[jira] [Commented] (YARN-366) Add a tracing async dispatcher to simplify debugging

["Sandy Ryza (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/YARN-366?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13570856#comment-13570856 ] 

Sandy Ryza commented on YARN-366:
---------------------------------

I've attached an initial patch that adds TracingAsyncDispatcher.  The basic idea of it is that it maintains a thread local reference to the event currently being handled, so that any other events that are fired off while it is being handled get tagged with it as a parent.  I've added an EventTrace field to the Event class that maintains trace and parentage information - if we want this feature to entirely not affect existing code, we could instead maintain a mapping inside the dispatcher.

I still need to add in configuration hooks to turn it on.
                
> Add a tracing async dispatcher to simplify debugging
> ----------------------------------------------------
>
>                 Key: YARN-366
>                 URL: https://issues.apache.org/jira/browse/YARN-366
>             Project: Hadoop YARN
>          Issue Type: New Feature
>          Components: nodemanager, resourcemanager
>    Affects Versions: 2.0.2-alpha
>            Reporter: Sandy Ryza
>            Assignee: Sandy Ryza
>         Attachments: YARN-366.patch
>
>
> Exceptions thrown in YARN/MR code with asynchronous event handling do not contain informative stack traces, as all handle() methods sit directly under the dispatcher thread's loop.
> This makes errors very difficult to debug for those who are not intimately familiar with the code, as it is difficult to see which chain of events caused a particular outcome.
> I propose adding an AsyncDispatcher that instruments events with tracing information.  Whenever an event is dispatched during the handling of another event, the dispatcher would annotate that event with a pointer to its parent.  When the dispatcher catches an exception, it could reconstruct a "stack" trace of the chain of events that led to it, and be able to log something informative.
> This would be an experimental feature, off by default, unless extensive testing showed that it did not have a significant performance impact.

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