Cloud Developer Tips

Play “Chicken” with Spot Instances

AWS Spot Instances have an interesting economic characteristic that make it possible to game the system a little. Like all EC2 instances, when you initiate termination of a Spot Instance then you incur a charge for the entire hour, even if you’ve used less than a full hour. But, when AWS terminates the instance due to the spot price exceeding the bid price, you do not pay for the current hour.

What if your Spot Instance could wait, after finishing its work, to see if AWS will terminate it involuntarily in this hour and avoid the hour’s cost? In the worst case, your instance can kill itself in the last few minutes of the hour and you will not have incurred any extra unplanned cost. In the best case, the spot price will rise above the instance’s bid price before the hour is up, AWS will terminate the instance involuntarily, and you will not be charged for that entire hour. Wouldn’t this technique reduce costs, especially when performed at large scale?

I call this technique Playing Chicken, based on the game of that name, because it shares similar characteristics to the game:

  • Whoever “swerves” (terminates) first, loses (pays for the hour)
  • If nobody “swerves” (terminates), then an undesirable situation occurs (the instance remains running)

How to Play Chicken

Playing Chicken is really as simple as running a script on the instance when you’re done with the work. Here’s such a script:

#! /bin/bash
if wget -q -O $t ; then
	# add 60 seconds artificially as a safety margin
	let runningSecs=$(( `date +%s` - `date -r $t +%s` ))+60
	rm -f $t
	let runningSecsThisHour=$runningSecs%3600
	let runningMinsThisHour=$runningSecsThisHour/60
	let leftMins=60-$runningMinsThisHour
	# start shutdown one minute earlier than actually required
	let shutdownDelayMins=$leftMins-1
	if [[ $shutdownDelayMins > 1 && $shutdownDelayMins < 60 ]]; then
		echo "Shutting down in $shutdownDelayMins mins."
		# TODO: Notify off-instance listener that the game of chicken has begun
		sudo shutdown -h +$shutdownDelayMins
		echo "Shutting down now."
		sudo shutdown -h now
	exit 0
echo "Failed to determine remaining minutes in this billable hour. Terminating now."
sudo shutdown -h now
exit 1

This script uses the technique published by Dmitriy Samovskiy to determine the launch time of the current instance without using the EC2 API, using the instance meta-data instead. We include a safety margin of two minutes: accounting for the remaining time conservatively adding 1 minute, and beginning the shutdown sequence one minute earlier.

You would run this script on the instance when the Spot Instance is done with its work instead of terminating the instance immediately. You also can add a hook at the indicated place to notify an off-instance listener that the game of chicken has begun, to allow you to track the savings delivered by this technique.

Warning: Make sure you really understand what this script does before you use it. If you mistakenly schedule an instance to be shut down you can cancel it with this command, run on the instance: sudo shutdown -c

How Much is Saved by Playing Chicken?

The extent to which you can benefit from playing chicken depends on a number of factors:

  • The difference between the spot price and your instance’s bid price. The further away the spot price is from your bid, the less likely it is that the spot price will hit the bid and save you money.
  • The volatility of the spot price. The more volatile the spot price, the more likely it will hit the bid and save you money.
  • The number of Spot Instances you terminate in a given period of time. If you normally don’t terminate any Spot Instances then you won’t save anything; if you terminate many then you can potentially save an hour’s worth of cost for each of them.
  • The EC2 Region and instance type. The actual spot price varies by region and instance type, so the potential savings depends on these factors as well.

I’m looking for help to work out a model that can describe the potential savings. If you are interested and able to help with the financial math, please get in touch.

Update: Hat tip to Simon Wardley who pointed out the site CloudExchange that shows great visualizations of the spot prices by region and instance type. This may help you formulate a bidding strategy.