Thread in 'parking to wait' state on Redis dequeue

Question

I have a tomcat - spring4.2 application that runs multiple threads. Each thread dequeues from only one queue, however there are more than one threads assigned to a queue.

Things start fine, but after few hours / ~500k dequeue operations, I find threads dequeue at extremely slow rate.

In jvisualvm I see the threads in orange i.e. park The thread dump is as follows:

"EMLT_2" - Thread t@64
   java.lang.Thread.State: WAITING
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for <2cf42d7> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at org.apache.commons.pool2.impl.LinkedBlockingDeque.takeFirst(LinkedBlockingDeque.java:583)
    at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:442)
    at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:363)
    at redis.clients.util.Pool.getResource(Pool.java:48)
    at redis.clients.jedis.JedisPool.getResource(JedisPool.java:86)
    at com.mycomp.sam.processors.SimpleDequeuer.dequeue(SimpleDequeuer.java:25)
    at com.mycomp.sam.processors.EMLT.run(EMLT.java:29)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None

"EMLT_1" - Thread t@63
   java.lang.Thread.State: WAITING
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for <2cf42d7> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at org.apache.commons.pool2.impl.LinkedBlockingDeque.takeFirst(LinkedBlockingDeque.java:583)
    at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:442)
    at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:363)
    at redis.clients.util.Pool.getResource(Pool.java:48)
    at redis.clients.jedis.JedisPool.getResource(JedisPool.java:86)
    at com.mycomp.sam.processors.SimpleDequeuer.dequeue(SimpleDequeuer.java:25)
    at com.mycomp.sam.processors.EMLT.run(EMLT.java:29)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None

The dequeue method is :

public String dequeue(String queue) {
        try (Jedis jedis = jedispool.getResource()) {
            List<String> str = jedis.blpop(10, queue);
            if(str!=null){
                return str.get(1);
            }
            else 
                return null; 
        }
    }

Will appreciate inputs. The app again works well for a while post restart. Pool conf:

<bean id="poolConfig" class="redis.clients.jedis.JedisPoolConfig">
    <property name="maxIdle" value="10" />
    <property name="maxTotal" value="70" />
    <property name="minIdle" value="10" />
</bean>
<bean id="jedispool" class="redis.clients.jedis.JedisPool">
    <constructor-arg name="poolConfig" ref="poolConfig" />
    <constructor-arg name="host" value="${REDIS_HOST}" />
    <constructor-arg name="port" value="6379" />
</bean>

Answer 1

It appears your jedis pool is running out of connections. Are you returning resources with returnResourceObject or returnResource? (I know both are deprecated, but they still work with the lates version of jedis).

Keep in mind that a jedis pool is not exactly like a DBCP/Apache Pool.

I had a similar issue and it ended when i called the methods above.

On the other hand, since Redis is pretty fast, maybe you should rethink your schema, and have very few threads calling blpop (one per queue) and the forward the values to the other threads, so those don't know about redis if they don't have to.