[MAPREDUCE-2062] speculative execution is too aggressive under certain conditions

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Component/s: jobtracker

Environment

hadoop-20 with HADOOP-2141

• Assignee: Unassigned
• Reporter: Joydeep Sen Sarma
• Created: 11/Sep/10 01:21
• Updated: 31/Jul/14 03:53
• Resolved: 30/Jul/14 23:56

Description

The function canBeSpeculated has subtle bugs that cause too much speculation in certain cases.

• it compares the current progress of the task with the last observed mean of all the tasks. if only one task is in question - then the progress rate decays as time progresses (in the absence of updates) and std-dev is zero. So a job with a single reducer or mapper is almost always speculated.
• is only a single task has reported progress - then the stddev is zero. so other tasks may be speculated aggressively.
• several tasks take a while to report progress initially. they seem to get speculated as soon as speculative-lag is over. the lag should be configurable at the minimum.