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@ -1,443 +0,0 @@
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/* An object allocator for Python.
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Here is an introduction to the layers of the Python memory architecture,
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showing where the object allocator is actually used (layer +2), It is
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called for every object allocation and deallocation (PyObject_New/Del),
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unless the object-specific allocators implement a proprietary allocation
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scheme (ex.: ints use a simple free list). This is also the place where
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the cyclic garbage collector operates selectively on container objects.
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Object-specific allocators
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_____ ______ ______ ________
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[ int ] [ dict ] [ list ] ... [ string ] Python core |
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+3 | <----- Object-specific memory -----> | <-- Non-object memory --> |
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_______________________________ | |
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[ Python's object allocator ] | |
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+2 | ####### Object memory ####### | <------ Internal buffers ------> |
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______________________________________________________________ |
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[ Python's raw memory allocator (PyMem_ API) ] |
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+1 | <----- Python memory (under PyMem manager's control) ------> | |
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__________________________________________________________________
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[ Underlying general-purpose allocator (ex: C library malloc) ]
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0 | <------ Virtual memory allocated for the python process -------> |
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=========================================================================
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_______________________________________________________________________
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[ OS-specific Virtual Memory Manager (VMM) ]
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-1 | <--- Kernel dynamic storage allocation & management (page-based) ---> |
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__________________________________ __________________________________
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[ ] [ ]
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-2 | <-- Physical memory: ROM/RAM --> | | <-- Secondary storage (swap) --> |
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*/
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/*==========================================================================*/
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/* A fast, special-purpose memory allocator for small blocks, to be used
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on top of a general-purpose malloc -- heavily based on previous art. */
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/* Vladimir Marangozov -- August 2000 */
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/*
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* "Memory management is where the rubber meets the road -- if we do the wrong
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* thing at any level, the results will not be good. And if we don't make the
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* levels work well together, we are in serious trouble." (1)
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*
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* (1) Paul R. Wilson, Mark S. Johnstone, Michael Neely, and David Boles,
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* "Dynamic Storage Allocation: A Survey and Critical Review",
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* in Proc. 1995 Int'l. Workshop on Memory Management, September 1995.
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*/
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#ifndef Py_INTERNAL_PYMALLOC_H
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#define Py_INTERNAL_PYMALLOC_H
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/* #undef WITH_MEMORY_LIMITS */ /* disable mem limit checks */
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/*==========================================================================*/
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/*
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* Allocation strategy abstract:
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*
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* For small requests, the allocator sub-allocates <Big> blocks of memory.
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* Requests greater than SMALL_REQUEST_THRESHOLD bytes are routed to the
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* system's allocator.
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*
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* Small requests are grouped in size classes spaced 8 bytes apart, due
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* to the required valid alignment of the returned address. Requests of
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* a particular size are serviced from memory pools of 4K (one VMM page).
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* Pools are fragmented on demand and contain free lists of blocks of one
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* particular size class. In other words, there is a fixed-size allocator
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* for each size class. Free pools are shared by the different allocators
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* thus minimizing the space reserved for a particular size class.
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*
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* This allocation strategy is a variant of what is known as "simple
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* segregated storage based on array of free lists". The main drawback of
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* simple segregated storage is that we might end up with lot of reserved
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* memory for the different free lists, which degenerate in time. To avoid
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* this, we partition each free list in pools and we share dynamically the
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* reserved space between all free lists. This technique is quite efficient
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* for memory intensive programs which allocate mainly small-sized blocks.
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*
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* For small requests we have the following table:
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*
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* Request in bytes Size of allocated block Size class idx
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* ----------------------------------------------------------------
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* 1-8 8 0
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* 9-16 16 1
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* 17-24 24 2
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* 25-32 32 3
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* 33-40 40 4
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* 41-48 48 5
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* 49-56 56 6
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* 57-64 64 7
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* 65-72 72 8
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* ... ... ...
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* 497-504 504 62
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* 505-512 512 63
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*
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* 0, SMALL_REQUEST_THRESHOLD + 1 and up: routed to the underlying
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* allocator.
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*/
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/*==========================================================================*/
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/*
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* -- Main tunable settings section --
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*/
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/*
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* Alignment of addresses returned to the user. 8-bytes alignment works
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* on most current architectures (with 32-bit or 64-bit address busses).
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* The alignment value is also used for grouping small requests in size
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* classes spaced ALIGNMENT bytes apart.
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*
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* You shouldn't change this unless you know what you are doing.
