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https://issues.apache.org/jira/browse/CURATOR-447?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16361667#comment-16361667
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ASF GitHub Bot commented on CURATOR-447:
----------------------------------------
Github user dragonsinth commented on a diff in the pull request:
https://github.com/apache/curator/pull/250#discussion_r167733216
--- Diff:
curator-recipes/src/main/java/org/apache/curator/framework/recipes/cache/TreeCache.java
---
@@ -702,19 +687,18 @@ private TreeNode find(String findPath)
ImmutableMap.Builder<String, ChildData> builder =
ImmutableMap.builder();
for ( Map.Entry<String, TreeNode> entry : map.entrySet() )
{
- TreeNode childNode = entry.getValue();
- ChildData childData = childNode.childData;
- // Double-check liveness after retreiving data.
- if ( childData != null && childNode.nodeState ==
NodeState.LIVE )
+ ChildData childData = entry.getValue().childData;
+ // Double-check liveness after retrieving data.
+ if ( isLive(childData) )
--- End diff --
It's subtle, but the point I'm trying to make is that you have to read
`.childData` exactly once, then check the liveness, then use only that value
going forward. It would be an error to remove the local variable here and
reference `entry.getValue().childData` twice.
It's a "double" check because _generally_ there shouldn't be any non-LIVE
children in the parent `children` map that we're iterating. But due to
volatility we have to "double check" that child nodes read from the map are
still alive after pulling `.childData`
> TreeCache: Improve memory usage and concurrent update logic
> -----------------------------------------------------------
>
> Key: CURATOR-447
> URL: https://issues.apache.org/jira/browse/CURATOR-447
> Project: Apache Curator
> Issue Type: Improvement
> Components: Recipes
> Affects Versions: 3.3.0, 2.12.0
> Reporter: Nick Hill
> Priority: Minor
> Labels: performance
>
> Jira https://issues.apache.org/jira/browse/CURATOR-374 reduced per-node
> memory usage in {{TreeCache}}. It can be improved further via removal of the
> {{nodeState}} field - its {{LIVE}} state corresponds exactly to the adjacent
> {{childData}} field being non-null, and a sentinel {{ChildData}} value can be
> used for the {{DEAD}} state. This simplification also reduces the room for
> bugs and state inconsistencies.
> Other improvements included:
> * A further simplification to have {{TreeNode}} extend {{AtomicReference}},
> which obviates the need for an explicit {{childData}} field
> * More robust cache update logic (in get-children and get-data event
> callbacks)
> * Avoid overhead of incrementing/decrementing the {{outstandingOps}} atomic
> integer post-initialization
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