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https://issues.apache.org/jira/browse/CASSANDRA-15202?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Aleksey Yeschenko updated CASSANDRA-15202:
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Source Control Link:
[2117e2af00603f5fb2181e53dbcba190b2eab861|https://github.com/apache/cassandra/commit/2117e2af00603f5fb2181e53dbcba190b2eab861]
Status: Resolved (was: Ready to Commit)
Resolution: Fixed
Cheers, committed to trunk as
[2117e2af00603f5fb2181e53dbcba190b2eab861|https://github.com/apache/cassandra/commit/2117e2af00603f5fb2181e53dbcba190b2eab861]
On commit fixed {{find()}} javadoc and extended `difference()` tests to cover
serde loops of a previously moved off-heap tree. It's not a code path we ever
use in trunk, but it is something we do in 3.0, when dumping trees for
debugging.
> Deserialize merkle trees off-heap
> ---------------------------------
>
> Key: CASSANDRA-15202
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15202
> Project: Cassandra
> Issue Type: Improvement
> Components: Consistency/Repair
> Reporter: Jeff Jirsa
> Assignee: Aleksey Yeschenko
> Priority: Normal
> Fix For: 4.0
>
> Attachments: offheap-mts-gc.png
>
>
> CASSANDRA-14096 made the first step to address the heavy on-heap footprint of
> merkle trees on repair coordinators - by reducing the time frame over which
> they are referenced, and by more intelligently limiting depth of the trees
> based on available heap size.
> That alone improves GC profile and prevents OOMs, but doesn’t address the
> issue entirely. The coordinator still must hold all the trees on heap at once
> until it’s done diffing them with each other, which has a negative effect,
> and, by reducing depth, we lose precision and thus cause more overstreaming
> than before.
> One way to improve the situation further is to build on CASSANDRA-14096 and
> move the trees entirely off-heap. This is a trivial endeavor, given that we
> are dealing with what should be full binary trees (though in practice aren’t
> quite, yet). This JIRA makes the first step towards there - by moving just
> deserialisation off-heap, leaving construction on the replicas on-heap still.
> Additionally, the proposed patch fixes the issue of replica coordinators
> sending merkle trees to itself over loopback, costing us a ser/deser loop per
> tree.
> Please note that there is more room for improvement here, and depending on
> 4.0 timeline those improvements may or may not land in time. To name a few:
> - with some minor modifications to init(), we can make sure that no matter
> the range, the tree is *always* perfectly full; this would allow us to get
> rid of child pointers in inner nodes, as child node addresses will be
> trivially calculatable given fixed size of nodes
> - the trees can be easily constructed off-heap so long as you run init() to
> pre-size the tree to find out how large a buffer you need
> - on-wire format doesn’t need to stream inner nodes, only leaves, and,
> really, only the hashes of the leaves
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