[jira] [Updated] (MAPREDUCE-5601) ShuffleHandler fadvises file regions as DONTNEED even when fetch fails

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

     [ https://issues.apache.org/jira/browse/MAPREDUCE-5601?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Sandy Ryza updated MAPREDUCE-5601:
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    Summary: ShuffleHandler fadvises file regions as DONTNEED even when fetch fails  (was: Fetches when reducer can't fit them result in unnecessary reads on shuffle server)

> ShuffleHandler fadvises file regions as DONTNEED even when fetch fails
> ----------------------------------------------------------------------
>
>                 Key: MAPREDUCE-5601
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5601
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>    Affects Versions: 2.2.0
>            Reporter: Sandy Ryza
>            Assignee: Sandy Ryza
>
> When a reducer initiates a fetch request, it does not know whether it will be able to fit the fetched data in memory.  The first part of the response tells how much data will be coming.  If space is not currently available, the reduce will abandon its request and try again later.  Unfortunately, this has some consequences on the server side - it forces unnecessary disk and network IO as the server begins to read the output data that will go nowhere.  Also, when the channel is closed, it triggers an fadvise DONTNEED that causes the data region to be evicted from the OS page cache.  Meaning that the next time it's asked for, it will definitely be read from disk, even if it happened to be in the page cache before the request.
> I noticed this when trying to figure out why my job was doing so much more disk IO in MR2 than in MR1.  When I turned the fadvise stuff off, I found that disk reads went to nearly 0 on machines that had enough memory to fit map outputs into the page cache.  I then straced the NodeManager noticed that there were over four times as many fadvise DONTNEED calls as map-reduce pairs.  Further logging showed the same map outputs being fetched about this many times.
> The fix would be to reserve space in the reducer before fetching the data.  Currently the fetching the size of the data and fetching the actual data happen in the same HTTP request.  Fixing it would require doing these in separate HTTP requests.  Or transferring the sizes through the AM.



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