nlm: improve concurrency of simple lock manager

Motivation:
when a large number of concurrent applications competing for file locks,
then SimpleLm will serialise all request as a single lock object is used
for all files. To reduce lock conjunction and improve concurrency a
serialization per lock subject (file) is a better option.

Modification:
Use Striped lock to partition locks. The number of partition is equal
4x#CPUs. It's guaranteed, that for the same file, the same lock will be
used.

Result:
Better lock throughput.

Acked-by: Paul Millar
Target: master
Require-book: no
Require-notes: no

Author: kofemann

core/src/main/java/org/dcache/nfs/v4/nlm/SimpleLm.java

@@ -22,9 +22,9 @@
 import com.google.common.collect.ArrayListMultimap;
 import com.google.common.collect.Multimap;
 import com.google.common.io.BaseEncoding;
+import com.google.common.util.concurrent.Striped;
 import java.util.Collection;
 import java.util.concurrent.locks.Lock;
-import java.util.concurrent.locks.ReentrantLock;
 
 /**
  * Simple non-distributed implementation of {@link LockManager}.
@@ -33,10 +33,17 @@
  */
 public class SimpleLm extends AbstractLockManager {
 
-    /**
-     * Exclusive lock on file object.
+    /*
+     * Use {@link Striped} here to split synchronized block on file locks into
+     * multiple partitions to increase concurrency, while guaranteeing atomicity
+     * on a single file.
+     *
+     * Use number of stripes equals to 4x#CPU. This matches to number of
+     * worker threads configured by default.
+     *
+     * FIXME: get number of threads from RPC service.
      */
-    private final Lock objLock = new ReentrantLock();
+    private final Striped<Lock> objLock = Striped.lock(Runtime.getRuntime().availableProcessors() * 4);
 
     /**
      * Exclusive lock on objects locks.
@@ -45,37 +52,42 @@ public class SimpleLm extends AbstractLockManager {
 
     @Override
     protected Lock getObjectLock(byte[] objId) {
-        return objLock;
+        String key = toKey(objId);
+        return objLock.get(key);
     }
 
     @Override
     protected Collection<NlmLock> getActiveLocks(byte[] objId) {
-        String key = BaseEncoding.base16().lowerCase().encode(objId);
+        String key = toKey(objId);
         return locks.get(key);
     }
 
     @Override
     protected void add(byte[] objId, NlmLock lock) {
-        String key = BaseEncoding.base16().lowerCase().encode(objId);
+        String key = toKey(objId);
         locks.put(key, lock);
     }
 
     @Override
     protected boolean remove(byte[] objId, NlmLock lock) {
-        String key = BaseEncoding.base16().lowerCase().encode(objId);
+        String key = toKey(objId);
         return locks.remove(key, lock);
     }
 
     @Override
     protected void addAll(byte[] objId, Collection<NlmLock> locks) {
-        String key = BaseEncoding.base16().lowerCase().encode(objId);
+        String key = toKey(objId);
         locks.forEach(l -> this.locks.put(key, l));
     }
 
     @Override
     protected void removeAll(byte[] objId, Collection<NlmLock> locks) {
-        String key = BaseEncoding.base16().lowerCase().encode(objId);
+        String key = toKey(objId);
         locks.forEach(l -> this.locks.remove(key, l));
     }
 
+    private final String toKey(byte[] objId) {
+        return BaseEncoding.base16().lowerCase().encode(objId);
+    }
+
 }