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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2758) Reduce memory copies when data is read from DFS
Date Sat, 01 Mar 2008 04:13:51 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2758?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12574066#action_12574066
] 

Raghu Angadi commented on HADOOP-2758:
--------------------------------------


More comparisions. I hope this shows the improvements.

Test : Run *6* instances of 'cat 5GbFile > /dev/null' using a single node cluster. All
the blocks are located on local disks (RAID0) I think.

The hdfs tests include *constant costs* : Client cpu and kernel cpu not on behalf of user
processes. Client cpu is at least as much as DataNodes. This implies, 25% improvement in wall
clock time implies more that 50% improvement in DataNode cpu.

||Test || Bound By || Run1 || Run2 || Run3 || Percentage || Avg || Note ||
| Trunk | CPU | 355 | 332 | 346 | 344 | 100% | |
| Trunk + patch | CPU | 225 | 213 | 228 | 222 | 65% | |
| cat command | Disk IO | 185 | 83 | 105 | 124 | 36% | Not really comparable|

Even 21 instances of 'cat allBlocksForFile > /dev/null' was not CPU bound. 'cat' takes
virtually zero cpu in user space. 

 



> Reduce memory copies when data is read from DFS
> -----------------------------------------------
>
>                 Key: HADOOP-2758
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2758
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: dfs
>            Reporter: Raghu Angadi
>            Assignee: Raghu Angadi
>             Fix For: 0.17.0
>
>         Attachments: HADOOP-2758.patch, HADOOP-2758.patch, HADOOP-2758.patch, HADOOP-2758.patch,
HADOOP-2758.patch, HADOOP-2758.patch
>
>
> Currently datanode and client part of DFS perform multiple copies of data on the 'read
path' (i.e. path from storage on datanode to user buffer on the client). This jira reduces
these copies by enhancing data read protocol and implementation of read on both datanode and
the client. I will describe the changes in next comment.
> Requirement is that this fix should reduce CPU used and should not cause regression in
any benchmarks. It might not improve the benchmarks since most benchmarks are not cpu bound.

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