From https://aws.amazon.com/blogs/aws/now-available-new-c4-instan... :
In some cases, your workload might not need all 18 of the cores (each of which
runs two hyperthreads, for a total of 36 vCPUs on c4.8xlarge). To tune your
application for better performance, you can manage the power consumption on a
per-core basis. This is known as C-state management, and gives you control over
the sleep level that a core may enter when idle. Let’s say that your code needs
just two cores. Your operating system can set the other 16 cores to a state that
draws little or no power, thereby creating some thermal headroom that will give
the remaining cores an opportunity to Turbo Boost. You also have control over
the desired performance (CPU clock frequency); this is known as P-state
management. You should consider changing C-state settings to decrease CPU
latency variability (cores in a sleep state consume less power, but deeper sleep
states require longer to become active when needed) and consider changing
P-state settings to adjust the variability in CPU frequency in order to best
meet the needs of your application.