[ https://issues.apache.org/jira/browse/ZOOKEEPER-2789?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16050139#comment-16050139 ]
ASF GitHub Bot commented on ZOOKEEPER-2789:
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Github user yunfan123 commented on the issue:
https://github.com/apache/zookeeper/pull/262
Hi, I think 48 bits low is better for large throughput zk cluster.
Another benefits is when use 48 bits low we assuming the epoch low than (1<<16), so we can 16 bits high to judge whether it is old version or new version.
So use 48 bits low we can make the upgrade progress smoothly
> 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
> Assignee: 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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