Hi,

I launched an AWS instance `t2.medium` (use CentOS 7 image "ami-7abd0209", product code: https://aws.amazon.com/marketplace/pp/B00O7WM7QW), which is supposed to have 4GB Memory in total, but turn out it is only "3.5GB". 

```
$ free -h
              total        used        free      shared  buff/cache   available
Mem:           3.5G        441M        1.4G         16M        1.6G        2.7G
Swap:            0B          0B          0B
```

It does not happen to AWS Ubuntu 16.04.

```
$ free -h
              total        used        free      shared  buff/cache   available
Mem:           3.9G         49M        3.3G        5.5M        518M        3.7G
Swap:            0B          0B          0B
```

This seems to happen to all AWS CentOS image, regardless which instance type it is.