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From "Henry Robinson (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2834) ByteBuffer-based read API for DFSInputStream
Date Tue, 06 Mar 2012 03:24:09 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2834?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13222969#comment-13222969
] 

Henry Robinson commented on HDFS-2834:
--------------------------------------

Thanks for the review! Per your first two questions:

* There's no significant difference in my benchmarks with the old copying path doing the same
experiment:
|| ||Native Checksums||	No Checksums||	Non-native Checksums||	Remote, Native Checksums||
|Copying (MB/s) - 32k buffer and request size|	2010.21	|2290.50|	721.52|	1412.20|								
|Old copying path - 32k buffer and request size	|2087.43	|2232.67|	708.67	|1365.60|

* I've run the modified TestParallelRead tests for a couple of hours, but I plan to do a soak
test overnight with the full suite before this gets committed. 
                
> ByteBuffer-based read API for DFSInputStream
> --------------------------------------------
>
>                 Key: HDFS-2834
>                 URL: https://issues.apache.org/jira/browse/HDFS-2834
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Henry Robinson
>            Assignee: Henry Robinson
>         Attachments: HDFS-2834-no-common.patch, HDFS-2834.3.patch, HDFS-2834.4.patch,
HDFS-2834.5.patch, HDFS-2834.6.patch, HDFS-2834.patch, HDFS-2834.patch, hdfs-2834-libhdfs-benchmark.png
>
>
> The {{DFSInputStream}} read-path always copies bytes into a JVM-allocated {{byte[]}}.
Although for many clients this is desired behaviour, in certain situations, such as native-reads
through libhdfs, this imposes an extra copy penalty since the {{byte[]}} needs to be copied
out again into a natively readable memory area. 
> For these cases, it would be preferable to allow the client to supply its own buffer,
wrapped in a {{ByteBuffer}}, to avoid that final copy overhead. 

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