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https://issues.apache.org/jira/browse/HDFS-2834?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13223052#comment-13223052
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Todd Lipcon commented on HDFS-2834:
-----------------------------------
Hi Dhruba. Looks like your implementation provides for "zero copy" reads, since
you can return mmapped memory directly. This is a neat trick, but a little
different from this JIRA, where it still copies (just once) into a
user-provided buffer. Also, looks like your implementation doesn't do any
checksumming for this API, right? Other concerns with the mmap approach are:
- it doesn't actually unmap when it goes "out of scope" as your comment
indicates -- you need to wait on an actual GC to call the finalizers, which can
cause the process to run out of address space on a 32-bit JVM if there isn't
actual pressure on the Java heap
- the mmap() call will cause a TLB shootdown across all the threads - I'd be
surprised if the API is actually faster for the case of smaller multi-threaded
reads. See section 5.1 of this paper for more info:
http://www.scribd.com/doc/59150636/C4-Continuously-Concurrent-Compacting-Collector
> 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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