[jira] [Commented] (MAPREDUCE-6628) Potential memory leak in CryptoOutputStream

["Chris Douglas (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MAPREDUCE-6628?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15468199#comment-15468199 ] 

Chris Douglas commented on MAPREDUCE-6628:
------------------------------------------

[~masokan] thank you for your patience with this.

The unit test looks useful for debugging, but it doesn't actually verify the fix. As written, it's also expensive to run (starts a cluster) and relies on a platform-dependent scan of {{/proc/self/status}}, rather than using {{java.lang.management}} APIs. That said, unit testing this corner of MapReduce is not straightforward, and your posted results demonstrate both the issue and the fix. We can commit this without a MR test.

Would it be possible to write a short unit test for {{CryptoOutputStream}} verifying the new {{closeOutputStream}} semantics? This should be very straightforward in Mockito, just checking that {{close}} behaves as expected when the flag is passed.

It's unfortunate that we're switching behavior based on object reference equality, to check whether the stream was wrapped. As designed, I don't see a cleaner way to improve this without refactoring the crypto implementation.

> Potential memory leak in CryptoOutputStream
> -------------------------------------------
>
>                 Key: MAPREDUCE-6628
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-6628
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: security
>    Affects Versions: 2.6.4
>            Reporter: Mariappan Asokan
>            Assignee: Mariappan Asokan
>         Attachments: MAPREDUCE-6628.001.patch, MAPREDUCE-6628.002.patch, MAPREDUCE-6628.003.patch, MAPREDUCE-6628.004.patch, MAPREDUCE-6628.005.patch, MAPREDUCE-6628.006.patch, MAPREDUCE-6628.007.patch
>
>
> There is a potential memory leak in {{CryptoOutputStream.java.}}  It allocates two direct byte buffers ({{inBuffer}} and {{outBuffer}}) that get freed when {{close()}} method is called.  Most of the time, {{close()}} method is called.  However, when writing to intermediate Map output file or the spill files in {{MapTask}}, {{close()}} is never called since calling so  would close the underlying stream which is not desirable.  There is a single underlying physical stream that contains multiple logical streams one per partition of Map output.  
> By default the amount of memory allocated per byte buffer is 128 KB and  so the total memory allocated is 256 KB,  This may not sound much.  However, if the number of partitions (or number of reducers) is large (in the hundreds) and/or there are spill files created in {{MapTask}}, this can grow into a few hundred MB. 
> I can think of two ways to address this issue:
> h2. Possible Fix - 1
> According to JDK documentation:
> {quote}
> The contents of direct buffers may reside outside of the normal garbage-collected heap, and so their impact upon the memory footprint of an application might not be obvious.  It is therefore recommended that direct buffers be allocated primarily for large, long-lived buffers that are subject to the underlying system's native I/O operations.  In general it is best to allocate direct buffers only when they yield a measureable gain in program performance.
> {quote}
> It is not clear to me whether there is any benefit of allocating direct byte buffers in {{CryptoOutputStream.java}}.  In fact, there is a slight CPU overhead in moving data from {{outBuffer}} to a temporary byte array as per the following code in {{CryptoOutputStream.java}}.
> {code}
>     /*
>      * If underlying stream supports {@link ByteBuffer} write in future, needs
>      * refine here. 
>      */
>     final byte[] tmp = getTmpBuf();
>     outBuffer.get(tmp, 0, len);
>     out.write(tmp, 0, len);
> {code}
> Even if the underlying stream supports direct byte buffer IO (or direct IO in OS parlance), it is not clear whether it will yield any measurable performance gain.
> The fix would be to allocate a ByteBuffer on the heap for inBuffer and wrap a byte array in a {{ByteBuffer}} for {{outBuffer}}.  By the way, the {{inBuffer}} and {{outBuffer}} have to be {{ByteBuffer}} as demanded by the {{encrypt()}} method in {{Encryptor}}.
> h2. Possible Fix - 2
> Assuming that we want to keep the buffers as direct byte buffers, we can create a new constructor to {{CryptoOutputStream}} and pass a boolean flag {{ownOutputStream}} to indicate whether the underlying stream will be owned by {{CryptoOutputStream}}. If it is true, then calling the {{close()}} method will close the underlying stream.  Otherwise, when {{close()}} is called only the direct byte buffers will be freed and the underlying stream will not be closed.
> The scope of changes for this fix will be somewhat wider.  We need to modify {{MapTask.java}}, {{CryptoUtils.java}}, and {{CryptoFSDataOutputStream.java}} as well to pass the ownership flag mentioned above.
> I can post a patch for either of the above.  I welcome any other ideas from developers to fix this issue.



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