[jira] [Created] (YARN-4964) Allow ShuffleHandler readahead without drop-behind

["Nathan Roberts (JIRA)" <ji...@apache.org>]

Nathan Roberts created YARN-4964:
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             Summary: Allow ShuffleHandler readahead without drop-behind
                 Key: YARN-4964
                 URL: https://issues.apache.org/jira/browse/YARN-4964
             Project: Hadoop YARN
          Issue Type: Improvement
          Components: nodemanager
    Affects Versions: 2.7.2, 3.0.0
            Reporter: Nathan Roberts
            Assignee: Nathan Roberts


Currently mapreduce.shuffle.manage.os.cache enables/disables both readahead (POSIX_FADV_WILLNEED) and drop-behind (POSIX_FADV_DONTNEED) logic within the ShuffleHandler.

It would be beneficial if these were separately configurable. 
- Running without readahead can lead to significant seek storms caused by large numbers of sendfiles() competing with one another.
- However, running with drop-behind can also lead to seek storms because there are cases where the server can successfully write the shuffle bytes to the network, BUT the client doesn't want the bytes right now (MergeManager wants to WAIT is an example) so it ignores them and asks for them again a bit later. This causes repeated reads of the same data from disk.

I'll attach a simple patch that enables/disables readahead based on mapreduce.shuffle.readahead.bytes==0, leaving mapreduce.shuffle.manage.os.cache controlling only the drop-behind.




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