Github user mateiz commented on a diff in the pull request:
https://github.com/apache/spark/pull/1460#discussion_r15078890
--- Diff: python/pyspark/rdd.py ---
@@ -168,6 +170,123 @@ def _replaceRoot(self, value):
self._sink(1)
+class Merger(object):
+ """
+ External merger will dump the aggregated data into disks when memory
usage is above
+ the limit, then merge them together.
+
+ >>> combiner = lambda x, y:x+y
+ >>> merger = Merger(combiner, 10)
+ >>> N = 10000
+ >>> merger.merge(zip(xrange(N), xrange(N)) * 10)
+ >>> merger.spills
+ 100
+ >>> sum(1 for k,v in merger.iteritems())
+ 10000
+ """
+
+ PARTITIONS = 64
+ BATCH = 1000
+
+ def __init__(self, combiner, memory_limit=256, path="/tmp/pyspark",
serializer=None):
+ self.combiner = combiner
+ self.path = os.path.join(path, str(os.getpid()))
+ self.memory_limit = memory_limit
+ self.serializer = serializer or
BatchedSerializer(AutoSerializer(), 1024)
+ self.item_limit = None
+ self.data = {}
+ self.pdata = []
+ self.spills = 0
+
+ def used_memory(self):
+ rss = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss
+ if platform.system() == 'Linux':
+ rss >>= 10
+ elif platform.system() == 'Darwin':
+ rss >>= 20
+ return rss
+
+ def merge(self, iterator):
+ iterator = iter(iterator)
+ d = self.data
+ comb = self.combiner
+ c = 0
+ for k, v in iterator:
+ if k in d:
+ d[k] = comb(d[k], v)
+ else:
+ d[k] = v
+
+ if self.item_limit is not None:
+ continue
+
+ c += 1
+ if c % self.BATCH == 0 and self.used_memory() >
self.memory_limit:
+ self.item_limit = c
+ self._first_spill()
+ self._partitioned_merge(iterator)
+ return
+
+ def _partitioned_merge(self, iterator):
+ comb = self.combiner
+ c = 0
+ for k, v in iterator:
+ d = self.pdata[hash(k) % self.PARTITIONS]
+ if k in d:
+ d[k] = comb(d[k], v)
+ else:
+ d[k] = v
+ c += 1
+ if c >= self.item_limit:
+ self._spill()
+ c = 0
+
+ def _first_spill(self):
+ path = os.path.join(self.path, str(self.spills))
+ if not os.path.exists(path):
+ os.makedirs(path)
+ streams = [open(os.path.join(path, str(i)), 'w')
+ for i in range(self.PARTITIONS)]
--- End diff --
The only constraint here is that it should be a library available in Python
by default, so that we don't ask users to install external packages. Maybe if
it's not available, we can fall back to GZIP with compression level 1, or to no
compression.
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