title: AWS ELB doesn't distribute requests to auto scaling group EC2 instances in some cases
body: <p>I'm trying to do performance testing for my AWS auto scaling group using jmeter.</p>
  
  <p>Firstly, I did scale-in/out testing. I set the threshold to be 70% cpu utilization for 2 periods, each period is 2 minutes. The ELB works fine, and the requests was distributed to all EC2 instances in auto scaling group, in spite of un-equality, after the system scale-out.</p>
  
  <p>In next, I want to test whether the two instances' load can be twice of one instance's.
  I fixed the instance number of auto scaling group, I set the min/max/desired instance count to be 2. When I push load from single JMeter, there are always just only one instance works and its cpu utilization reach almost 100 percent, but the cpu utilization of the other instance is still zero.... If I push load from an JMeter cluster which contains several slaves, all instances take load. </p>
  
  <p>Somebody said, maybe the load is not heavy enough, so the ELB considered that just one instance can handle it and didn't dispatch requests to other instance. I don't think so, because I push load from just one slave of this JMeter cluster, however I increase the load, just only one instance handle requests.</p>
  
  <p>I found an blog which said ELB is great in HA but not load balancing.
  <a href="https://www.stackdriver.com/elb-affinity-problems" rel="noreferrer">https://www.stackdriver.com/elb-affinity-problems</a>
  But, I don't think the behavior, only one instance handle requests, is normal.</p>
  
  <p>What the hell in the ELB load balance mechanism? I'm confused. </p>