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From "Liang Xie (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6735) A minor optimization to avoid pread() be blocked by read() inside the same DFSInputStream
Date Wed, 23 Jul 2014 06:24:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6735?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14071409#comment-14071409
] 

Liang Xie commented on HDFS-6735:
---------------------------------

bq.  We'd not check in the test since it does not assert anything? We'd just check it in as
a utility testing concurrent pread throughput?
In the last of testing code snippet, there're assertion, see:
{code}
      assertTrue(readLatency.readMs > readLatency.readMs);
      //because we issued a pread already, so the second one should not hit
      //disk, even consider running on a slow VM, 1 second should be fine?
      assertTrue(readLatency.preadMs < 1000);
{code}
Per "assertTrue(readLatency.preadMs < 1000);", we could know weather the pread() be blocked
by read() or not :)

> A minor optimization to avoid pread() be blocked by read() inside the same DFSInputStream
> -----------------------------------------------------------------------------------------
>
>                 Key: HDFS-6735
>                 URL: https://issues.apache.org/jira/browse/HDFS-6735
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 3.0.0
>            Reporter: Liang Xie
>            Assignee: Liang Xie
>         Attachments: HDFS-6735.txt
>
>
> In current DFSInputStream impl, there're a couple of coarser-grained locks in read/pread
path, and it has became a HBase read latency pain point so far. In HDFS-6698, i made a minor
patch against the first encourtered lock, around getFileLength, in deed, after reading code
and testing, it shows still other locks we could improve.
> In this jira, i'll make a patch against other locks, and a simple test case to show the
issue and the improved result.
> This is important for HBase application, since in current HFile read path, we issue all
read()/pread() requests in the same DFSInputStream for one HFile. (Multi streams solution
is another story i had a plan to do, but probably will take more time than i expected)



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