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heap.py
# # Module which supports allocation of memory from an mmap # # multiprocessing/heap.py # # Copyright (c) 2006-2008, R Oudkerk # Licensed to PSF under a Contributor Agreement. # import bisect from collections import defaultdict import mmap import os import sys import tempfile import threading from .context import reduction, assert_spawning from . import util __all__ = ['BufferWrapper'] # # Inheritable class which wraps an mmap, and from which blocks can be allocated # if sys.platform == 'win32': import _winapi class Arena(object): """ A shared memory area backed by anonymous memory (Windows). """ _rand = tempfile._RandomNameSequence() def __init__(self, size): self.size = size for i in range(100): name = 'pym-%d-%s' % (os.getpid(), next(self._rand)) buf = mmap.mmap(-1, size, tagname=name) if _winapi.GetLastError() == 0: break # We have reopened a preexisting mmap. buf.close() else: raise FileExistsError('Cannot find name for new mmap') self.name = name self.buffer = buf self._state = (self.size, self.name) def __getstate__(self): assert_spawning(self) return self._state def __setstate__(self, state): self.size, self.name = self._state = state # Reopen existing mmap self.buffer = mmap.mmap(-1, self.size, tagname=self.name) # XXX Temporarily preventing buildbot failures while determining # XXX the correct long-term fix. See issue 23060 #assert _winapi.GetLastError() == _winapi.ERROR_ALREADY_EXISTS else: class Arena(object): """ A shared memory area backed by a temporary file (POSIX). """ if sys.platform == 'linux': _dir_candidates = ['/dev/shm'] else: _dir_candidates = [] def __init__(self, size, fd=-1): self.size = size self.fd = fd if fd == -1: # Arena is created anew (if fd != -1, it means we're coming # from rebuild_arena() below) self.fd, name = tempfile.mkstemp( prefix='pym-%d-'%os.getpid(), dir=self._choose_dir(size)) os.unlink(name) util.Finalize(self, os.close, (self.fd,)) os.ftruncate(self.fd, size) self.buffer = mmap.mmap(self.fd, self.size) def _choose_dir(self, size): # Choose a non-storage backed directory if possible, # to improve performance for d in self._dir_candidates: st = os.statvfs(d) if st.f_bavail * st.f_frsize >= size: # enough free space? return d return util.get_temp_dir() def reduce_arena(a): if a.fd == -1: raise ValueError('Arena is unpicklable because ' 'forking was enabled when it was created') return rebuild_arena, (a.size, reduction.DupFd(a.fd)) def rebuild_arena(size, dupfd): return Arena(size, dupfd.detach()) reduction.register(Arena, reduce_arena) # # Class allowing allocation of chunks of memory from arenas # class Heap(object): # Minimum malloc() alignment _alignment = 8 _DISCARD_FREE_SPACE_LARGER_THAN = 4 * 1024 ** 2 # 4 MB _DOUBLE_ARENA_SIZE_UNTIL = 4 * 1024 ** 2 def __init__(self, size=mmap.PAGESIZE): self._lastpid = os.getpid() self._lock = threading.Lock() # Current arena allocation size self._size = size # A sorted list of available block sizes in arenas self._lengths = [] # Free block management: # - map each block size to a list of `(Arena, start, stop)` blocks self._len_to_seq = {} # - map `(Arena, start)` tuple to the `(Arena, start, stop)` block # starting at that offset self._start_to_block = {} # - map `(Arena, stop)` tuple to the `(Arena, start, stop)` block # ending at that offset self._stop_to_block = {} # Map arenas to their `(Arena, start, stop)` blocks in use self._allocated_blocks = defaultdict(set) self._arenas = [] # List of pending blocks to free - see comment in free() below self._pending_free_blocks = [] # Statistics self._n_mallocs = 0 self._n_frees = 0 @staticmethod def _roundup(n, alignment): # alignment must be a power of 2 mask = alignment - 1 return (n + mask) & ~mask def _new_arena(self, size): # Create a new arena with at least the given *size* length = self._roundup(max(self._size, size), mmap.PAGESIZE) # We carve larger and larger arenas, for efficiency, until we # reach a large-ish size (roughly L3 cache-sized) if self._size < self._DOUBLE_ARENA_SIZE_UNTIL: self._size *= 2 util.info('allocating a new mmap of length %d', length) arena = Arena(length) self._arenas.append(arena) return (arena, 0, length) def _discard_arena(self, arena): # Possibly delete the given (unused) arena length = arena.size # Reusing an existing arena is faster than creating a new one, so # we only reclaim space if it's large enough. if length < self._DISCARD_FREE_SPACE_LARGER_THAN: return blocks = self._allocated_blocks.pop(arena) assert not blocks del self._start_to_block[(arena, 