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AWS EC2 ECS - How many tasks should I place on a single instance?

At the moment, I have a single c4.large (3.75GB RAM, 2 vCPU) instance in my workers cluster, currently running 21 tasks for 16 services. These tasks range from image processing, to data transformation, most sending HTTP requests too. As you can see, the instance is quite well utilisated.

CPU and memory utilisation are also quite high

My question is, how do I know how many tasks to place on an instance? I am placing up to 8 tasks for a service, but I'm unsure as to whether this results in a speed increase, given they are using the same underlying instance. How do I find the optimal placement?

Should I put many chefs in my kitchen, or will just two get the food out to customers faster?

Solution

  • We typically run lots of smaller sized server in our clusters. Like 4-6 t2.small for our workers and place 6-7 tasks on each. The main reason for this is not to speed up processing but reduce the blast radius of servers going down.

    We've seen it quite often for a server to simply fail an instance health check and AWS take it down. Having the workers spread out reduces the effect on the system.