hadoop-hdfs-issues mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Gopal V (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4710) Turning off HDFS short-circuit checksums unexpectedly slows down Hive
Date Wed, 24 Apr 2013 20:55:17 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4710?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13640942#comment-13640942
] 

Gopal V commented on HDFS-4710:
-------------------------------

I did consider that as a fix, but a regular BufferedInputStream isn't seekable (particularly
backwards), which would need to be reimplemented on top of it.

This needs some of the same ByteBuffer juggling used in the verify checksum mode - to buffer
data in chunks, but drop them when seeking.
                
> Turning off HDFS short-circuit checksums unexpectedly slows down Hive
> ---------------------------------------------------------------------
>
>                 Key: HDFS-4710
>                 URL: https://issues.apache.org/jira/browse/HDFS-4710
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs-client
>    Affects Versions: 2.0.4-alpha
>         Environment: Centos (EC2) + short-circuit reads on
>            Reporter: Gopal V
>            Priority: Minor
>              Labels: perfomance
>
> When short-circuit reads are on, HDFS client slows down when checksums are turned off.
> With checksums on, the query takes 45.341 seconds and with it turned off, it takes 56.345
seconds. This is slower than the speeds observed when short-circuiting is turned off.
> The issue seems to be that FSDataInputStream.readByte() calls are directly transferred
to the disk fd when the checksums are turned off.
> Even though all the columns are integers, the data being read will be read via DataInputStream
which does
> {code}
> public final int readInt() throws IOException {
>         int ch1 = in.read();
>         int ch2 = in.read();
>         int ch3 = in.read();
>         int ch4 = in.read();
> {code}
> To confirm, an strace of the Yarn container shows
> {code}
> 26690 read(154, "B", 1)                 = 1
> 26690 read(154, "\250", 1)              = 1
> 26690 read(154, ".", 1)                 = 1
> 26690 read(154, "\24", 1)               = 1
> {code}
> To emulate this without the entirety of Hive code, I have written a simpler test app

> https://github.com/t3rmin4t0r/shortcircuit-reader
> The jar will read a file in -bs <n> sized buffers. Running it with 1 byte blocks
gives similar results to the Hive test run.

--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators
For more information on JIRA, see: http://www.atlassian.com/software/jira

Mime
View raw message