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_r167180844
 
 

 ##########
 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())
+                    .build(readChannelLoader);
+        }
+    };
+
+    @VisibleForTesting
+    long getReadChannelCacheExpireTimeMs() {
+        return readChannelCacheExpireTimeMs;
+    }
+
+    @VisibleForTesting
+    CacheLoader<Long, BufferedReadChannel> getReadChannelLoader() {
+        return readChannelLoader;
+    }
+
+    private final  CacheLoader<Long, BufferedReadChannel> readChannelLoader =
+            new CacheLoader<Long, BufferedReadChannel> () {
+        public BufferedReadChannel load(Long entryLogId) throws Exception {
+
+            return getChannelForLogId(entryLogId);
+
         }
     };
 
     /**
-     * Each thread local buffered read channel can share the same file handle 
because reads are not relative
-     * and don't cause a change in the channel's position. We use this map to 
store the file channels. Each
-     * file channel is mapped to a log id which represents an open log file.
+     * FileChannelBackingCache used to cache RefCntFileChannels for read.
+     * In order to avoid get released file, adopt design of 
FileInfoBackingCache.
+     * @see FileInfoBackingCache
      */
-    private final ConcurrentMap<Long, FileChannel> logid2FileChannel = new 
ConcurrentHashMap<Long, FileChannel>();
+    class FileChannelBackingCache {
+        static final int DEAD_REF = -0xdead;
+
+        final ConcurrentHashMap<Long, CachedFileChannel> fileChannels = new 
ConcurrentHashMap<>();
+
+        CachedFileChannel loadFileChannel(long logId) throws IOException {
+            CachedFileChannel cachedFileChannel = fileChannels.get(logId);
+            if (cachedFileChannel != null) {
+                boolean retained = cachedFileChannel.tryRetain();
+                assert(retained);
+                return cachedFileChannel;
+            }
+            File file = findFile(logId);
+            // get channel is used to open an existing entry log file
+            // it would be better to open using read mode
+            FileChannel newFc = new RandomAccessFile(file, "r").getChannel();
+            cachedFileChannel = new CachedFileChannel(logId, newFc);
+            fileChannels.put(logId, cachedFileChannel);
+            boolean retained = cachedFileChannel.tryRetain();
+            assert(retained);
+            return cachedFileChannel;
+        }
+
+        /**
+         * close FileChannel and remove from cache when possible.
+         * @param logId
+         * @param fc
+         */
+        private void releaseFileChannel(long logId, CachedFileChannel fc) {
+            if (fc.markDead()) {
+                try {
+                    fc.fc.close();
+                } catch (IOException e) {
+                    LOG.warn("Exception occurred in 
ReferenceCountedFileChannel"
+                            + " while closing channel for log file: {}", fc);
+                } finally {
+                    IOUtils.close(LOG, fc.fc);
+                }
+                fileChannels.remove(logId);
 
 Review comment:
   it shouldn't need locking. there's a remove method that takes the key and 
the value, and only removes if both match.

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