pool + POOL_OVERHEAD;
pool->freeblock = bp + size;
*(block **)(pool->freeblock) = NULL;
pool->capacity = (POOL_SIZE - POOL_OVERHEAD) / size;
UNLOCK();
return (void *)bp;
}
/*
* Allocate new pool
*/
if (watermark < ARENA_NB_POOLS) {
/* commit malloc(POOL_SIZE) from the current arena */
commit_pool:
watermark++;
pool = (poolp )arenabase;
arenabase += POOL_SIZE;
pool->pooladdr = pool;
pool->magic = (uint )POOL_MAGIC;
pool->szidx = DUMMY_SIZE_IDX;
goto init_pool;
}
/*
* Allocate new arena
*/
#ifdef WITH_MEMORY_LIMITS
if (!(arenacnt < MAX_ARENAS)) {
UNLOCK();
goto redirect;
}
#endif
/*
* With malloc, we can't avoid loosing one page address space
* per arena due to the required alignment on page boundaries.
*/
bp = (block *)_SYSTEM_MALLOC(ARENA_SIZE + SYSTEM_PAGE_SIZE);
if (bp == NULL) {
UNLOCK();
goto redirect;
}
/*
* Keep a reference in the list of allocated arenas. We might
* want to release (some of) them in the future. The first
* word is never used, no matter whether the returned address
* is page-aligned or not, so we safely store a pointer in it.
*/
*(block **)bp = arenalist;
arenalist = bp;
arenacnt++;
watermark = 0;
/* Page-round up */
arenabase = bp + (SYSTEM_PAGE_SIZE -
((off_t )bp & SYSTEM_PAGE_SIZE_MASK));
goto commit_pool;
}
/* The small block allocator ends here. */
redirect:
/*
* Redirect the original request to the underlying (libc) allocator.
* We jump here on bigger requests, on error in the code above (as a
* last chance to serve the request) or when the max memory limit
* has been reached.
*/
return (void *)_SYSTEM_MALLOC(nbytes);
}
/* free */
void
_THIS_FREE(void *p)
{
poolp pool;
poolp next, prev;
uint size;
off_t offset;
#ifdef WITH_MALLOC_HOOKS
if (free_hook != NULL) {
(*free_hook)(p);
return;
}
#endif
if (p == NULL) /* free(NULL) has no effect */
return;
offset = (off_t )p & POOL_SIZE_MASK;
pool = (poolp )((block *)p - offset);
if (pool->pooladdr != pool || pool->magic != (uint )POOL_MAGIC) {
_SYSTEM_FREE(p);
return;
}
LOCK();
/*
* At this point, the pool is not empty
*/
if ((*(block **)p = pool->freeblock) == NULL) {
/*
* Pool was full
*/
pool->freeblock = (block *)p;
--pool->ref.count;
/*
* Frontlink to used pools
* This mimics LRU pool usage for new allocations and
* targets optimal filling when several pools contain
* blocks of the same size class.
*/
size = pool->szidx;
next = usedpools[size + size];
prev = next->prevpool;
pool->nextpool = next;
pool->prevpool = prev;
next->prevpool = pool;
prev->nextpool = pool;
UNLOCK();
return;
}
/*
* Pool was not full
*/
pool->freeblock = (block *)p;
if (--pool->ref.count != 0) {
UNLOCK();
return;
}
/*
* Pool is now empty, unlink from used pools
*/
next = pool->nextpool;
prev = pool->prevpool;
next->prevpool = prev;
prev->nextpool = next;
/*
* Frontlink to free pools
* This ensures that previously freed pools will be allocated
* later (being not referenced, they are perhaps paged out).
*/
pool->nextpool = freepools;
freepools = pool;
UNLOCK();
return;
}
/* realloc */
void *
_THIS_REALLOC(void *p, size_t nbytes)
{
block *bp;
poolp pool;
uint size;
#ifdef WITH_MALLOC_HOOKS
if (realloc_hook != NULL)
return (*realloc_hook)(p, nbytes);
#endif
if (p == NULL)
return _THIS_MALLOC(nbytes);
/* realloc(p, 0) on big blocks is redirected. */
pool = (poolp )((block *)p - ((off_t )p & POOL_SIZE_MASK));
if (pool->pooladdr != pool || pool->magic != (uint )POOL_MAGIC) {
/* We haven't allocated this block */
if (!(nbytes > SMALL_REQUEST_THRESHOLD) && nbytes) {
/* small request */
size = nbytes;
goto malloc_copy_free;
}
bp = (block *)_SYSTEM_REALLOC(p, nbytes);
}
else {
/* We're in charge of this block */
size = (pool->szidx + 1) << ALIGNMENT_SHIFT; /* block size */
if (size >= nbytes) {
/* Don't bother if a smaller size was requested
except for realloc(p, 0) == free(p), ret NULL */
if (nbytes == 0) {
_THIS_FREE(p);
bp = NULL;
}
else
bp = (block *)p;
}
else {
malloc_copy_free:
bp = (block *)_THIS_MALLOC(nbytes);
if (bp != NULL) {
memcpy(bp, p, size);
_THIS_FREE(p);
}
}
}
return (void *)bp;
}
/* calloc */
/* -- unused --
void *
_THIS_CALLOC(size_t nbel, size_t elsz)
{
void *p;
size_t nbytes;
#ifdef WITH_MALLOC_HOOKS
if (calloc_hook != NULL)
return (*calloc_hook)(nbel, elsz);
#endif
nbytes = nbel * elsz;
p = _THIS_MALLOC(nbytes);
if (p != NULL)
memset(p, 0, nbytes);
return p;
}
*/
/*==========================================================================*/
/*
* Hooks
*/
#ifdef WITH_MALLOC_HOOKS
void
_SET_HOOKS( void *(*malloc_func)(size_t),
void *(*calloc_func)(size_t, size_t),
void *(*realloc_func)(void *, size_t),
void (*free_func)(void *) )
{
LOCK();
malloc_hook = malloc_func;
calloc_hook = calloc_func;
realloc_hook = realloc_func;
free_hook = free_func;
UNLOCK();
}
void
_FETCH_HOOKS( void *(**malloc_funcp)(size_t),
void *(**calloc_funcp)(size_t, size_t),
void *(**realloc_funcp)(void *, size_t),
void (**free_funcp)(void *) )
{
LOCK();
*malloc_funcp = malloc_hook;
*calloc_funcp = calloc_hook;
*realloc_funcp = realloc_hook;
*free_funcp = free_hook;
UNLOCK();
}
#endif /* !WITH_MALLOC_HOOKS */
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