[HADOOP-5271] JobConf option for minimum progress threshold before reducers are assigned

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Assignee: Unassigned
• Reporter: Tim Williamson
• Created: 17/Feb/09 06:36
• Updated: 25/May/10 04:58
• Resolved: 25/May/10 04:58

Description

A specific sub-case of the general priority inversion problem noted in HADOOP-4557 is when many lower priority jobs are submitted and are waiting for mappers to free up. Even though they haven't actually done any work, they will be assigned any free reducers. If a higher priority job is submitted, priority inversion results not just due to the lower priority tasks that are in the midst of completing, but also due to the ones that haven't yet started but have claimed all the free reducers. A simple workaround is to require a job to complete some useful work before assigning it a reducer. This can be done in a tunable and backwards compatible manner by adding a "minimum map progress percentage before assigning a reducer" option to the JobConf. Setting this to 0 would eliminate the common case above, and setting it to 100 would technically eliminate the inversion of HADOOP-4557, though likely at an unacceptably high cost.

Issue Links

• relates to: MAPREDUCE-314 Avoid priority inversion that could result due to scheduling running jobs in an order sorted by priority