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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2789) Reassign `ZXID` for solving 32bit overflow problem
Date Wed, 24 May 2017 07:02:04 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2789?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16022407#comment-16022407
] 

ASF GitHub Bot commented on ZOOKEEPER-2789:
-------------------------------------------

Github user asdf2014 commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/262#discussion_r118176130
  
    --- Diff: src/contrib/loggraph/src/java/org/apache/zookeeper/graph/JsonGenerator.java
---
    @@ -121,8 +121,8 @@ public JsonGenerator(LogIterator iter) {
     		} else if ((m = newElectionP.matcher(e.getEntry())).find()) {
     		    Iterator<Integer> iterator = servers.iterator();
     		    long zxid = Long.valueOf(m.group(2));
    -		    int count = (int)zxid;// & 0xFFFFFFFFL;
    -		    int epoch = (int)Long.rotateRight(zxid, 32);// >> 32;
    +		    long count = zxid & 0xffffffffffL;
    +		    int epoch = (int)Long.rotateRight(zxid, 40);// >> 40;
    --- End diff --
    
    Already unify all code those processing `ZXID` into using `ZixdUtils`.


> Reassign `ZXID` for solving 32bit overflow problem
> --------------------------------------------------
>
>                 Key: ZOOKEEPER-2789
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2789
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: quorum
>    Affects Versions: 3.5.3
>            Reporter: Benedict Jin
>             Fix For: 3.6.0
>
>   Original Estimate: 168h
>  Remaining Estimate: 168h
>
> If it is `1k/s` ops, then as long as $2^32 / (86400 * 1000) \approx 49.7$ days ZXID will
exhausted. But, if we reassign the `ZXID` into 16bit for `epoch` and 48bit for `counter`,
then the problem will not occur until after  $Math.min(2^16 / 365, 2^48 / (86400 * 1000 *
365)) \approx Math.min(179.6, 8925.5) = 179.6$ years.
> However, i thought the ZXID is `long` type, reading and writing the long type (and `double`
type the same) in JVM, is divided into high 32bit and low 32bit part of the operation, and
because the `ZXID` variable is not  modified with `volatile` and is not boxed for the corresponding
reference type (`Long` / `Double`), so it belongs to [non-atomic operation] (https://docs.oracle.com/javase/specs/jls/se8
/html/jls-17.html#jls-17.7). Thus, if the lower 32 bits of the upper 32 bits are divided into
the entire 32 bits of the `long`, there may be a concurrent problem.



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