0)] del self._stop_to_block[(arena, length)] self._arenas.remove(arena) seq = self._len_to_seq[length] seq.remove((arena, 0, length)) if not seq: del self._len_to_seq[length] self._lengths.remove(length) def _malloc(self, size): # returns a large enough block -- it might be much larger i = bisect.bisect_left(self._lengths, size) if i == len(self._lengths): return self._new_arena(size) else: length = self._lengths[i] seq = self._len_to_seq[length] block = seq.pop() if not seq: del self._len_to_seq[length], self._lengths[i] (arena, start, stop) = block del self._start_to_block[(arena, start)] del self._stop_to_block[(arena, stop)] return block def _add_free_block(self, block): # make block available and try to merge with its neighbours in the arena (arena, start, stop) = block try: prev_block = self._stop_to_block[(arena, start)] except KeyError: pass else: start, _ = self._absorb(prev_block) try: next_block = self._start_to_block[(arena, stop)] except KeyError: pass else: _, stop = self._absorb(next_block) block = (arena, start, stop) length = stop - start try: self._len_to_seq[length].append(block) except KeyError: self._len_to_seq[length] = [block] bisect.insort(self._lengths, length) self._start_to_block[(arena, start)] = block self._stop_to_block[(arena, stop)] = block def _absorb(self, block): # deregister this block so it can be merged with a neighbour (arena, start, stop) = block del self._start_to_block[(arena, start)] del self._stop_to_block[(arena, stop)] length = stop - start seq = self._len_to_seq[length] seq.remove(block) if not seq: del self._len_to_seq[length] self._lengths.remove(length) return start, stop def _remove_allocated_block(self, block): arena, start, stop = block blocks = self._allocated_blocks[arena] blocks.remove((start, stop)) if not blocks: # Arena is entirely free, discard it from this process self._discard_arena(arena) def _free_pending_blocks(self): # Free all the blocks in the pending list - called with the lock held. while True: try: block = self._pending_free_blocks.pop() except IndexError: break self._add_free_block(block) self._remove_allocated_block(block) def free(self, block): # free a block returned by malloc() # Since free() can be called asynchronously by the GC, it could happen # that it's called while self._lock is held: in that case, # self._lock.acquire() would deadlock (issue #12352). To avoid that, a # trylock is used instead, and if the lock can't be acquired # immediately, the block is added to a list of blocks to be freed # synchronously sometimes later from malloc() or free(), by calling # _free_pending_blocks() (appending and retrieving from a list is not # strictly thread-safe but under CPython it's atomic thanks to the GIL). if os.getpid() != self._lastpid: raise ValueError( "My pid ({0:n}) is not last pid {1:n}".format( os.getpid(),self._lastpid)) if not self._lock.acquire(False): # can't acquire the lock right now, add the block to the list of # pending blocks to free self._pending_free_blocks.append(block) else: # we hold the lock try: self._n_frees += 1 self._free_pending_blocks() self._add_free_block(block) self._remove_allocated_block(block) finally: self._lock.release() def malloc(self, size): # return a block of right size (possibly rounded up) if size < 0: raise ValueError("Size {0:n} out of range".format(size)) if sys.maxsize <= size: raise OverflowError("Size {0:n} too large".format(size)) if os.getpid() != self._lastpid: self.__init__() # reinitialize after fork with self._lock: self._n_mallocs += 1 # allow pending blocks to be marked available self._free_pending_blocks() size = self._roundup(max(size, 1), self._alignment) (arena, start, stop) = self._malloc(size) real_stop = start + size if real_stop < stop: # if the returned block is larger than necessary, mark # the remainder available self._add_free_block((arena, real_stop, stop)) self._allocated_blocks[arena].add((start, real_stop)) return (arena, start, real_stop) # # Class wrapping a block allocated out of a Heap -- can be inherited by child process # class BufferWrapper(object): _heap = Heap() def __init__(self, size): if size < 0: raise ValueError("Size {0:n} out of range".format(size)) if sys.maxsize <= size: raise OverflowError("Size {0:n} too large".format(size)) block = BufferWrapper._heap.malloc(size) self._state = (block, size) util.Finalize(self, BufferWrapper._heap.free, args=(block,)) def create_memoryview(self): (arena, start, stop), size = self._state return memoryview(arena.buffer)[start:start+size]
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