sijie 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_r171943920
 
 

 ##########
 File path: 
bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/EntryLogger.java
 ##########
 @@ -326,59 +353,45 @@ private int readFromLogChannel(long entryLogId, 
BufferedReadChannel channel, Byt
         return channel.read(buff, pos);
     }
 
+    final ConcurrentLinkedQueue<Cache<Long, EntryLogBufferedReadChannel>> 
readChannelCaches =
+            new ConcurrentLinkedQueue<>();
+
     /**
      * 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>>() {
+    final ThreadLocal<Cache<Long, EntryLogBufferedReadChannel>> 
logid2ReadChannel =
+            new ThreadLocal<Cache<Long, EntryLogBufferedReadChannel>>() {
         @Override
-        public Map<Long, BufferedReadChannel> initialValue() {
+        public Cache<Long, EntryLogBufferedReadChannel> 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();
+            Cache<Long, EntryLogBufferedReadChannel> cache =
+                    CacheBuilder.newBuilder().concurrencyLevel(1)
+                    .expireAfterAccess(readChannelCacheExpireTimeMs, 
TimeUnit.MILLISECONDS)
+                    //decrease the refCnt
+                    .removalListener(( RemovalListener<Long, 
EntryLogBufferedReadChannel>) removal
 
 Review comment:
   nit:
   
   it is a bit hard to read. can you write it in a better format? also remove 
space before `RemovalListener`.
   
   ```
   .removal((RemovalListener<Long, EntryLogBufferedReadChannel>) notification ->
       removal.getValue().release()
   )
   .ticker(getTicker())
   .build();

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