What is the last optimization you can make?

You need the absolute highest possible network performance for a cluster computing application. You already selected
homogeneous instance types supporting 10 gigabit enhanced networking, made sure that your workload was network
bound, and put the instances in a placement group. What is the last optimization you can make?

A.
Use 9001 MTU instead of 1500 for Jumbo Frames, to raise packet body to packet overhead ratios.

B.
Segregate the instances into different peered VPCs while keeping them all in a placement group, so each one has its own Internet
Gateway.

C.
Bake an AMI for the instances and relaunch, so the instances are fresh in the placement group and do not have noisy neighbors.

D.
Turn off SYN/ACK on your TCP stack or begin using UDP for higher throughput.

Explanation:
For instances that are collocated inside a placement group, jumbo frames help to achieve the maximum network
throughput possible, and they are recommended in this case. For more information, see Placement Groups.
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/network_mtu.html#jumbo_frame_instances