[jira] [Commented] (SPARK-21551) pyspark's collect fails when getaddrinfo is too slow

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

    [ https://issues.apache.org/jira/browse/SPARK-21551?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16111564#comment-16111564 ] 

Hyukjin Kwon commented on SPARK-21551:
--------------------------------------

User 'peay' has created a pull request for this issue:
https://github.com/apache/spark/pull/18752

> 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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