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From "Nishkam Ravi (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-11152) Better random number generator
Date Tue, 30 Sep 2014 07:11:34 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-11152?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14152884#comment-14152884

Nishkam Ravi commented on HADOOP-11152:

Unlike Random, round-robin isn't stateless and gets complicated in the presence of node failures.
The code change would be significant as well. A variant of power-of-two: pick a random node,
if the load is higher than a threshold, try again until a good DN is found, with a limit on
the number of trials. This should provide good probabilistic guarantees on the worst-case

> Better random number generator
> ------------------------------
>                 Key: HADOOP-11152
>                 URL: https://issues.apache.org/jira/browse/HADOOP-11152
>             Project: Hadoop Common
>          Issue Type: Improvement
>            Reporter: Luke Lu
>              Labels: newbie++
> HDFS-7122 showed that naive ThreadLocal usage of simple LCG based j.u.Random creates
unacceptable distribution of random numbers for block placement. Similarly, ThreadLocalRandom
in java 7 (same static thread local with synchronized methods overridden) has the same problem.

> "Better" is defined as better quality and faster than j.u.Random (which is already much
faster (20x) than SecureRandom).
> People (e.g. Numerical Recipes) have shown that by combining LCG and XORShift we can
have a better fast RNG. It'd be worthwhile to investigate a thread local version of these
"better" RNG.

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