race condition on stalled map output fetches
--------------------------------------------
Key: HADOOP-750
URL: http://issues.apache.org/jira/browse/HADOOP-750
Project: Hadoop
Issue Type: Bug
Components: mapred
Affects Versions: 0.8.0
Reporter: Owen O'Malley
Assigned To: Owen O'Malley
Fix For: 0.9.0
I've seen reduces getting killed because of a race condition in the
ReduceTaskRunner. In the logs it looks like:
2006-11-27 08:40:44,795 WARN org.apache.hadoop.mapred.TaskRunner: Map output
copy stalled on
http://kry2296.inktomisearch.com:7030/mapOutput?map=task_0001_m_015626_0
...
2006-11-27 09:16:41,361 INFO org.apache.hadoop.mapred.TaskRunner:
task_0001_r_000658_0 Need 52 map output(s)
2006-11-27 09:16:41,361 INFO org.apache.hadoop.mapred.TaskRunner:
task_0001_r_000658_0 Got 39 known map output location(s); scheduling...
2006-11-27 09:16:41,361 INFO org.apache.hadoop.mapred.TaskRunner:
task_0001_r_000658_0 Scheduled 0 of 39 known outputs (0 slow hosts and 39 dup
hosts)
...
2006-11-27 09:16:47,071 INFO org.apache.hadoop.mapred.TaskTracker:
task_0001_r_000658_0 0.3328575% reduce > copy (28679 of 28720 at 0.76 MB/s) >
...
2006-11-27 09:16:47,338 INFO org.apache.hadoop.mapred.TaskRunner:
task_0001_r_000658_0 done copying task_0001_m_015462_0 output from node1
...
2006-11-27 09:36:51,398 INFO org.apache.hadoop.mapred.TaskTracker:
task_0001_r_000658_0: Task failed to report status for 1204 seconds. Killing.
Basically, the handling of the stall has a race condition that leaves the
fetcher in a bad state. At the end of the fetch, all of the tasks finish and
their results never get handled. When the thread times out, all of the map
output copiers are waiting for things to fetch and the prepare thread is
waiting for results.
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