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https://issues.apache.org/jira/browse/HDFS-17813?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18009880#comment-18009880
]
ASF GitHub Bot commented on HDFS-17813:
---------------------------------------
khazhen opened a new pull request, #7829:
URL: https://github.com/apache/hadoop/pull/7829
### Description of PR
Refer to [HDFS-17813](https://issues.apache.org/jira/browse/HDFS-17813).
The NameCache class is used to cache frequently used names in namenode, it
promotes a name used more than useThreshold to the cache, the promote logic:
```
K put(final K name) {
K internal = cache.get(name);
if (internal != null) {
lookups++;
return internal;
}
// Track the usage count only during initialization
if (!initialized) {
UseCount useCount = transientMap.get(name);
if (useCount != null) {
useCount.increment();
if (useCount.get() >= useThreshold) {
promote(name); // name got promoted
}
return useCount.value;
}
useCount = new UseCount(name);
transientMap.put(name, useCount);
}
return null;
}
```
When promoting, the cache stores the `name` parameter from put() instead of
the existing useCount.value. This causes the returned value to change after a
name is promoted, resulting in memory duplication.
> NameCache promote wrong name to cache map
> -----------------------------------------
>
> Key: HDFS-17813
> URL: https://issues.apache.org/jira/browse/HDFS-17813
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namenode
> Reporter: kazan
> Priority: Minor
>
> The NameCache class is used to cache frequently used names in namenode, it
> promotes a name used more than useThreshold to the cache, the promote logic:
> {code:java}
> K put(final K name) {
> K internal = cache.get(name);
> if (internal != null) {
> lookups++;
> return internal;
> }
> // Track the usage count only during initialization
> if (!initialized) {
> UseCount useCount = transientMap.get(name);
> if (useCount != null) {
> useCount.increment();
> if (useCount.get() >= useThreshold) {
> promote(name); // name got promoted
> }
> return useCount.value;
> }
> useCount = new UseCount(name);
> transientMap.put(name, useCount);
> }
> return null;
> } {code}
> When promoting, the cache stores the `name` parameter from put() instead of
> the existing useCount.value. This causes the returned value to change after a
> name is promoted, resulting in memory duplication.
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