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*/
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#define ALIGNMENT 8 /* must be 2^N */
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#define ALIGNMENT_SHIFT 3
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/* Return the number of bytes in size class I, as a uint. */
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#define INDEX2SIZE(I) (((unsigned int)(I) + 1) << ALIGNMENT_SHIFT)
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/*
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* Max size threshold below which malloc requests are considered to be
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* small enough in order to use preallocated memory pools. You can tune
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* this value according to your application behaviour and memory needs.
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*
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* Note: a size threshold of 512 guarantees that newly created dictionaries
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* will be allocated from preallocated memory pools on 64-bit.
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*
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* The following invariants must hold:
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* 1) ALIGNMENT <= SMALL_REQUEST_THRESHOLD <= 512
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* 2) SMALL_REQUEST_THRESHOLD is evenly divisible by ALIGNMENT
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*
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* Although not required, for better performance and space efficiency,
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* it is recommended that SMALL_REQUEST_THRESHOLD is set to a power of 2.
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*/
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#define SMALL_REQUEST_THRESHOLD 512
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#define NB_SMALL_SIZE_CLASSES (SMALL_REQUEST_THRESHOLD / ALIGNMENT)
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#if NB_SMALL_SIZE_CLASSES > 64
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#error "NB_SMALL_SIZE_CLASSES should be less than 64"
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#endif /* NB_SMALL_SIZE_CLASSES > 64 */
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/*
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* The system's VMM page size can be obtained on most unices with a
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* getpagesize() call or deduced from various header files. To make
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* things simpler, we assume that it is 4K, which is OK for most systems.
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* It is probably better if this is the native page size, but it doesn't
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* have to be. In theory, if SYSTEM_PAGE_SIZE is larger than the native page
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* size, then `POOL_ADDR(p)->arenaindex' could rarely cause a segmentation
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* violation fault. 4K is apparently OK for all the platforms that python
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* currently targets.
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*/
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#define SYSTEM_PAGE_SIZE (4 * 1024)
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#define SYSTEM_PAGE_SIZE_MASK (SYSTEM_PAGE_SIZE - 1)
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/*
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* Maximum amount of memory managed by the allocator for small requests.
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*/
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#ifdef WITH_MEMORY_LIMITS
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#ifndef SMALL_MEMORY_LIMIT
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#define SMALL_MEMORY_LIMIT (64 * 1024 * 1024) /* 64 MiB -- more? */
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#endif
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#endif
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/*
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* The allocator sub-allocates <Big> blocks of memory (called arenas) aligned
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* on a page boundary. This is a reserved virtual address space for the
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* current process (obtained through a malloc()/mmap() call). In no way this
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* means that the memory arenas will be used entirely. A malloc(<Big>) is
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* usually an address range reservation for <Big> bytes, unless all pages within
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* this space are referenced subsequently. So malloc'ing big blocks and not
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* using them does not mean "wasting memory". It's an addressable range
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* wastage...
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*
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* Arenas are allocated with mmap() on systems supporting anonymous memory
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* mappings to reduce heap fragmentation.
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*/
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#define ARENA_SIZE (256 << 10) /* 256 KiB */
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#ifdef WITH_MEMORY_LIMITS
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#define MAX_ARENAS (SMALL_MEMORY_LIMIT / ARENA_SIZE)
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#endif
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/*
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* Size of the pools used for small blocks. Should be a power of 2,
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* between 1K and SYSTEM_PAGE_SIZE, that is: 1k, 2k, 4k.
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*/
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#define POOL_SIZE SYSTEM_PAGE_SIZE /* must be 2^N */
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#define POOL_SIZE_MASK SYSTEM_PAGE_SIZE_MASK
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/*
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* -- End of tunable settings section --
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*/
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/*==========================================================================*/
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/*
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* Locking
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*
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* To reduce lock contention, it would probably be better to refine the
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* crude function locking with per size class locking. I'm not positive
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* however, whether it's worth switching to such locking policy because
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* of the performance penalty it might introduce.
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*
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* The following macros describe the simplest (should also be the fastest)
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* lock object on a particular platform and the init/fini/lock/unlock
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* operations on it. The locks defined here are not expected to be recursive
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* because it is assumed that they will always be called in the order:
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* INIT, [LOCK, UNLOCK]*, FINI.
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*/
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/*
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* Python's threads are serialized, so object malloc locking is disabled.
