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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2834) ByteBuffer-based read API for DFSInputStream
Date Thu, 08 Aug 2013 06:12:48 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2834?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13733186#comment-13733186
] 

Lars Hofhansl commented on HDFS-2834:
-------------------------------------

Just for reference with many open files one can easily OOM on direct buffer memory. See: HBASE-8143.
1MB seems to be a rather large default.

                
> ByteBuffer-based read API for DFSInputStream
> --------------------------------------------
>
>                 Key: HDFS-2834
>                 URL: https://issues.apache.org/jira/browse/HDFS-2834
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client, performance
>            Reporter: Henry Robinson
>            Assignee: Henry Robinson
>             Fix For: 2.0.2-alpha
>
>         Attachments: HDFS-2834.10.patch, HDFS-2834.11.patch, HDFS-2834.3.patch, HDFS-2834.4.patch,
HDFS-2834.5.patch, HDFS-2834.6.patch, HDFS-2834.7.patch, HDFS-2834.8.patch, HDFS-2834.9.patch,
hdfs-2834-libhdfs-benchmark.png, HDFS-2834-no-common.patch, HDFS-2834.patch, HDFS-2834.patch
>
>
> 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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