ivankelly commented on a change in pull request #832: Issue 620: Close the 
fileChannels for read when they are idle
URL: https://github.com/apache/bookkeeper/pull/832#discussion_r167220014
 
 

 ##########
 File path: 
bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/EntryLogger.java
 ##########
 @@ -330,54 +333,216 @@ private int readFromLogChannel(long entryLogId, 
BufferedReadChannel channel, Byt
      * A thread-local variable that wraps a mapping of log ids to 
bufferedchannels
      * These channels should be used only for reading. logChannel is the one
      * that is used for writes.
+     * We use this Guava cache to store the BufferedReadChannel.
+     * When the BufferedReadChannel is removed, the underlying fileChannel's 
refCnt decrease 1,
+     * temporally use 1h to relax replace after reading.
      */
-    private final ThreadLocal<Map<Long, BufferedReadChannel>> logid2Channel =
-            new ThreadLocal<Map<Long, BufferedReadChannel>>() {
+    private final ThreadLocal<Cache<Long, BufferedReadChannel>> logid2Channel =
+            new ThreadLocal<Cache<Long, BufferedReadChannel>>() {
         @Override
-        public Map<Long, BufferedReadChannel> initialValue() {
+        public Cache<Long, BufferedReadChannel> initialValue() {
             // Since this is thread local there only one modifier
             // We dont really need the concurrency, but we need to use
             // the weak values. Therefore using the concurrency level of 1
-            return new MapMaker().concurrencyLevel(1)
-                .weakValues()
-                .makeMap();
+            return CacheBuilder.newBuilder().concurrencyLevel(1)
+                    .expireAfterAccess(readChannelCacheExpireTimeMs, 
TimeUnit.MILLISECONDS)
+                    //decrease the refCnt
+                    .removalListener(removal -> logid2FileChannel.get((Long) 
removal.getKey()).release())
 
 Review comment:
   A lock would just tangle it up more.
   
   You don't have to modify BufferedReadChannel. There's a couple of options. 
   1. You could create a specialization of BufferedReadChannel local to this 
class which has the CachedFileChannel as a member and use that in the tls cache.
   2. You could create a container class which references both the 
bufferedReadChannel and the CachedFileChannel and use that in the TLS cache. 
   
   I think 1 is the better option, as you'll only have to change a few 
signatures.

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