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https://issues.apache.org/jira/browse/SOLR-18176?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18069664#comment-18069664
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David Smiley commented on SOLR-18176:
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note: it's tempting to call this a bug but practically speaking, user's will
see this as a performance boost. Had this regression existed for only a minor
release or two, I'd not say this. I wouldn't bother changing in JIRA (I think
a new JIRA issue is created if you trie) but at least consider this for the
changelog. IMO this is a "changed", which is where improvements go generally
> HttpShardHandler query throughput bottleneck from ZooKeeper
> -----------------------------------------------------------
>
> Key: SOLR-18176
> URL: https://issues.apache.org/jira/browse/SOLR-18176
> Project: Solr
> Issue Type: Bug
> Affects Versions: 10.0, 9.10.1
> Reporter: Matthew Biscocho
> Assignee: Matthew Biscocho
> Priority: Major
> Labels: pull-request-available
> Attachments: image-2026-03-24-13-14-15-761.png
>
> Time Spent: 40m
> Remaining Estimate: 0h
>
> I found significant throughput performance bottlenecking from queries with a
> Solr cloud containing nodes sharing collections and heavily sharded. What I
> noticed was as Solr query load increased, ZooKeeper CPU utilization followed
> linearly. Taking a JFR dump, it showed that every distrib query in
> HttpShardHandler was doing a synchronized get [without allowCache=true here
> |https://github.com/apache/solr/blob/2ea21db9af976eee8ed10c08fb95e071889387be/solr/core/src/java/org/apache/solr/handler/component/CloudReplicaSource.java#L192]for
> collection state from ZooKeeper which eventually started bottlenecking
> zookeeper reads and holding QTP threads drastically making query latency
> worse.
> Changing to use cache resulting in a huge boost in query throughput and
> reduction in ZooKeeper CPU utilization.
> !image-2026-03-24-13-14-15-761.png!
> PR to follow.
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