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https://issues.apache.org/jira/browse/ZOOKEEPER-2024?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15284001#comment-15284001
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Alexander Shraer commented on ZOOKEEPER-2024:
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[~fpj], ideally I'd also like it to be the case. But, many people have reviewed
and gave comments on this patch, and we can't possibly wait for all of them.
Specifically your comments have been addressed 13 Apr and since then we've been
pinging on this thread.
If there's something wrong with the patch then by all means revert it, but
dragging the review process for months and years (this patch is been
at works since 2014!) is not the way this community should work.
> Major throughput improvement with mixed workloads
> -------------------------------------------------
>
> Key: ZOOKEEPER-2024
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2024
> Project: ZooKeeper
> Issue Type: Improvement
> Components: quorum, server
> Reporter: Kfir Lev-Ari
> Assignee: Kfir Lev-Ari
> Fix For: 3.6.0
>
> Attachments: ZOOKEEPER-2024.patch, ZOOKEEPER-2024.patch,
> ZOOKEEPER-2024.patch, ZOOKEEPER-2024.patch, ZOOKEEPER-2024.patch,
> ZOOKEEPER-2024.patch, ZOOKEEPER-2024.patch, ZOOKEEPER-2024.patch,
> ZOOKEEPER-2024.patch, ZOOKEEPER-2024.patch, ZOOKEEPER-2024.patch,
> ZOOKEEPER-2024.patch, ZOOKEEPER-2024.patch
>
>
> The patch is applied to the commit processor, and solves two problems:
> 1. Stalling - once the commit processor encounters a local write request, it
> stalls local processing of all sessions until it receives a commit of that
> request from the leader.
> In mixed workloads, this severely hampers performance as it does not allow
> read-only sessions to proceed at faster speed than read-write ones.
> 2. Starvation - as long as there are read requests to process, older remote
> committed write requests are starved.
> This occurs due to a bug fix
> (https://issues.apache.org/jira/browse/ZOOKEEPER-1505) that forces processing
> of local read requests before handling any committed write. The problem is
> only manifested under high local read load.
> Our solution solves these two problems. It improves throughput in mixed
> workloads (in our tests, by up to 8x), and reduces latency, especially higher
> percentiles (i.e., slowest requests).
> The main idea is to separate sessions that inherently need to stall in order
> to enforce order semantics, from ones that do not need to stall. To this end,
> we add data structures for buffering and managing pending requests of stalled
> sessions; these requests are moved out of the critical path to these data
> structures, allowing continued processing of unaffected sessions.
> Please see the docs:
> 1) https://goo.gl/m1cINJ - includes a detailed description of the new commit
> processor algorithm.
> 2) The attached patch implements our solution, and a collection of related
> unit tests (https://reviews.apache.org/r/25160)
> 3) https://goo.gl/W0xDUP - performance results.
> (See https://issues.apache.org/jira/browse/ZOOKEEPER-2023 for the
> corresponding new system test that produced these performance measurements)
>
> See also https://issues.apache.org/jira/browse/ZOOKEEPER-1609
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