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https://issues.apache.org/jira/browse/HDFS-4710?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13635501#comment-13635501
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Gopal V commented on HDFS-4710:
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It is not really a one-size-fits-all fix, I guess.
For large reads, a direct read would be the right option to avoid having
another copy.
But for small repeated reads, it does make sense to bounce buffer.size chunks
through a buffer to avoid syscall overhead.
> 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.
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