Map-side sort is hampered by io.sort.record.percent
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Key: HADOOP-5108
URL: https://issues.apache.org/jira/browse/HADOOP-5108
Project: Hadoop Core
Issue Type: Bug
Components: mapred
Affects Versions: 0.20.0
Reporter: Arun C Murthy
Currently io.sort.record.percent is a fairly obscure, per-job configurable,
expert-level parameter which controls how much accounting space is available
for records in the map-side sort buffer (io.sort.mb). Typically values for
io.sort.mb (100) and io.sort.record.percent (0.05) imply that we can store
~350,000 records in the buffer before necessitating a sort/combine/spill.
However for many applications which deal with small records e.g. the
world-famous wordcount and it's family this implies we can only use 5-10% of
io.sort.mb i.e. (5-10M) before we spill inspite of having _much_ more memory
available in the sort-buffer. The word-count for e.g. results in ~12 spills
(given hdfs block size of 64M). The presence of a combiner exacerbates the
problem by piling serialization/deserialization of records too...
Sure, jobs can configure io.sort.record.percent, but it's tedious and obscure;
we really can do better by getting the framework to automagically pick it by
using all available memory (upto io.sort.mb) for either the data or accounting.
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