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https://issues.apache.org/jira/browse/SPARK-21551?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16103543#comment-16103543
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Reynold Xin commented on SPARK-21551:
-------------------------------------
Sure.
> pyspark's collect fails when getaddrinfo is too slow
> ----------------------------------------------------
>
> Key: SPARK-21551
> URL: https://issues.apache.org/jira/browse/SPARK-21551
> Project: Spark
> Issue Type: Bug
> Components: PySpark
> Affects Versions: 2.1.0
> Reporter: peay
> Priority: Critical
>
> Pyspark's {{RDD.collect}}, as well as {{DataFrame.toLocalIterator}} and
> {{DataFrame.collect}} all work by starting an ephemeral server in the driver,
> and having Python connect to it to download the data.
> All three are implemented along the lines of:
> {code}
> port = self._jdf.collectToPython()
> return list(_load_from_socket(port, BatchedSerializer(PickleSerializer())))
> {code}
> The server has **a hardcoded timeout of 3 seconds**
> (https://github.com/apache/spark/blob/e26dac5feb02033f980b1e69c9b0ff50869b6f9e/core/src/main/scala/org/apache/spark/api/python/PythonRDD.scala#L695)
> -- i.e., the Python process has 3 seconds to connect to it from the very
> moment the driver server starts.
> In general, that seems fine, but I have been encountering frequent timeouts
> leading to `Exception: could not open socket`.
> After investigating a bit, it turns out that {{_load_from_socket}} makes a
> call to {{getaddrinfo}}:
> {code}
> def _load_from_socket(port, serializer):
> sock = None
> # Support for both IPv4 and IPv6.
> # On most of IPv6-ready systems, IPv6 will take precedence.
> for res in socket.getaddrinfo("localhost", port, socket.AF_UNSPEC,
> socket.SOCK_STREAM):
> .. connect ..
> {code}
> I am not sure why, but while most such calls to {{getaddrinfo}} on my machine
> only take a couple milliseconds, about 10% of them take between 2 and 10
> seconds, leading to about 10% of jobs failing. I don't think we can always
> expect {{getaddrinfo}} to return instantly. More generally, Python may
> sometimes pause for a couple seconds, which may not leave enough time for the
> process to connect to the server.
> Especially since the server timeout is hardcoded, I think it would be best to
> set a rather generous value (15 seconds?) to avoid such situations.
> A {{getaddrinfo}} specific fix could avoid doing it every single time, or do
> it before starting up the driver server.
>
> cc SPARK-677 [~davies]
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