158 lines
4.6 KiB
Python
158 lines
4.6 KiB
Python
"""
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This module contains multithread-safe cache implementations.
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Caches mainly have a
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__getitem__ and getorbuild() method
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where the latter either just return a cached value or
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first builds the value.
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These are the current cache implementations:
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BuildcostAccessCache tracks building-time and accesses. Evicts
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by product of num-accesses * build-time.
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"""
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import py
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gettime = py.std.time.time
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class WeightedCountingEntry(object):
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def __init__(self, value, oneweight):
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self.num = 1
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self._value = value
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self.oneweight = oneweight
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def weight():
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def fget(self):
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return (self.num * self.oneweight, self.num)
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return property(fget, None, None, "cumulative weight")
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weight = weight()
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def value():
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def fget(self):
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# you need to protect against mt-access at caller side!
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self.num += 1
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return self._value
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return property(fget, None, None)
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value = value()
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def __repr__(self):
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return "<%s weight=%s>" % (self.__class__.__name__, self.weight)
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class BasicCache(object):
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def __init__(self, maxentries=128):
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self.maxentries = maxentries
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self.prunenum = int(maxentries - maxentries/8)
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self._lock = py.std.threading.RLock()
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self._dict = {}
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def getentry(self, key):
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lock = self._lock
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lock.acquire()
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try:
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return self._dict.get(key, None)
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finally:
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lock.release()
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def putentry(self, key, entry):
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self._lock.acquire()
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try:
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self._prunelowestweight()
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self._dict[key] = entry
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finally:
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self._lock.release()
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def delentry(self, key, raising=False):
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self._lock.acquire()
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try:
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try:
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del self._dict[key]
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except KeyError:
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if raising:
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raise
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finally:
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self._lock.release()
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def getorbuild(self, key, builder, *args, **kwargs):
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entry = self.getentry(key)
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if entry is None:
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entry = self.build(key, builder, *args, **kwargs)
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return entry.value
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def _prunelowestweight(self):
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""" prune out entries with lowest weight. """
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# note: must be called with acquired self._lock!
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numentries = len(self._dict)
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if numentries >= self.maxentries:
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# evict according to entry's weight
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items = [(entry.weight, key) for key, entry in self._dict.iteritems()]
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items.sort()
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index = numentries - self.prunenum
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if index > 0:
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for weight, key in items[:index]:
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del self._dict[key]
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class BuildcostAccessCache(BasicCache):
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""" A BuildTime/Access-counting cache implementation.
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the weight of a value is computed as the product of
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num-accesses-of-a-value * time-to-build-the-value
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The values with the least such weights are evicted
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if the cache maxentries threshold is superceded.
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For implementation flexibility more than one object
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might be evicted at a time.
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"""
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# time function to use for measuring build-times
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_time = gettime
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def __init__(self, maxentries=64):
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super(BuildcostAccessCache, self).__init__(maxentries)
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def build(self, key, builder, *args, **kwargs):
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start = self._time()
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val = builder(*args, **kwargs)
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end = self._time()
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entry = WeightedCountingEntry(val, end-start)
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self.putentry(key, entry)
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return entry
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class AgingCache(BasicCache):
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""" This cache prunes out cache entries that are too old.
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"""
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def __init__(self, maxentries=128, maxseconds=10.0):
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super(AgingCache, self).__init__(maxentries)
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self.maxseconds = maxseconds
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def getentry(self, key):
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self._lock.acquire()
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try:
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try:
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entry = self._dict[key]
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except KeyError:
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entry = None
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else:
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if entry.isexpired():
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del self._dict[key]
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entry = None
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return entry
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finally:
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self._lock.release()
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def build(self, key, builder, *args, **kwargs):
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ctime = gettime()
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val = builder(*args, **kwargs)
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entry = AgingEntry(val, ctime + self.maxseconds)
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self.putentry(key, entry)
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return entry
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class AgingEntry(object):
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def __init__(self, value, expirationtime):
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self.value = value
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self.weight = expirationtime
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def isexpired(self):
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t = py.std.time.time()
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return t >= self.weight
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