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https://issues.apache.org/jira/browse/HADOOP-19346?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17901357#comment-17901357
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ASF GitHub Bot commented on HADOOP-19346:
-----------------------------------------
ctrezzo merged PR #7187:
URL: https://github.com/apache/hadoop/pull/7187
> ViewFileSystem.InnerCache: Replaced ReentrantReadWriteLock with
> ConcurrentHashMap/putIfAbsent()
> -----------------------------------------------------------------------------------------------
>
> Key: HADOOP-19346
> URL: https://issues.apache.org/jira/browse/HADOOP-19346
> Project: Hadoop Common
> Issue Type: Improvement
> Components: common
> Reporter: Xing Lin
> Assignee: Xing Lin
> Priority: Minor
> Labels: pull-request-available
>
> Use of ReentrantReadWriteLock + Map can be replaced/simplified with a
> ConcurrentHashMap.
> Right now, we are using a ReentrantReadWriteLock to protect concurrent
> updates to InnerCache.map().
> As a performance optimization, to reduce acquiring writeLock, we first
> acquire readLock, check existence of the key and then acquire writeLock, to
> insert the new key.
> {code:java}
> FileSystem get(URI uri, Configuration config) throws IOException {
> Key key = new Key(uri);
> FileSystem fs = null;
> try {
> rwLock.readLock().lock();
> fs = map.get(key);
> if (fs != null) {
> return fs;
> }
> } finally {
> rwLock.readLock().unlock();
> }
> try {
> rwLock.writeLock().lock();
> fs = map.get(key);
> if (fs != null) {
> return fs;
> }
> fs = fsCreator.getNewInstance(uri, config);
> map.put(key, fs);
> return fs;
> } finally {
> rwLock.writeLock().unlock();
> }
> }
> {code}
> The above function is already available as computeIfAbsent() from
> ConcurrentHashMap, which atomically checks existence of a key and inserts the
> new key if not there. This greatly simplifies the code. On the other hand, it
> should improve performance as well: concurrentHashMap supports finer-grain
> concurrency vs. a single ReadWrite Lock.
> We can replace the above code with the following one and it is much simpler.
> {code:java}
> FileSystem get(URI uri, Configuration config) throws IOException {
> Key key = new Key(uri);
> FileSystem fs = map.computeIfAbsent(key, k -> getNewFileSystem(uri,
> config));
> if (fs == null) {
> throw new IOException("Failed to create new FileSystem instance for "
> + uri);
> }
> return fs;
> {code}
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