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From "Walter Su (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8901) Use ByteBuffer in striping positional read
Date Wed, 18 Nov 2015 01:46:11 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8901?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15010009#comment-15010009
] 

Walter Su commented on HDFS-8901:
---------------------------------

bq. in the worst case, if the user provided read buffer isn't the right buffer type desired
by the erasure coder, then all cells will need one time copy.
I think the buffer given by caller is always byte\[\]? There's no bytebuffer version of pread
API.
{code}
public int read(long position, byte[] buffer, int offset, int length)
{code}
Besides, Even if the caller provides a bytebuffer, the data copy from _byte\[\] buffer_ to
_decodeInputs_ still needed.

Assume schema is 3+2, then
{noformat}
byte[] buf is      64k 64k 64k 64k 64k 64k 64k 64k 64k ...
decodeInputs[0] is 64k         64k         64k         ...
decodeInputs[1] is     64k         64k         64k     ...
decodeInputs[2] is         64k         64k         64k ...
{noformat}
We can't feed encoder the original _byte\[\] buffer_ and only call encoding one time.
We can encode in one time if we provide decodeInputs, then we need data copy.
We can also encode many times stripe by stripe like stateful read, then data copy isn't needed.

We are not pursuing fewer times of data copy since the performance is bounded by IO speed
and coding speed. 

> Use ByteBuffer in striping positional read
> ------------------------------------------
>
>                 Key: HDFS-8901
>                 URL: https://issues.apache.org/jira/browse/HDFS-8901
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Kai Zheng
>            Assignee: Kai Zheng
>         Attachments: HDFS-8901-v2.patch, initial-poc.patch
>
>
> Native erasure coder prefers to direct ByteBuffer for performance consideration. To prepare
for it, this change uses ByteBuffer through the codes in implementing striping position read.
It will also fix avoiding unnecessary data copying between striping read chunk buffers and
decode input buffers.



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