Amazon EC2: Difference between Micro and Small Instance types


I recently did a small load test a micro and a small instance type on Amazon EC2. I had some surprising results and they were the exact opposite of what I was expecting and was hoping someone could shed some light on it or give me some ideas to figure out what is going on.

Setup: Micro instance setup the way I wanted with a trunk, inbound routes, extensions and some very basic call handling rules.

Micro instance test: I used soft phones and some help from a friend with 2 cell phones, I had a total of 10 active calls going to a conference bridge. The call quality was fine and the latency wasn’t even noticeable and we were able to have a normal conversation. The Micro instance ran at 60% CPU with these 10 calls I used the CloudWatch metrics to track CPU usage.

I was satisfied with the performance of the Micro instance so I decided to scale it up and create and AMI out of the Micro instance I had used to the do the test and start a Small instance.

Small instance test: When I first started the instance I had to reconfigure NAT. I did the same type of test with the conference call bridge, but this time at about the 4th or 5th active call we started to experience 3-5 second lag. Meaning that I would say something on my Verizon cell phone and would hear it come through the soft phone on my laptop 3-5 seconds later. My friend with the cell phones also experienced this as well. They would say something, I and their second cell phone would hear it 3-5 seconds later. I tested it on two different availability zones east-1d and east-1c with the exact same result. While we were experiencing the lag I did a *43 echo test and there was almost no latency, things were repeated back to me almost instantly.

I verified that the kernel is a 1000hz one and am using asterick 1.8 with freepbx 2.1. I used the a pre-built ami: ami-96d076ff

The only major modification I made to the AMI is added a 1GB partition that I setup as swap, but this was never used in either tests.

I am planning on trying a couple things: Copy the existing AMI and make it a Micro and do a test. Create the AMI from the ami-96d076ff image and make it a small instead of copying my pre-existing ami.

Does anyone have any ideas? or Anything for me to try?