[MAPREDUCE-5601] ShuffleHandler fadvises file regions as DONTNEED even when fetch fails

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.2.0
• Fix Version/s: 2.3.0
• Hadoop Flags: Reviewed
• Assignee: Sandy Ryza
• Reporter: Sandy Ryza
• Created: 30/Oct/13 00:23
• Updated: 24/Feb/14 20:57
• Resolved: 01/Nov/13 08:55

Description

When a reducer initiates a fetch request, it does not know whether it will be able to fit the fetched data in memory. The first part of the response tells how much data will be coming. If space is not currently available, the reduce will abandon its request and try again later. When this occurs, the ShuffleHandler still fadvises the file region as DONTNEED. Meaning that the next time it's asked for, it will definitely be read from disk, even if it happened to be in the page cache before the request.

I noticed this when trying to figure out why my job was doing so much more disk IO in MR2 than in MR1. When I turned the fadvise stuff off, I found that disk reads went to nearly 0 on machines that had enough memory to fit map outputs into the page cache. I then straced the NodeManager and noticed that there were over four times as many fadvise DONTNEED calls as map-reduce pairs. Further logging showed the same map outputs being fetched about this many times.

This is a regression from MR1, which only did the fadvise DONTNEED after all the bytes were transferred.