Kafka read balance between logstash instances

I'm using logstash2.1(logstash-2.1.1-1.noarch.rmp). I hit a issue about the balancing between consumer threads of multiple logstash instance. I have 1 topic with 24 partitions, 3 logstash instances to consume this topic. The kafka input config on each logstash instance is:

```
input {
kafka {
zk_connect => "x.x.x.x:2181, y.y.y.y:2181, z.z.z.z:2181"
topic_id => "test"
consumer_threads => 8
fetch_message_max_bytes => 20971520
}
}
```

However, I found the topic was consumed not evenly from Kafka:

```
$./kafka-run-class.sh kafka.tools.ConsumerOffsetChecker --group logstash --topic test
Group Topic Pid Offset logSize Lag Owner
logstash test 0 15915 16401 486 host1
logstash test 1 17535 20657 3122 host1
logstash test 2 9833 9833 0 host1
logstash test 3 12822 12871 49 host1
logstash test 4 7599 7705 106 host1
logstash test 5 18055 18057 2 host1
logstash test 6 14273 14395 122 host1
logstash test 7 15870 16292 422 host1
logstash test 8 2211 2211 0 host2
logstash test 9 4442 4442 0 host2
logstash test 10 8854 8854 0 host2
logstash test 11 5618 5618 0 host2
logstash test 12 4460 4460 0 host2
logstash test 13 6607 6607 0 host2
logstash test 14 4432 4432 0 host2
logstash test 15 8882 8882 0 host2
logstash test 16 4536 6347 1811 host2
logstash test 17 8867 41720 32853 host2
logstash test 18 5420 10002 4582 host2
logstash test 19 5106 8267 3161 host2
logstash test 20 18694 18694 0 host3
logstash test 21 9054 9054 0 host3
logstash test 22 4444 4444 0 host3
logstash test 23 4400 4402 2 host3
```

host1 consumed 8 partitions, but host2 consumed 12 partitions, it should be 8 partitions. So host3 could only consume 4 partitions.
We could also see that the additional 4 partitions consumed by host2 have many messages lagged behind.

Any idea to balance consumers across multiple hosts?

Thanks,
Kenny