[MAPREDUCE-286] Optimize the last merge of the map output files

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Assignee: Unassigned
• Reporter: Devaraj Das
• Created: 01/Mar/08 09:24
• Updated: 17/Jul/14 21:22
• Resolved: 17/Jul/14 21:22

Description

In ReduceTask, today we do merges of io.sort.factor number of files everytime we merge and write the result back to disk. The last merge can probably be better. For example, if there are io.sort.factor + 10 files at the end, today we will merge 100 files into one and then return an iterator over the remaining 11 files. This can be improved (in terms of disk I/O) to merge the smallest 11 files and then return an iterator over the 100 remaining files. Other option is to not do any single level merge when we have io.sort.factor + n files remaining (where n << io.sort.factor) but just return the iterator directly. Thoughts?