112 lines
3.2 KiB
Python
112 lines
3.2 KiB
Python
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"""
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This module contains multithread-safe cache implementations.
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All Caches have
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getorbuild(key, builder)
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delentry(key)
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methods and allow configuration when instantiating the cache class.
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"""
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from time import time as gettime
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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._dict = {}
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def _getentry(self, key):
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return self._dict[key]
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def _putentry(self, key, entry):
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self._prunelowestweight()
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self._dict[key] = entry
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def delentry(self, key, raising=False):
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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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def getorbuild(self, key, builder):
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try:
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entry = self._getentry(key)
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except KeyError:
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entry = self._build(key, builder)
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self._putentry(key, entry)
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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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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)
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for key, entry in self._dict.items()]
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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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# in MT situations the element might be gone
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self.delentry(key, raising=False)
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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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def _build(self, key, builder):
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start = gettime()
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val = builder()
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end = gettime()
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return WeightedCountingEntry(val, end-start)
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class WeightedCountingEntry(object):
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def __init__(self, value, oneweight):
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self._value = value
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self.weight = self._oneweight = oneweight
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def value(self):
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self.weight += self._oneweight
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return self._value
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value = property(value)
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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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entry = self._dict[key]
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if entry.isexpired():
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self.delentry(key)
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raise KeyError(key)
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return entry
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def _build(self, key, builder):
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val = builder()
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entry = AgingEntry(val, gettime() + self.maxseconds)
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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 = gettime()
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return t >= self.weight
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