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*/
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#define SIMPLELOCK_DECL(lock) /* simple lock declaration */
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#define SIMPLELOCK_INIT(lock) /* allocate (if needed) and initialize */
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#define SIMPLELOCK_FINI(lock) /* free/destroy an existing lock */
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#define SIMPLELOCK_LOCK(lock) /* acquire released lock */
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#define SIMPLELOCK_UNLOCK(lock) /* release acquired lock */
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/* When you say memory, my mind reasons in terms of (pointers to) blocks */
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typedef uint8_t pyblock;
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/* Pool for small blocks. */
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struct pool_header {
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union { pyblock *_padding;
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unsigned int count; } ref; /* number of allocated blocks */
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pyblock *freeblock; /* pool's free list head */
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struct pool_header *nextpool; /* next pool of this size class */
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struct pool_header *prevpool; /* previous pool "" */
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unsigned int arenaindex; /* index into arenas of base adr */
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unsigned int szidx; /* block size class index */
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unsigned int nextoffset; /* bytes to virgin block */
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unsigned int maxnextoffset; /* largest valid nextoffset */
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};
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typedef struct pool_header *poolp;
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/* Record keeping for arenas. */
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struct arena_object {
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/* The address of the arena, as returned by malloc. Note that 0
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* will never be returned by a successful malloc, and is used
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* here to mark an arena_object that doesn't correspond to an
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* allocated arena.
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*/
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uintptr_t address;
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/* Pool-aligned pointer to the next pool to be carved off. */
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pyblock* pool_address;
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/* The number of available pools in the arena: free pools + never-
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* allocated pools.
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*/
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unsigned int nfreepools;
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/* The total number of pools in the arena, whether or not available. */
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unsigned int ntotalpools;
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/* Singly-linked list of available pools. */
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struct pool_header* freepools;
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/* Whenever this arena_object is not associated with an allocated
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* arena, the nextarena member is used to link all unassociated
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* arena_objects in the singly-linked `unused_arena_objects` list.
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* The prevarena member is unused in this case.
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*
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* When this arena_object is associated with an allocated arena
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* with at least one available pool, both members are used in the
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* doubly-linked `usable_arenas` list, which is maintained in
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* increasing order of `nfreepools` values.
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*
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* Else this arena_object is associated with an allocated arena
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* all of whose pools are in use. `nextarena` and `prevarena`
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* are both meaningless in this case.
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*/
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struct arena_object* nextarena;
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struct arena_object* prevarena;
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};
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#define POOL_OVERHEAD _Py_SIZE_ROUND_UP(sizeof(struct pool_header), ALIGNMENT)
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#define DUMMY_SIZE_IDX 0xffff /* size class of newly cached pools */
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/* Round pointer P down to the closest pool-aligned address <= P, as a poolp */
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#define POOL_ADDR(P) ((poolp)_Py_ALIGN_DOWN((P), POOL_SIZE))
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/* Return total number of blocks in pool of size index I, as a uint. */
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#define NUMBLOCKS(I) \
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((unsigned int)(POOL_SIZE - POOL_OVERHEAD) / INDEX2SIZE(I))
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/*==========================================================================*/
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/*
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* This malloc lock
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*/
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SIMPLELOCK_DECL(_malloc_lock)
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#define LOCK() SIMPLELOCK_LOCK(_malloc_lock)
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#define UNLOCK() SIMPLELOCK_UNLOCK(_malloc_lock)
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#define LOCK_INIT() SIMPLELOCK_INIT(_malloc_lock)
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#define LOCK_FINI() SIMPLELOCK_FINI(_malloc_lock)
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/*
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* Pool table -- headed, circular, doubly-linked lists of partially used pools.
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This is involved. For an index i, usedpools[i+i] is the header for a list of
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all partially used pools holding small blocks with "size class idx" i. So
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usedpools[0] corresponds to blocks of size 8, usedpools[2] to blocks of size
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16, and so on: index 2*i <-> blocks of size (i+1)<<ALIGNMENT_SHIFT.
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Pools are carved off an arena's highwater mark (an arena_object's pool_address
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member) as needed. Once carved off, a pool is in one of three states forever
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after:
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used == partially used, neither empty nor full
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At least one block in the pool is currently allocated, and at least one
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block in the pool is not currently allocated (note this implies a pool
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has room for at least two blocks).
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This is a pool's initial state, as a pool is created only when malloc
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needs space.
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The pool holds blocks of a fixed size, and is in the circular list headed
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at usedpools[i] (see above). It's linked to the other used pools of the
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same size class via the pool_header's nextpool and prevpool members.
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If all but one block is currently allocated, a malloc can cause a
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transition to the full state. If all but one block is not currently
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allocated, a free can cause a transition to the empty state.
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full == all the pool's blocks are currently allocated
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On transition to full, a pool is unlinked from its usedpools[] list.
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It's not linked to from anything then anymore, and its nextpool and
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prevpool members are meaningless until it transitions back to used.
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A free of a block in a full pool puts the pool back in the used state.
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Then it's linked in at the front of the appropriate usedpools[] list, so
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that the next allocation for its size class will reuse the freed block.
