Create fewer copies of buffer data during sort/spill
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Key: HADOOP-2919
URL: https://issues.apache.org/jira/browse/HADOOP-2919
Project: Hadoop Core
Issue Type: Improvement
Components: mapred
Reporter: Chris Douglas
Attachments: 2919-0.patch
Currently, the sort/spill works as follows:
Let r be the number of partitions
For each call to collect(K,V) from map:
* If buffers do not exist, allocate a new DataOutputBuffer to collect K,V
bytes, allocate r buffers for collecting K,V offsets
* Write K,V into buffer, noting offsets
* Register offsets with associated partition buffer, allocating/copying
accounting buffers if nesc
* Calculate the total mem usage for buffer and all partition collectors by
iterating over the collectors
* If total mem usage is greater than half of io.sort.mb, then start a new
thread to spill, blocking if another spill is in progress
For each spill (assuming no combiner):
* Save references to our K,V byte buffer and accounting data, setting the
former to null (will be recreated on the next call to collect(K,V))
* Open a SequenceFile.Writer for this partition
* Sort each partition separately (the current version of sort reuses, but still
requires wrapping, indices in IntWritable objects)
* Build a RawKeyValueIterator of sorted data for the partition
* Deserialize each key and value and call SequenceFile::append(K,V) on the
writer for this partition
There are a number of opportunities for reducing the number of copies,
creations, and operations we perform in this stage, particularly since growing
many of the buffers involved requires that we copy the existing data to the
newly sized allocation.
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