Python memory management

obmalloc.c

/*
 * Python的内存分为
 * Block
 * Pool
 * arena
 * 内存池几个部分
*/

#undef PyObject_Malloc
void *
PyObject_Malloc(size_t nbytes)
{
	block *bp;
	poolp pool;
	poolp next;
	uint size;

	/*
	 * 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;
			assert(bp != NULL);
			if ((pool->freeblock = *(block **)bp) != NULL) {
				UNLOCK();
				return (void *)bp;
			}
			/*
			 * Reached the end of the free list, try to extend it.
			 */
			if (pool->nextoffset <= pool->maxnextoffset) {
				/* There is room for another block. */
				pool->freeblock = (block*)pool +
						  pool->nextoffset;
				pool->nextoffset += INDEX2SIZE(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;
		}

		/* There isn't a pool of the right size class immediately
		 * available:  use a free pool.
		 */
		if (usable_arenas == NULL) {
			/* No arena has a free pool:  allocate a new arena. */
#ifdef WITH_MEMORY_LIMITS
			if (narenas_currently_allocated >= MAX_ARENAS) {
				UNLOCK();
				goto redirect;
			}
#endif
			usable_arenas = new_arena();
			if (usable_arenas == NULL) {
				UNLOCK();
				goto redirect;
			}
			usable_arenas->nextarena =
				usable_arenas->prevarena = NULL;
		}
		assert(usable_arenas->address != 0);

		/* Try to get a cached free pool. */
		pool = usable_arenas->freepools;
		if (pool != NULL) {
			/* Unlink from cached pools. */
			usable_arenas->freepools = pool->nextpool;

			/* This arena already had the smallest nfreepools
			 * value, so decreasing nfreepools doesn't change
			 * that, and we don't need to rearrange the
			 * usable_arenas list.  However, if the arena has
			 * become wholly allocated, we need to remove its
			 * arena_object from usable_arenas.
			 */
			--usable_arenas->nfreepools;
			if (usable_arenas->nfreepools == 0) {
				/* Wholly allocated:  remove. */
				assert(usable_arenas->freepools == NULL);
				assert(usable_arenas->nextarena == NULL ||
				       usable_arenas->nextarena->prevarena ==
					   usable_arenas);

				usable_arenas = usable_arenas->nextarena;
				if (usable_arenas != NULL) {
					usable_arenas->prevarena = NULL;
					assert(usable_arenas->address != 0);
				}
			}
			else {
				/* nfreepools > 0:  it must be that freepools
				 * isn't NULL, or that we haven't yet carved
				 * off all the arena's pools for the first
				 * time.
				 */
				assert(usable_arenas->freepools != NULL ||
				       usable_arenas->pool_address <=
				           (block*)usable_arenas->address +
				               ARENA_SIZE - POOL_SIZE);
			}
		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, set up the free list to
			 * contain just the second block, and return the first
			 * block.
			 */
			pool->szidx = size;
			size = INDEX2SIZE(size);
			bp = (block *)pool + POOL_OVERHEAD;
			pool->nextoffset = POOL_OVERHEAD + (size << 1);
			pool->maxnextoffset = POOL_SIZE - size;
			pool->freeblock = bp + size;
			*(block **)(pool->freeblock) = NULL;
			UNLOCK();
			return (void *)bp;
		}

		/* Carve off a new pool. */
		assert(usable_arenas->nfreepools > 0);
		assert(usable_arenas->freepools == NULL);
		pool = (poolp)usable_arenas->pool_address;
		assert((block*)pool <= (block*)usable_arenas->address +
		                       ARENA_SIZE - POOL_SIZE);
		pool->arenaindex = usable_arenas - arenas;
		assert(&arenas[pool->arenaindex] == usable_arenas);
		pool->szidx = DUMMY_SIZE_IDX;
		usable_arenas->pool_address += POOL_SIZE;
		--usable_arenas->nfreepools;

		if (usable_arenas->nfreepools == 0) {
			assert(usable_arenas->nextarena == NULL ||
			       usable_arenas->nextarena->prevarena ==
			       	   usable_arenas);
			/* Unlink the arena:  it is completely allocated. */
			usable_arenas = usable_arenas->nextarena;
			if (usable_arenas != NULL) {
				usable_arenas->prevarena = NULL;
				assert(usable_arenas->address != 0);
			}
		}

		goto init_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.
	 */
	if (nbytes == 0)
		nbytes = 1;
	return (void *)malloc(nbytes);
}