Chao created HIVE-8457:
--------------------------
Summary: MapOperator initialization when multiple Spark threads is
enabled. [Spark Branch]
Key: HIVE-8457
URL: https://issues.apache.org/jira/browse/HIVE-8457
Project: Hive
Issue Type: Bug
Components: Spark
Reporter: Chao
Currently, on the Spark branch, each thread it is bound with a thread-local
IOContext, which gets initialized when we generates a input {{HadoopRDD}}, and
later used in {{MapOperator}}, {{FilterOperator}}, etc.
And, given the introduction of HIVE-8118, we may have multiple downstream RDDs
that share the same input {{HadoopRDD}}, and we would like to have the
{{HadoopRDD}} to be cached, to avoid scanning the same table multiple times. A
typical case would be like the following:
{noformat}
inputRDD inputRDD
| |
MT_11 MT_12
| |
RT_1 RT_2
{noformat}
Here, {{MT_11}} and {{MT_12}} are {{MapTran}}s from a splitted {{MapWork}},
and {{RT_1}} and {{RT_2}} are two {{ReduceTran}}s. Note that, this example is
simplified, as we may also have {{ShuffleTran}} between {{MapTran}} and
{{ReduceTran}}.
When multiple Spark threads are running, {{MT_11} may be executed first, and it
will ask for an iterator from the {{HadoopRDD}} will trigger the creation of
the iterator, which in turn triggers the initialization of the {{IOContext}}
associated with that particular thread.
Now, before {{MT_12}} starts executing, it will also ask for an iterator from
the
{{HadoopRDD}}, and since the RDD is already cached, instead of creating a new
iterator, it will just fetch it from the cached result. However, the problem
is, this will skip the initialization of the IOContext associated with this
particular thread. When {{MT_12}} starts executing, it will first initialize
the {{MapOperator}}, but since the {{IOContext}} is not initialized, this will
fail miserably.
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