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empty == all the pool's blocks are currently available for allocation
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On transition to empty, a pool is unlinked from its usedpools[] list,
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and linked to the front of its arena_object's singly-linked freepools list,
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via its nextpool member. The prevpool member has no meaning in this case.
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Empty pools have no inherent size class: the next time a malloc finds
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an empty list in usedpools[], it takes the first pool off of freepools.
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If the size class needed happens to be the same as the size class the pool
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last had, some pool initialization can be skipped.
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Block Management
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Blocks within pools are again carved out as needed. pool->freeblock points to
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the start of a singly-linked list of free blocks within the pool. When a
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block is freed, it's inserted at the front of its pool's freeblock list. Note
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that the available blocks in a pool are *not* linked all together when a pool
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is initialized. Instead only "the first two" (lowest addresses) blocks are
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set up, returning the first such block, and setting pool->freeblock to a
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one-block list holding the second such block. This is consistent with that
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pymalloc strives at all levels (arena, pool, and block) never to touch a piece
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of memory until it's actually needed.
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So long as a pool is in the used state, we're certain there *is* a block
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available for allocating, and pool->freeblock is not NULL. If pool->freeblock
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points to the end of the free list before we've carved the entire pool into
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blocks, that means we simply haven't yet gotten to one of the higher-address
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blocks. The offset from the pool_header to the start of "the next" virgin
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block is stored in the pool_header nextoffset member, and the largest value
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of nextoffset that makes sense is stored in the maxnextoffset member when a
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pool is initialized. All the blocks in a pool have been passed out at least
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once when and only when nextoffset > maxnextoffset.
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Major obscurity: While the usedpools vector is declared to have poolp
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entries, it doesn't really. It really contains two pointers per (conceptual)
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poolp entry, the nextpool and prevpool members of a pool_header. The
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excruciating initialization code below fools C so that
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usedpool[i+i]
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"acts like" a genuine poolp, but only so long as you only reference its
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nextpool and prevpool members. The "- 2*sizeof(block *)" gibberish is
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compensating for that a pool_header's nextpool and prevpool members
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immediately follow a pool_header's first two members:
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union { block *_padding;
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uint count; } ref;
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block *freeblock;
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each of which consume sizeof(block *) bytes. So what usedpools[i+i] really
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contains is a fudged-up pointer p such that *if* C believes it's a poolp
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pointer, then p->nextpool and p->prevpool are both p (meaning that the headed
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circular list is empty).
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It's unclear why the usedpools setup is so convoluted. It could be to
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minimize the amount of cache required to hold this heavily-referenced table
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(which only *needs* the two interpool pointer members of a pool_header). OTOH,
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referencing code has to remember to "double the index" and doing so isn't
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free, usedpools[0] isn't a strictly legal pointer, and we're crucially relying
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on that C doesn't insert any padding anywhere in a pool_header at or before
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the prevpool member.
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**************************************************************************** */
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#define MAX_POOLS (2 * ((NB_SMALL_SIZE_CLASSES + 7) / 8) * 8)
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/*==========================================================================
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Arena management.
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`arenas` is a vector of arena_objects. It contains maxarenas entries, some of
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which may not be currently used (== they're arena_objects that aren't
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currently associated with an allocated arena). Note that arenas proper are
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separately malloc'ed.
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Prior to Python 2.5, arenas were never free()'ed. Starting with Python 2.5,
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we do try to free() arenas, and use some mild heuristic strategies to increase
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the likelihood that arenas eventually can be freed.
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unused_arena_objects
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This is a singly-linked list of the arena_objects that are currently not
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being used (no arena is associated with them). Objects are taken off the
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head of the list in new_arena(), and are pushed on the head of the list in
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PyObject_Free() when the arena is empty. Key invariant: an arena_object
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is on this list if and only if its .address member is 0.
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usable_arenas
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This is a doubly-linked list of the arena_objects associated with arenas
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that have pools available. These pools are either waiting to be reused,
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or have not been used before. The list is sorted to have the most-
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allocated arenas first (ascending order based on the nfreepools member).
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This means that the next allocation will come from a heavily used arena,
|
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|
which gives the nearly empty arenas a chance to be returned to the system.
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In my unscientific tests this dramatically improved the number of arenas
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that could be freed.
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Note that an arena_object associated with an arena all of whose pools are
|
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currently in use isn't on either list.
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|
*/
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/* How many arena_objects do we initially allocate?
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* 16 = can allocate 16 arenas = 16 * ARENA_SIZE = 4 MiB before growing the
|
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* `arenas` vector.
|
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|
|
*/
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#define INITIAL_ARENA_OBJECTS 16
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#endif /* Py_INTERNAL_PYMALLOC_H */
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Victor Stinner
GitHub