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https://issues.apache.org/jira/browse/CASSANDRA-15202?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16885370#comment-16885370
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Benedict commented on CASSANDRA-15202:
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LGTM. Some minor suggestions:
* Use {{ByteOrder.LITTLE_ENDIAN}} for off heap?
* {{RandomPartitioner.MAXIMUM_TOKEN_SIZE}}: use {{(bitLength + 7) / 8}}?
* {{differenceHelper}}: replace int return values with explicit Enum?
* {{OffHeapNode.attachRef}}: use {{null}} referent? Should probably (perhaps
another time) introduce a special variant for these pure leak-tracking use
cases.
* {{OnHeapLeaf.deserialize}}: verify {{in.readByte() == HASH_SIZE}}?
* Extract shared method {{OffHeapInner.child}} from {{left}} and {{right}}?
* Extract shared method for off heap disabled warning?
* {{prefer_offheap_merkle_trees}} - why prefer?
* {{FBUtilities.xor}} and {{FBUtilities.xorOntoLeft}} - move to {{MerkleTree}}?
* Missing line break: {{MerkleTree.release}}, {{OffHeapInner.maxOffHeapSize}}?
* Rename test-only methods to {{unsafeMethodName}} and disappear to bottom of
class?
* "TODO: reset computed flag on OnHeapInners" - simply ignore/remove TODO if
renamed to {{unsafeInvalidateHelper}}?
> 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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