Disabling Hyperthreading in EC2 Cluster Compute instance:

#!/bin/bash
for cpunum in $(
cat /sys/devices/system/cpu/cpu*/topology/thread_siblings_list |
cut -s -d, -f2- | tr ',' '\n' | sort -un); do
echo 0 > /sys/devices/system/cpu/cpu$cpunum/online
done

https://forums.aws.amazon.com/message.jspa?messageID=189757