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https://issues.apache.org/jira/browse/SPARK-2962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14092427#comment-14092427
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Mridul Muralidharan commented on SPARK-2962:
--------------------------------------------
To give more context;
a) Our jobs start with load data from dfs as starting point : and so this is
the first stage that gets executed.
b) We are sleeping for 1 minute before starting the jobs (in case cluster is
busy, etc) - unfortunately, this is not sufficient and iirc there is no
programmatic way to wait more deterministically for X% of node (was something
added to alleviate this ? I did see some discussion)
c) This becomes more of a problem because spark does not honour preferred
location anymore while running in yarn. See SPARK-208 - due to 1.0 interface
changes.
[ Practically, if we are using large enough number of nodes (with replication
of 3 or higher), usually we do end up with quite of lot of data local tasks
eventually - so (c) is not an immediate concern for our current jobs assuming
(b) is not an issue, though it is suboptimal in general case ]
> Suboptimal scheduling in spark
> ------------------------------
>
> Key: SPARK-2962
> URL: https://issues.apache.org/jira/browse/SPARK-2962
> Project: Spark
> Issue Type: Bug
> Components: Spark Core
> Affects Versions: 1.1.0
> Environment: All
> Reporter: Mridul Muralidharan
>
> In findTask, irrespective of 'locality' specified, pendingTasksWithNoPrefs
> are always scheduled with PROCESS_LOCAL
> pendingTasksWithNoPrefs contains tasks which currently do not have any alive
> locations - but which could come in 'later' : particularly relevant when
> spark app is just coming up and containers are still being added.
> This causes a large number of non node local tasks to be scheduled incurring
> significant network transfers in the cluster when running with non trivial
> datasets.
> The comment "// Look for no-pref tasks after rack-local tasks since they can
> run anywhere." is misleading in the method code : locality levels start from
> process_local down to any, and so no prefs get scheduled much before rack.
> Also note that, currentLocalityIndex is reset to the taskLocality returned by
> this method - so returning PROCESS_LOCAL as the level will trigger wait times
> again. (Was relevant before recent change to scheduler, and might be again
> based on resolution of this issue).
> Found as part of writing test for SPARK-2931
>
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