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https://issues.apache.org/jira/browse/KAFKA-7019?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16580021#comment-16580021
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ASF GitHub Bot commented on KAFKA-7019:
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lindong28 opened a new pull request #5505: Test KAFKA-7019
URL: https://github.com/apache/kafka/pull/5505
This PR is opened solely to debug test failure for PR #5221. No need to
review.
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> Reduction the contention between metadata update and metadata read operation
> ----------------------------------------------------------------------------
>
> Key: KAFKA-7019
> URL: https://issues.apache.org/jira/browse/KAFKA-7019
> Project: Kafka
> Issue Type: Improvement
> Components: core
> Reporter: Dong Lin
> Assignee: Radai Rosenblatt
> Priority: Major
> Fix For: 2.1.0
>
>
> Currently MetadataCache.updateCache() grabs a write lock in order to process
> the UpdateMetadataRequest from controller. And a read lock is needed in order
> to handle the MetadataRequest from clients. Thus the handling of
> MetadataRequest and UpdateMetadataRequest blocks each other and the broker
> can only process such request at a time even if there are multiple request
> handler threads. Note that broker can not process MetadataRequest in parallel
> if there is a UpdateMetadataRequest waiting for the write lock, even if
> MetadataRequest only requires the read lock to e processed.
> For large cluster which has tens of thousands of partitions, it can take e.g.
> 200 ms to process UpdateMetadataRequest and MetadataRequest from large
> clients (e.g. MM). During the period when user is rebalancinng cluster, the
> leadership change will cause both UpdateMetadataRequest from controller and
> also MetadataRequest from client. If a broker receives 10 MetadataRequest per
> second and 2 UpdateMetadataRequest per second on average, since these
> requests need to be processed one-at-a-time, it can reduce the request
> handler thread idle ratio to 0 which makes this broker unavailable to user.
> We can address this problem by removing the read lock in MetadataCache. The
> idea is that MetadataCache.updateCache() can instantiate a new copy of the
> cache as method local variable when it is processing the
> UpdateMetadataRequest and replace the class private varaible with newly
> instantiated method local varaible at the end of MetadataCache.updateCache().
> The handling of MetadataRequest only requires access to the read-only
> class-private variable.
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