[jira] [Assigned] (SPARK-26549) PySpark worker reuse take no effect for parallelize xrange

["Hyukjin Kwon (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SPARK-26549?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Hyukjin Kwon reassigned SPARK-26549:
------------------------------------

    Assignee: Yuanjian Li

> PySpark worker reuse take no effect for parallelize xrange
> ----------------------------------------------------------
>
>                 Key: SPARK-26549
>                 URL: https://issues.apache.org/jira/browse/SPARK-26549
>             Project: Spark
>          Issue Type: Bug
>          Components: PySpark
>    Affects Versions: 3.0.0
>            Reporter: Yuanjian Li
>            Assignee: Yuanjian Li
>            Priority: Major
>
> During [the follow-up work|https://github.com/apache/spark/pull/23435#issuecomment-451079886] for PySpark worker reuse scenario, we found that the worker reuse takes no effect for `sc.parallelize(xrange(...))`.
> It happened because of the specialize rdd.parallelize for xrange(SPARK-4398) generated data by xrange, which don't need to use the passed-in iterator. But this will break the end of stream checking in python worker and finally cause worker reuse takes no effect.
> Relative code block and more details listing below:
> Current specialize logic of xrange don't need the passed-in iterator, context.py:
> {code:java}
> if isinstance(c, xrange):
>     ...
>     def f(split, iterator):
>         return xrange(getStart(split), getStart(split + 1), step)
>     ...
>     return self.parallelize([], numSlices).mapPartitionsWithIndex(f)
> {code}
> We got an unexpected value -1 which refers to END_OF_DATA_SECTION while check end of stream. See the code in worker.py:
> {code:java}
> # check end of stream
> if read_int(infile) == SpecialLengths.END_OF_STREAM:
>     write_int(SpecialLengths.END_OF_STREAM, outfile)
> else:
>     # write a different value to tell JVM to not reuse this worker
>     write_int(SpecialLengths.END_OF_DATA_SECTION, outfile)
>     sys.exit(-1)
> {code}
> The code works well for parallelize(range) because the END_OF_DATA_SECTION has been handled during load iterator from the socket stream, see the code in FramedSerializer:
> {code:java}
> def load_stream(self, stream):
>     while True:
>         try:
>             yield self._read_with_length(stream)
>         except EOFError:
>             return
> ...
> def _read_with_length(self, stream):
>     length = read_int(stream)
>     if length == SpecialLengths.END_OF_DATA_SECTION:
>         raise EOFError #END_OF_DATA_SECTION raised EOF here and catched in load_stream
>     elif length == SpecialLengths.NULL:
>         return None
>     obj = stream.read(length)
>     if len(obj) < length:
>         raise EOFError
>     return self.loads(obj)
> {code}



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